Abstract
Electrical reexcitation of rat papillary muscle after a short interval (50 to 80 milliseconds) results in action potentials with no significant mechanical counterpart. The mechanical response recovers as the interval is increased beyond 80 milliseconds. The rate of recovery is slowed at low external calcium levels. It appears that the coupling mechanism passes through a refractory stage owing to the depletion of an intracellular "releasable calcium" fraction.
