Maximal Twitch Tension in Intact Length-Clamped Ferret Papillary Muscles Evoked by Modified Postextrasystolic Potentiation

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Maximal Twitch Tension in Intact Length-Clamped Ferret Papillary Muscles Evoked by Modified Postextrasystolic Potentiation
65

                     Maximal Twitch Tension in Intact
                  Length-Clamped Ferret Papillary Muscles
                    Evoked by Modified Postextrasystolic
                               Potentiation
                Ferdinand Urthaler, Alfred A. Walker, David N.S. Reeves, and Lloyd L. Hefner

          A modified test of postextrasystolic potentiation achieved with a brief episode of rapid pacing followed
          by a 6-second pause (RPP maneuver) was used to evoke maximal force in isolated intact ferret right
          ventricular papillary muscles. Maximal RPP tensions were examined under length-clamped conditions
          and compared with the steady-state forces obtained when further increases in [Ca2+]0, did not further
          increase force and to the tensions recorded at the point of saturation of force when similarly
          length-clamped muscles were subjected to caffeine-induced tetanization. The results show that the
          calculated maximal twitch tension achieved with RPP is comparable to the 25-35 g/nun2 observed in
          intact single skeletal muscle fibers. The study also shows that the beat-to-beat decay of the potentiated
          contraction is exponential. While the amount of the constant fractional beat-to-beat decay is a function
          of [Ca^L, it is not influenced by length. During the decay of potentiation, the ratio of the potentiation
          of any beat divided by that of the previous beat is a constant, called (x). With certain assumptions,
          it is shown that (x) is a measure of the fraction of activator calcium taken up by the sarcoplasmic
          reticulum in each beat and, in the steady state, the fraction of activator calcium that comes from the
          sarcoplasmic reticulum. The (x) amounted to 33%, 50%, and 65% when [Ca 2+ ], was 1.25, 2.50, and
          5.0 mM, respectively. Thus, at 1.25 mM [Ca2+]., some two thirds of the total calcium required to
          activate the myofilaments comes from the extracellular compartment during excitation and only one
          third is contributed via release from the sarcoplasmic reticulum. In the region of optimal myofilament
          overlap, RPP force-length curves are remarkably shallow and almost indistinguishable from the
          sarcomere length-tension relation observed in skinned single cardiac cells. Tetanus plateau tensions
          are significantly smaller than RPP forces at any length, and the slope of the tetanus force-length curves
          is greater than that obtained with RPP. Thus, and by exclusion, we also suggest that caffeine may exert
          significant downstream inhibitory effects. (Circulation Research 1988;62:65-74)

        ytosolic free calcium concentration, [Ca2+],,

C
                                                                       In the present study, we have used a modified test of
         modulates the contraction and relaxation of                postextrasystolic potentiation to achieve maximal
         myocardial myofibrils, and the resting [Ca2*],             force. We have measured these maximal tensions under
of 0.05-0.5 /xM will increase by one or two orders of               length-clamped conditions and compared the magni-
magnitude when the cells are electrically or chemically             tude of these peak twitches with the amplitude
stimulated.1 To examine the relation between [Ca2+];                of the beats recorded when further increases in extra-
and force, steady-state levels of calcium activation of             cellular calcium, [Ca 2+ ] o , did not cause further in-
contractile proteins must be achieved. Since in the                 creases in force and with the tensions recorded at
normally beating heart the duration of the contraction-             the point of saturation of force when similarly length-
relaxation cycle is too brief to allow such steady state            controlled muscles were subjected to caffeine-induced
to develop,2 various investigators5"" resorted to studies           tetanization.
of the pCa-force relation in hyperpermeable3"* or                      The results show that the calculated maximal twitch
mechanically9"" skinned cardiac muscle fibers. More                 tension that can be achieved in an intact length-clamped
recently, rapid repetitive stimulation (tetanization) of            papillary muscle with a modified postextrasystolic
ryanodine-pretreated but otherwise intact papillary                 potentiation is comparable to the peak tensions of
muscles1213 was successfully used to achieve both                   25-35 g/mm2 observed in intact single skeletal muscle
steady contractile activations and maximal calcium-                 fibers.l413 These forces, also observed by others,16"" are
activated force.1213                                                considerably larger than the maximal calcium-activated
                                                                    forces observed in either mechanically skinned cardiac
  From the Division of Cardiology, Department of Medicine,
University of Alabama at Birmingham, Birmingham, Ala.               fibers6 or pharmacologically pretreated intact muscle
  Supported by the National Heart, Lung, and Blood Institute, HL    during tetanization.'213 The study also shows that the
31310 and SCOR on Ischemic Heart Disease Grant 5P50HL17667.         beat-to-beat decay of the force after the potentiated
  Address for reprints: Ferdinand Urthaler, MD, Division of         contraction is exponential and that the amount of the
Cardiology, Department of Medicine, University of Alabama at
Birmingham, Birmingham, AL 35294.                                   constant fraction of beat-to-beat decay is a function of
  Received February 26, 1987; accepted July 28, 1987.               [Ca 2+ ] 0 .

