The effects of ryanodine, EGTA and low-sodium on action potentials in rat and guinea-pig ventricular myocytes: evidence for two inward currents during the plateau.

Action potentials were recorded from single cells isolated from rat and guinea-pig ventricular muscle. In rat cells the repolarization showed two distinct phases, referred to as the early and late phases. In guinea-pig cells there was a maintained plateau. Reducing external sodium by replacement with lithium or choline suppressed the late phase of the action potential in rat cells, and shortened the plateau of the action potential in guinea-pig cells. Intracellular EGTA abolished contraction while suppressing the late phase of the action potential in rat cells, and shortening the plateau in guinea-pig cells. Ryanodine (1 microM), which is thought to inhibit the release of calcium from internal stores, suppressed contraction and the late phase of the action potential in rat cells. In guinea-pig cells, there was no substantial effect of ryanodine (1 microM) on either contraction or the time course of the action potential. The late phase of the action potential in rat cells was suppressed by increasing the external potassium concentration to 12 mM, and enhanced by reducing external potassium to 1.2 mM. It is concluded that an inward current activated by internal calcium contributes to the late phase of the action potential in rat cells, and to the plateau in guinea-pig cells. Two possibilities are a current arising from electrogenic sodium-calcium exchange, and a current through ion channels activated by calcium. The effects of reducing external sodium would be consistent with either mechanism. The contribution of such an inward current would be expected to be modified by outward currents through a rectifying potassium conductance which varies with external potassium concentration. In the rat, but not the guinea-pig, the rise in internal calcium which activates the inward current seems to be largely dependent on ryanodine-sensitive release of calcium from internal stores.