In rat papillary muscle, the mechanical response to paired pulse stimulation depends on a recovery period >80 ms, suggesting excitation-contraction coupling is limited by the recharging of a rapidly releasable calcium fraction.
Myocardial fibers in rat papillary muscle preparations are electrically reexcitable 60 ms or less after previous excitation. Electrical reexcitation at this early stage has no mechanical counterpart and does not increase peak tension, area under the tension curve, or dT/dt. When the isometric tension response to a paired stimulus is superimposed upon the isometric tension response to a single stimulus, no differences in the shape or amplitude of the curves can be detected while the pulse interval is less than 80 ms. As the paired pulse interval is increased beyond 80 ms a small additional response to the second pulse is seen, superimposed upon the response to the first. The response to the second pulse increases progressively as the pulse interval is increased. The increase in response with increasing pulse interval was used to plot the time course of recovery. Recovery is accelerated by epinephrine and decreased by low calcium. The evidence suggests that the limiting factor in the recovery process is in the excitation–contraction coupling system. It is postulated that the recovery of the coupling system depends upon the recharging of the 'rapidly releasable' calcium fraction within the fiber which is completely discharged by the triggering action of a single action potential.
Lee et al. (Wed,) studied this question.