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66                                                                       Circulation Research        Vol 62, No 1, January 1988

               Materials and Methods                             that 1.5%. In the few muscles with larger angles of
   Right ventricular papillary muscles (cross-sectional          rotation as estimated visually, the pins are readjusted.
area
Urthaler et al Maximal Force of Intact Cardiac Muscle                                                                              67

                  Modified Post-Extrasystolk Potentiation                puting p values.24 Throughout each repeated measures
     12
                                                                         analysis of variance, we chose a conservative estimate
                                                                         of p, as reflected by the Geisser-Greenhouse value.23

                                                                                                Results
     8-           fca'+lo: 2.50                                          Paired Pacing Versus Modified Postextrasystolic
                    Temp: 25*                                            Potentiation
                     CSL: 0.98L m a x
                                                                            Twitch tensions evoked by maximally effective
                                                                         paired pacing (PP) were compared with the potentiated
 —   4
                                                                         contractions achieved with the rapid pace pause (RPP)
 >
 ©                                                                       maneuver. Each muscle (n = 5) was studied at two
                                                                         central segment lengths (0.900 and0.984% of L J and
                                                                         exposed to three increasing [Ca2+]0 (Table 1). At low
                                                                         length and 1.25 mM Ca, the control force averaged
             I IiiIi iiI I
                                                                         2.34 ± 0.52 g/mm2. PP and RPP increased the force by
            400      800     1200    1600     0       400   800   1200    179% and 216%, respectively, above control. At 2.5
                                    Time in msec
                                                                         mM Ca, these increments were 98% and 102%, and at
FIGURE 1. Example of modified postextrasystolic potentiation             5.0 mM Ca, the respective potentiations were 44% and
(PC) achieved with rapid pace pause (RPP) maneuver. Control              48% above control. At 0.984% of 1 ^ and 1.25 mM
steady-state twitch (left) generates 4.1 glnvn2 offorce. Appli-          Ca, the control force averaged 3.94 ± 1.18 g/mm2. PP
cation of 10 repetitive stimuli (short vertical bars) beginning at       and RPP increased the force by 128% and 130%,
time of half relaxation of steady-state beatfollowed by 6-second         respectively, above control. With exposure to 2.5 mM
pause leads to PC of 11.6 g/mnf.                                         Ca, PP and RPP increased twitch tension by 64% and
                                                                         70%, respectively, and at 5.0 mM Ca, the increments
                                                                         of force still averaged 31% and 33%, respectively.
consists of a series of rapid stimuli beginning at half                  Except for one paired comparison between PP and RPP
relaxation time of the control steady-state beat, fol-                   evoked potentiations (see low length and 1.25 mM Ca,
lowed by a pause (Figure 1). The pause is terminated                     Table 1, there were no significant differences between
by the potentiated beat. In addition to the interval                     PP and RPP beats at any length or [ C a ] o .
between the stimulus of the preceding beat and the                          On instituting paired pacing, the magnitude of the
onset of rapid repetitive stimulation at half relaxation,                potentiated contractions first typically increases over
we selected 10 stimuli (5-msec stimulus duration)                        some 4 - 8 beats before reaching a peak and then
delivered at a frequency of 10-15 Hz followed by a                       stabilizes some 6-12 beats after onset of PP at a level
pause of 6 seconds. The actual execution of this                         slightly lower than peak. In contrast, the RPP elicited
modified postextrasystolic potentiation using the preset                 maximal potentiation is always achieved with the first
parameters requires a single keyboard command.                           beat after the pause, and it can easily be repeated with
Reference control beats (usually two), the potentiated                   less than 5% variability. Thus, twitch potentiation
contraction, and at least 10 consecutive beats fol lowing                evoked with RPP is a convenient and ideally suited
the PC were routinely recorded and stored by the                         intervention whenever a reliable, immediate, and
computer for subsequent analysis.                                        rapidly reversible potentiation of contractile perfor-
                                                                         mance is desired.
Statistics
   The results are expressed as means ± S D . The                        Maximal Twitch Tension
repeated measures analysis of variance, section P2V,                        Using bicarbonate-phosphate buffer. In four other
Biomedical Data Package (BMDP), was used for com-                        length-clamped muscles (0.989 L ^ ) , [Ca 2+ ] 0 was

