[Effect of (Na)e concentration on action potentials of myocardial fibers of Pseudemys].

The occurrence, in the myocardial fibers of several vertebrates, of two components, a fast and a slow one, in the depolarizing phase of the action potential, as shown by several workers, has been tested on freshwater turtle myocardium. In order to test the dependence of the fast component on external sodium concentrations, the maximum rate of depolarization and the level of the notch between the two phases were analyzed in hyposodic solutions. No dependence was found between the height of the overshoot of the action potential and [Na]e, either in atrial or in ventricular fibers. Instead, the maximum depolarization rate and the level of the notch between the two phases showed a linear dependence on the logarithm of [Na]e. These findings support the hypothesis that the fast component is sodium-dependent, while the slow one, responsible for the reaching of the overshoot peak, does not depend on [Na]e and has a more positive equilibrium potential.