Influence of varied [Ca 2+] o and [Na +] o on slow responses of newborn and adult rat hearts

Abstract

The electrophysiological properties of slow responses of rat hearts were investigated in newborn (two-day-old) and adult (few-month-old) rats under conditions of high [K+]0 (25 mM) with isoproterenol (1 X 10(-7)M) using the standard techniques of intracellular microelectrode recording. The potentials of slow responses were measured in varied [Ca2+]0: in adults the peak potential varied linearly with a slope of about 31 mV for a 10-fold change in [Ca2+]0, while in newborns it varied, but not linearly, and showed a positive peak potential in low [Ca2+]0 (0.5 mM). In a low [Na+]0 solution (55% [Na+]0), with constant [Ca2+]0 (2 mM), the peak potential was reduced in newborn hearts, while it was little affected in adult hearts. These results suggest that the peak potential of the slow response depends mainly on [Ca2+]0 in adult rat hearts, while it depends on both [Ca2+]0 and [Na+]0 in newborns.