          Table 1. Effects of Length and [Ca3*], on Standard and Modified Postextrasystolic Potentiations Studied in
          Five Ferret Papillary Muscles
                                                              Central segment length (% L,^)
                                                 0.900                                                0.984
                               DVLT                PP              RPP                DVLT              PP            RPP
          [Ca- + ] 0          (g/mm2)          (g/mm2)           (g/mm2)             (g/mm2)         (g/mm2)        (g/mm2)
          1.25 mM           2 .34±0.52       6 .52±1.44        7.40+1.37            3.94±1.18      8.97 ±1.80      9.10±1.79
          2.50 mM           4 .38±0.53       8.66 ±1.08        8.86±0.96            6.29±1.10     10.32±1.37      10.72± 1.29
          5.00 mM           6 .66±0.87       9 .60±1.40        9.86±1.30            8.54±0.73     11.20+1.50      11.40±1.49
            DVLT, steady-state developed tension; PP, postextrasystolic potentiation evoked by paired pacing; RPP, modified
          postextrasystolic potentiation evoked by rapid pace pause maneuver. Repeated measures analysis of variance for all the
          above conditions (3 x 2) indicates that there is no significant difference between PP and RPP except at low length and
          1.25 mM Ca.

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68                                                                                               Circulation Research         Vol 62, No 1, Janua

  Effect of [Ca* + ] o 0n Steady State Forces and RPP Evoked Potentiated Contractions      Decay of potentiated contraction. At any
  12                                                                                    [Ca 2+ ] o (0.625-5.0 mM), the potentiated cont
                                                                                        decays fractionally with each beat. Figures 3
                                                                                        illustrate such a characteristic decay. At low [
                                                                                        (0.625 mM) (n = 5), return to steady-state controlforce
                                                                                        required 2-3 beats, while 6-10 beats were nee
                                                                                        return to control at 5.0 mM [Ca 2+ ] o (n = 8). The n
                                                                                        of beats rather than time is the critical determin
                                                                                        this decay. Thus, whether paced at 12-24 beats/
                                                                                        any other rate between 6 and 30 beats/min, it ta
                                                                                        any of the [Ca 2+ ] 0 studied, the same number o
                                                                                        for the potentiated contraction to return to contr
                                                                                        fractional decay of each beat was then further
                                                                                        acterized by plotting on semilogarithmic pap
                                                                                        difference between the potentiated force and the c
FIGURE 2. Maximal potentiated contraction evoked with RPP                               force versus the number of the beat (Figu
(twitches without hatching) is observed at 5 mM [Ca2*],, both                           Irrespective of the [ C a ] 0 , each of these plots a
in HEPES and in phosphate-bicarbonate-containing buffer.                                imates a straight line indicating that the decay o
Maximal steady-state twitch tensions (hatched twitches) are                             potentiated contractions is exponential. The low
achieved when [Ca1*], is increased to 14 mM. Peak twitch                                [ C a ] 0 , however, the steeper the slope. Thus
tensions achieved with RPP are consistently, although slightly,                         possible to directly determine from these grap
higher than maximal steady-state twitch tensions.                                       rate constant, K, which is the number of
                                                                                        required for the potentiated contraction to decay
                                                                                        or =
Urthaler et al Maxima) Force of Intact Cardiac Muscle                                                                                                   69

                    Effect of Low and High [Ca' + ] 0 on the Beat-Dependent
                           Decay of Potentiated Contraction

                                                                                          FIGURE4. Effect of low (0.625 mM)andhigh (5.0
                                                                                          mM) [Co2*], on normalized beat-dependent decay
                                                                                          of RPP-induced potentiated contraction (PC).
                                                                 Rate: 12
                                                                Temp: 25* C
                                                                                          x-Axis, beat numberfollowing PC; y-axis, ratio of
                                                                                          ADVLT/bDVLT^; &DVLT, developed tension
                                                                                          minus control steady-state tension; SDVLT^,
                                                                                          developed tension ofPC minus control steady-state
                                                                                          tension.

