Restitution of the action potential in cat papillary muscle.

BASS, kz G. htitution of the action potential in cat papillary muscle. Am. J. Physiol. Z%(6) : 17 17-l 724. 1975.-Membrane recovery (electrical restitution) following an action potential (AP) was studied in isolated, electrically driven cat papillary muscle by interpolation of test extrasystoles at various delays in the interval between two AP’s (recorded with glass microelectrodes). Curves describing the time course of restitution were derived by plotting extrasystolic AP area versus delay preceding the extrasystole. Electrical restitution begins with slow dV/dt, short plateau, and small AP area, then proceeds through a phase of large AP area with long positive plateau &ass A of electrical restitution) before control AP characteristics recur. AP’s during phase A resemble low-Ca++ AP’s. AP characteristics reciprocal to those during phase A persist for three to four postextrasystolic AP’s and resemble high-Ca++ AP’s. Phase A and postextrasystolic AP changes are exaggerated in low Ca+f and minimal in high Ca++. These observations suggest an inverse relationship between membrane Ca++ content and AP area and the existence of a negative feedback loop by which AP configuration is controlled. It is proposed that phase A, which occurs approximately concurrently with phase 1 of mechanical restitution (I), may be due to a transient post-AP decrease in membrane Ca++ content associated with the intracellular Ca++ elevation thought to be responsible in part for phase 1.