Effects of Na-Ca Exchange Mechanism on the Action Potential and Membrane Currents in the Single Cells of the Guinea-Pig and the Rabbit Heart

ABSTRACT

In single atrial and ventricular cells isolated from the guinea-pig and the rabbit heart, action potentials and membrane currents were recorded by using the whole cell voltage clamp technique. In rabbit atrial cells the repolarization showed two distinctive phases, referred as the early and late phases(early and late plateau phase), but in guinea-pig atrial cells there was a maintained plateau and less distinctive two phases of repolartization. Increasing intracellular sodium or reducing external sodium by replacement with lithium suppressed the late phase of the action potential in rabbit atrial cells and shortened the plateau of action potential in rabbit ventricle and guinea-pig atrial cells. Reducing external sodium decreased Ca-current and late inward current in voltage clamp. Ouabain in the concentration of 10(-5)M shortened the duration of action potential and shifted the holding current level to outward direction, decreased Ca-current and moved late inward current to outward direction. Ryanodine 10(-6)M which is known to be an inhibitor of Ca-release in the intracellular store, suppressed the late phase of action potential in rabbit atrial cells and shortened the plateau of action potential in rabbit ventricular cells. Ryanodine also decreased Ca-current and shifted late inward current to outward direction. It is concluded that an inward current activated by intracellular calcium contributes to the late Phase of the action potential in rabbit atrial cells and to the late plateau in rabbit ventricular cells and in guinea-pig atrial cells. It may be carried by the Na-Ca exchange precess and/or by calcium-activated non-specific channels but preferably Na-Ca exchange machanism.