                                    2                 3
                                        Beat Number

   Effect of length on maximum twitch tension evoked                          relations in this region of lengths. In the tetanized
with RPP maneuver. The results of this study are                              caffeine-pretreated muscles at 5.0 mM [Ca2+]o, the
summarized in Table 2. In 7 muscles exposed to 5 mM                           normalized increase in force is still 26% with a slope
Ca, twitch tension elicited with the RPP maneuver                             of 3.1 (r = 0.95). In contrast, the effect of length on the
steadily increased by some 14% when length was                                RPP-potentiated contraction at 5.0 mM [Ca2+]0 showed
incremented from 0.900 to 0.986 L ^ , although at L ^ ,                       a 12% increase in force with a slope of 1.6 (r = 0.99),
an increase in [Ca2+]o from 5.0 to 8.0 mM had no                              which is significantly less (/?
70                                                                         Circulation Research           Vol 62, No 1, January 1988

         Table 2A. Effect of Length on Amplitude of Tetanic Plateau Force (n=12)
         Central segment length (% L . , )   0.907          0.926         0.943               0.964                  0.986
                                        2
         Tetanus plateau tension (g/mm )   7.04±1.86     7.63 ±1.85     8.27+1.97          8.86 + 2.20             9.58±2.59

         Table 2B. Effect of Length on Maximum Twitch Tension Evoked With RPP Maneuver (n = 7)
         Central segment length (%                 0.900         0.930         0.950            0.950                0.986
         RPP maximum twitch tension (g/mm2)     10.69 ±1.08   11.43±0.81    11.76±0.75       11.76 ± 0.75         12.14±0.77

                        Discussion                                different experimental approaches. Developed force
Maximum Twitch Tension                                            increases steadily and directly with increases in [Ca2+]0.
   The present study shows that the maximum uncor-                Maximum twitch tensions areregularlyachieved when
rected twitch tension of an intact ferret right ventricular       [Ca2+]ois raised to 11-14 mM. Similar peak forces are
papillary muscle, length-clamped at 0.99 L^, and                  obtained from the frequency-potentiated contractions
contracting 12 times per minute at 25° C, is about 12             of the RPP maneuver at 5 mM [Ca2+]0. Under condi-
g/mm2. After taking into consideration the ratio of               tions of maximal steady-state contractility (L,,^ and
cellular-to-total volume, which amounts to 0.6 accord-            11-14 mM [Ca2+]0, further increases in [Ca2+]0 led to
ing to Page,2* the maximum corrected twitch tension is            a fall in developed tension. The same declines in peak
about 20 g/mm2. This peak developed force compares                force also occurred when muscles underwent RPP
favorably with the maximal calcium-activated isomet-              testing, provided the [Ca2+]0 is 5 mM or more. Hence,
ric tension of mechanically skinned skeletal muscle               by definition, the muscles exhibited calcium over-
fibers,272* which have variably been reported to average          load .M It has recently been shown that calcium overload
about 11 g/mm2 u or 14 g/mm2 w or to vary between 15              is related to a diastolic release of calcium from the
and 20 g/mm2.27 Moreover, since the volume fraction               sarcoplasmic reticulum into the cytosol.* Such release
of mitochondria (17-37%) is considerably larger in                will diminish the amount of sarcoplasmic reticulum
cardiac*031 than in skeletal muscle cells (2%), peak              calcium available for release in the ensuing beat(s) or,
cardiac stress, normalized for myofibrillar cross-sec-            perhaps, partially inactivate the subsequent calcium-
tional area, will be in the vicinity of 25-30 g/mm.               induced calcium release of calcium.
These tensions are comparable to those observed by                   From these considerations, we conclude that in-
Gordon et al in the intact frog skeletal musclefiberand           creases in [Ca2+]0 alone can increase developed tension
are very close to the 35 g/mm measured by Ramsey                  to near maximal levels achievable by ferret papillary
and Street" in the intact single fibers of frog semiten-          muscles. We also conclude that peak cardiac stress per
dinosus. The overall agreement between the peak                   unit cross-sectional area of myofibrils at the point of
stresses of intact length-clamped papillary muscles and           saturation of twitch force is around 30 g/mm2 and that
intact single skeletal fibers is probably as good as can          this peak force is reliably achieved in one twitch with
be reasonably expected from such measurements and                 the RPP maneuver. Because peak forces evoked by
calculations, especially considering that the multicel-           RPP are virtually indistinguishable from maximal
lular preparation with its complex cell-to-cell attach-           calcium-activated force of a single skeletal fiber1428
ments and branchings" cannot achieve the structural               and because these tensions were usually much larger
arrangement of the more uniform longitudinal striation            than the maximal calcium-activated forces of skinned
of the single skeletal fiber. Thus, we suspect that               cardiac cells6 or tetanized papillary muscles,1213 we
cardiac and skeletal contractile proteins are capable of          also suspect that peak segment-controlled RPP ten-
generating about the same amount of peak force as
previously suggested by Brady.                                                       Effect of Clungti In l«nfth on Ampbtud*
                                                                                               of TtUnic Ptatuu Forct

