Heterogeneity of the Action Potential in Isolated Rat Ventricular Myocytes and Tissue

The objectives of this study were to measure action potential parameters in enzyme- dissociated, adult rat ventricular myocytes stimulated at 1 Hz, to compare these measurements with those obtained from intact ventricular tissue, and to determine myocyte and tissue responses at stimulus frequencies between 0.1 and 5 Hz. Action potentials were characterized in terms of amplitude, overshoot, resting potential, duration at 25% and 75% repolarization (APD25, APD75), and Vmax. Based on statistical differences in APD25 and APD75, myocyte action potentials were classified as type 1 (3.1 ± 1.0 and 21.5 ± 3.6 msec), type II (7.4 ± 1.1 and 38.2 ± 6.7 msec), or type III (14.5 ± 1.9 and 46.0 ± 4.1 msec). Action potentials corresponding to type I were found in right ventricular endocardium and right papillary muscles, and those corresponding to types II and III in the left ventricular endocardium [apex, middle (II); base (III)] and left papillary muscles (II). Myocytes and papillary muscles responded to increases in driving rate with nearly identical lengthening of APD25 and shortening of APD75. The one exception was at 5 Hz where a lengthening of the APD75 occurred in some myocytes. We conclude that action potential configuration in rat ventricle is heterogeneous, and that this is reflected by the different types of action potentials in isolated myocytes. It is likely that the magnitude of a transient outward current is a determinant of action potential configuration, and that slow reactivation of this current is a significant factor underlying the stimulus frequency response.