   In rat ventricular trabeculae in which central segment
sarcomere shortening was prevented17 and in rabbit
papillary muscles in which the sarcolemma was ren-
dered hyperpermeable with EDTA,3 the peak tensions
wereremarkablysimilar to those observed in our study.
In contrast, the maximal calcium-activated forces of
chemically skinned single rabbit and rat ventricular
cells were considerably smaller.6 In a preparation
similar to ours but without length control, the maxi-
mum plateau forces evoked by ryanodine-induced
tetanization1213 averaged only half of the peak tensions
elicited by the RPP maneuver. Uncontrolled internal                                                   800
shortening at the expense of stretch of the damaged                                                 Timt «i

ends17"-54-33 is a likely reason for prominent inhibitory         FIGURE 7. At Lma, increases in [Ca2*]. beyond 5 mM did not
effects.                                                         further increase amplitude of tetanic plateau. However, changes
   The findings of this study further illustrate that             in length from 90 to 98% ofL^ clearly increased amplitude of
similar maximum twitch tensions can be achieved using             tetanic force (see text for details).

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Urthaler et al        Maximal Force of Intact Cardiac Muscle                                                                                              71

                           Comparison between RPP and Tetanus
                    Evoked Force Central Segment Lengths Relationships

                                                                              A RPP                FIGURE 8. Comparison between RPP- and tetanus-
                                                                              • Tetanus
                                                                                                   evokedforce-length relations. Panel A: Forces expressed
                                                                                                   in g/mnr. Panel B: Forces normalized and expressed in
  12                                                                                               % offorce at SL^. Dotted line in Panel B represents a
                                                                                                   reference steady-state twitch tension-length curve ob-
                                                                                           1.0 J
                                                                                                   tained in 10 muscles exposed to 1.25 mM [Ca2*]..
                                                                                                   Calculated slope of this reference curve is 5.3 (T - 0.99),
                                                                                                   and percent increase of force between lowest and highest
                                                                                                   length is 45%. •, Force-length curve obtained in 12
                                                                                                   caffeine-pretreated tetanized muscles; normalized slope
                                                                                                   (Panel B) is 3.1 (r = 0.95). A, Force-length curve from
                                           C«'+]
72                                                                       Circulation Research        Vol 62, No 1, January 1988

not altogether clear whether the brief initial positive          influx through the voltage-dependent slow channels as
inotropic response should be ascribed to an immediate            well as the calcium entry via the membrane potential-
transient release of calcium from an internal store4* or         dependent Na-Ca exchange.
to an increase in sensitivity of the myofilaments to                2) It is assumed that with each beat, the SR and the
calcium.43 On the other hand, it appears that the                SL compete for reuptake of calcium. In our model, the
prolonged decrease in contractility can be largely               disposal of calcium by the SL includes the calcium
attributed to a reduction of calcium release into the            extrusion via both the Na-Ca exchanger and an ATPase-
cytoplasm from the sarcoplasmic reticulum.42-43                  operated pump. It is further assumed that under given
   Recently, it has also been shown that in papillary            experimental conditions, reuptake of calcium by the SR
muscles varying [Ca 2+ ] 0 caused marked changes in the          will be a constant fraction, x, of the total activator
level of tetanic force.1213 Our study is in accord with          calcium.
these observations since increases in [Ca 2+ ] o from 2.5           3) It is assumed that the SR always releases the same
to 5.0 mM consistently raised the level of the tetanus           amount of calcium that it took up on the previous beat.
plateau tensions. However, while further increases in               4) It is assumed that the amount of calcium entering
[Ca 2+ ] 0 beyond 5 mM caused no further increments in           through the SL is a constant amount, A, at least for the
tetanic force, presumably because of maximal activa-             6-8 beats following the RPP maneuver, implying that
tion, the amplitudes of the tetanic plateau tension still        the RPP maneuver per se does not alter the fractional
varied appreciably with changes in length. In other              amount of transsarcolemmal flux of calcium during
words, at any length between 0.90 and 0.98 L^,, the              these 6-8 beats. It can be some other constant (and
amplitude of the tetanus plateau was no greater at 8 or          should be) under different conditions.
 10 mM [Ca 2+ ] 0 than at 5 mM [Ca 2+ ] 0 , although
increments in length caused the same amount of                   Mathematical Derivation
increase in tetanic force at all three [Ca 2+ ] 0 .                 5) Let the amount of calcium for the potentiated beat
   In the region of optimal myofilament overlap, the             be denoted by Co (C for calcium, 0 for reference beat).
normalized tetanus force-length curves exhibited                 For the first beat after the maximal potentiated beat, the
slopes that were considerably steeper than those                 amount of calcium from the SR is xC0 (assumptions 2
obtained with the RPP maneuver. Because the tetanus              and 3), and the amount of calcium entering through the
slopes were identical at 5, 8, and 10 mM [Ca 2+ ] 0 , it         sarcolemma is A (assumption 4). The total activator
is unlikely that caffeine reduced sarcolemmal trans-             calcium for this beat, C,, is the sum of these (assump-
location of calcium so as to precisely offset the                tion 1):
increments of the transsarcolemmal calcium gradient
that occurred when [Ca 2+ ] 0 increased. Taken together,
these observations raise the important question of why                                 C, = xC0 + A
the slopes of the force-length curves are steeper with             For beat 2,
tetanus than with RPP. This difference in slope cannot
be ascribed to physical factors since it is consistently                               C2 = xC, + A
demonstrable when the same muscles are examined                    where C2 is the total amount of activator calcium,
with RPP and tetanus. Furthermore, if one accepts                xC, of which comes from the SR and A through the SL.
that muscles exposed to 5 mM caffeine and 5 mM                     In general, for beat (n+ 1),
[Ca 2+ ] 0 are subjected to maximal activation, then
changes in length should not be associated with                                       Cn+l=xC, + A                          (1)
changes in force resulting from altered myofilament                 Eventually, a new steady state will be reached in
sensitivity to calcium. The observation that further             which the amount of activator calcium is essentially
increases in [ C a ] 0 to 8 and even 10 mM had no effect         the same for every beat. This amount of calcium is
of their own on the slope of the tetanus force-length            denoted by Ca. Equation 1 must still be satisfied, so
curves strongly supports this assertion. Thus, other
mechanism(s) must be invoked to explain the steeper                                        = xCL + A                       (2)
length-dependent maximal activation of force induced               To eliminate A from Equation 1, Equation 2 is
by tetanization with caffeine. By exclusion, we                  solved for A:
arrived at the suggestion that caffeine may well exert
significant downstream inhibitory effect(s), i.e., that                                A = C. - xC.
caffeine could, in fact, diminish the response of the               Equation 1 now becomes
contractile proteins to a given level of occupancy of
the calcium binding sites on troponin C.
                                                                                        = x(Cn - C J + C.
                                                                                                                           (3)
                      Appendix
Excitation-Contraction Coupling Model                                                                 • =   X

   1) It is assumed that activator calcium (C) comes                                     c —c
from the sarcoplasmic reticulum (SR) and through the             Final Assumption
sarcolemma (SL), which refers to all sources of                    6) A final assumption is that to a reasonable ap-
transsarcolemmal calcium entry such as the calcium               proximation, the force developed in a beat is propor-

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Urthaler et al   Maximal Force of Intact Cardiac Muscle                                                                                  73

tional to the activator calcium, at least in the range seen                  of single skinned cardiac cells. J Physiol (Lond) 1975;
                                                                             249:469-495
in our experiments. If k is the constant of proportional-              11.   Fabiato A: Calcium release in skinned cardiac cells: Variation
ity, then                                                                    with species, tissues and development. Fed Proc 1982;41:
                        C       = JtF                                        2238-2244
                                                                       12.   Yue DT, Marban E, Wier WG: Relationship between force and
                             *-•„= krn                                       intracellular [Ca2+] in tetanized mammalian heart muscle. J
                                                                             Gen Physiol 1986;87:223-242
                                                                       13.   Marban E, Kusuoka H, Yue DT, Weisfeldt ML, Wier WG:
where F n + , is developed force in (n+ l)st beat, F, is                     Maximal Ca:+-activated force elicited by tetanization of ferret
developed force in the immediately preceding beat,                           papillary muscle and whole heart: Mechanism and character-
and F» is developed force in the steady-state beat.                          istics of steady contractile activation in intact myocardium.
                                                                             Circ Res 1986;59:262-269
Substitute these into Equation 3:
                                                                       14.   Gordon AM, Huxley AF, Julian FJ: The variation in isometric
            —C         JtF      — JtF      F     —F         AF               tension with sarcomere length in vertebrate muscle fibers. J
 X =                                                                         Physiol (Lond) 1966; 184:170-192
                                          F -F.           AF.          15.   Ramsey RW, Street SF: The isometric length-tension diagram
       c.-c.                                                                 of isolated skeletal muscle fibers of the frog. J Cell Comp
      AF n+1                                                                 Physiol 1940;15:ll-34
 X =                                                   (4)             16.   Pollack GH, Krueger JW: Sarcomere dynamics in intact cardiac
       AF                                                                    muscle. Eur J Cardiol 1976;4(suppl):53-65
                                                                       17.   Ter Keurs HEDJ, Rijnsburger WH, Van Henningen R, Nagel-
   This means that one can determine x simply by                             smith MJ: Tension development and sarcomere length in rat
forming the ratio shown. For any two successive beats,                       cardiac trabeculae. Evidence of length-dependent activation.
the steady-state force is subtracted and then the ratio of                   Circ Res 1980;46:703-714
the differences is calculated.                                         18.   Gordon AM, Pollack GH: Effect of calcium on the sarco-
                                                                             mere length-tension relation in rat cardiac muscle. Implica-
   It can also be shown37 that x, as defined above and                       tions for the Frank-Starling mechanism. Circ Res 1980;47:
using the previous assumption, is related to K (Figure                       610-619
6), the beat constant for decay, by the equation:                      19.   Donald TC, Reeves DNS, Reeves RC, Walker AA, Hefner LL:
                                                                             Effect of damaged ends in papillary muscle preparations. Am
                                                                             J Physiol 1980;238:H14-H23
                                                                       20.   Urthaler F, Walker AA, James TN: Changing negative ino-
   For each decay, with a mathematical approach, sev-                        tropic effect of acetylcholine in maturing canine cardiac
eral values of x can be directly calculated and thus, by                     muscle. Am J Physiol 198O;238:H1-H7
averaging, a reliable estimate of x can be obtained. In                21.   Urthaler F, Walker AA: Indirect stimulatory action of the
contrast, with the graphic approach, only one value of                       calcium channel blockers AQA-39. J Pharmacol Exp Ther
                                                                              1984;230:336-340
K (1/e) can be obtained; hence, only one value of x can                22.   Forman R, Ford LE, Sonnenblick EH: Effect of muscle length
be obtained.                                                                 on the force-velocity relationship of tetanized cardiac muscle.
                                                                             Circ Res 1972;31:195-206
                                                                       23.   Ford LE, Forman R: Tetanized cardiac muscle, in Ciba
                          References                                         Foundation Symposium 24: The Physiological Basis of Star-
 1. Reuter H: Calcium channel modulation by neurotransmitters,               ling's Law of the Heart. Amsterdam, North Holland, Elsevier-
    enzymes and dnigs. Nature 1983;301:569-574                               Excerpta Medica, 1974, pp 137-154
 2. Robertson SP, Johnson JD, Potter JD: The time course of CaI+       24.   Dixon TC: BMDP Statistical Software. Berkeley, University of
    exchange with caJmodulin, troponin, parvalbumin and myosin               California Press, 1981
    in response to transient increases in Ca2 + . Biophys J            25.   Geisser S, Greenhouse S: An extension of Box's results on the
    1981 ;34:559-569                                                         use of the F distribution in multivariate analysis. Ann Math
 3. WinegradS: Studies of cardiac muscle with a high permeability            Statist 1958;29:885-891
    to calcium produced by treatment with ethylene diamine-            26.   Page E: Cat heart muscle in vitro. III. The extracellular space.
    tetraacetic acid. J Gen Physiol 1971 ;58:71-93                           J Gen Physiol 1962;46:201-213
 4. Endo M, Kitazawa T: E-C coupling studies on skinned cardiac        27.   Endo M: Regulation of contraction-relaxation cycle of muscle
    fibers, in Morad M, Tabatabai M (eds): Biophysical Aspects of            (in Japanese). Proc XVII Gen Assoc Japan Med Congress
    Cardiac Muscle. New York, Academic Press, 1978, pp                       1967;1:193
    307-327                                                            28.   Gordon AM, Godt RE, Donaldson SKB, Harris CE: Tension
 5. Kentish JC, Nayler WG: Ca!+-dependent tension generation in              in skinned frog muscle fibres in solutions of varying ionic
    chemically "skinned" cardiac trabeculae: Effect of pH. J                 strength and neutral salt composition. J Gen Physiol 1973;
    Physiol (Lond) 1978;284:9O-91P                                           62:550-574
 6. Fabiato A: Myoplasmic free calcium concentration reached           29.   Hellam DC, Podolsky RJ: Force measurements in skinned
    during the twitch of an intact cardiac cell and during calcium-          muscle fibres. J Physiol (Lond) 1969;2O0:807-819
    induced release of calcium from the sarcoplasmic reticulum of      30.   Urthaler F, Walker AA, Kawamura K, Hefner LL, James TN:
    a skinned cardiac cell from the adult rat or rabbit ventricle. J         Canine atrial and ventricular muscle mechanics studied as a
    Gen Physiol 1981;78:457-497                                              function of age. Circ Res 1978;42:703-713
 7. Kentish JC: The inhibitory effect of monovalent ions on force      31.   Canale ED, Campbell GR, Smolich JJ, Campbell JH: Morphol-
    development in detergent-skinned ventricular muscle from                 ogy of cardiac muscle, in Oksche A, Vollrath L (eds): Hand-
    guinea-pig. J Physiol (Lond) 1984;352:353-374                            book of Microscopic Anatomy, vol FI/7: Cardiac Muscle.
 8. Kentish JC: The effects of inorganic phosphate and creatine              Berlin, Heidelberg, New York, Tokyo, Springer-Verlag, 1986,
    phosphate on force production in skinned muscles from rat                pp8-51
    ventricle. J Physiol (Lond) 1986;370:585-6O4                       32.   Page E, McCallister LP, Power B: Stereological measurements
 9. Fabiato A, Fabiato F: Activation of skinned cardiac cells.               of cardiac ultrastructures implicated in excitation-contraction
    Subcellular effects of cardioactive drugs. Eur J Cardiol                 coupling sarcotubules and T-system. Proc Natl Acad Sci USA
    1973; 1:143-155                                                          1971;68; 1465-1466
10. Fabiato A, Fabiato F: Contractions induced by a calcium-           33.   Brady AJ: Contractile and mechanical properties of the
    triggered release of calcium from the sarcoplasmic reticulum             myocardium, Sperelakis N (ed): Physiology and Pathophys-

                              Downloaded from http://circres.ahajournals.org/ by guest on March 6, 2015
74                                                                                 Circulation Research       Vol 62, No I, January 1988

      iology of the Heart, Boston, Martinus Nijhoff Publisher, 1984,           of skinned cardiac cells on the sarcomere length. Nature
      pp 279-299                                                                1975;256:54-56
34.   Julian FJ, Sollins MR, Moss RL: Absence of a plateau in            42.   Blinks JR, Olson CB, Jewell BR, Braveny P: Influence of
      length-tension relations of rabbit papillary muscle when                 caffeine and other methylxanthines on mechanical properties of
      internal shortening is prevented. Nature 1976;260:340-342                isolated mammalian heart muscle. Circ Res 1972;30:367-392
35.   Huntsman LL, Joseph DS, Oiye MY, Nichols GL: Auxotonic             43.   Hess P, Wier WG: Excitation-contraction coupling in cardiac
      contractions in cardiac muscle segments. Am J Physiol                    Purkinje fibers. J Gen Physiol 1984;83:417-433
      1979;237:131-138                                                   44.   Carmeliet E, Vereecke J: Adrenaline and the plateau phase of
36.   Allen DG, Eisner DA, Pirolo JS, Smith GL: The relationship               the cardiac action potential. PflugersArch 1969;313:3O0-315
      between intracellular calcium and contraction in calcium-          45.   Fabiato A, Fabiato F: Techniques of skinned cardiac cells and
      overloaded ferret papillary muscles. / Physiol 1985;364:                 of isolated caridac fibers with disrupted sarcolemma with
      169-182                                                                  reference to the effects of catecholamines and of caffeine.
37.   Morad M, Goldman Y: Excitation-contraction coupling in                   Recent Adv Stud Card Struc Metab 1976;9:71-94
      heart muscle: Membrane control of development of tension.          46.   Chapman RA, Leoty C: The time-dependent and dose-
      Prog Biophys Mol Biol 1975;27:257-313                                    dependent effects of caffeine on the contraction of the ferret
38.   Rosin H, Farah A: Post-stimulation potentiation of contractility         heart. J Physiol (Lond) 1976;256:287-314
      in the isolated auricle of the rabbit. Am J Physiol 1955;          47.   Blinks JR, Endoh M: Modification of myofibrillar responsive-
      180:75-82                                                                ness to CA + + as an inotropic mechanism. Circulation
39.   Hoffman BF, Bindler E, Suckling EE: Postextrasystolic po-                 1986;73(suppl III):III-85-III-98
      tentiation of contraction in cardiac muscle. Am J Physiol
      1956; 185:95-102                                                   KEY WORDS • modified       postextrasystolic   potentiation •
40.   Wood EH, Heppner RL, Weidmann S: Inotropic effects of              force-length curves • maximal twitch tension • tetaniza-
      electric currents. Circ Res 1969;24:409-445                        tion • central     segment   length    control • caffeine •
41.   Fabiato A, Fabiato F: Dependence of the contractile activation     E-C coupling model

                              Downloaded from http://circres.ahajournals.org/ by guest on March 6, 2015
Maximal twitch tension in intact length-clamped ferret papillary muscles evoked by
                        modified postextrasystolic potentiation.
                   F Urthaler, A A Walker, D N Reeves and L L Hefner

                                          Circ Res. 1988;62:65-74
                                        doi: 10.1161/01.RES.62.1.65
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