Effect of stimulation and veratrine on total cellular calcium in rat and guinea-pig ventricular myocytes

In rat cardiac myocytes, calcium efflux by Na+/Ca(2+)-exchange is expected only during ventricular systole following initial action potential repolarization. In contrast, in guinea-pigs, calcium influx via Na+/Ca(2+)-exchange is expected only during the initial portion of the action potential. Thus electrical stimulation is expected to result in reduced intracellular calcium ([Ca2+]i) in rat and an increase in guinea pig. We tested this hypothesis by measuring total cellular calcium ([Ca]tot) using 45Ca following stimulation of isolated rat and guinea-pig ventricular myocytes. Many studies have also emphasized that the rate and the direction of Na+/Ca(2+)-exchange across the sarcolemma are in part dependent on the magnitude of the transsarcolemmal sodium gradient. Thus, increasing intracellular sodium ([Na+]i) is expected to result in an increased [Ca2+]i. This hypothesis was also tested by measuring [Ca]tot following veratrine administration. Enzymatically isolated rat and guinea-pig ventricular myocytes were divided into two groups; non-stimulated and stimulated (1 Hz). The concentration-dependent effects of veratrine (1,10,100 micrograms/ml) on [Ca]tot were determined in both these groups. In the absence of veratrine, non-stimulated rat myocytes had a significantly higher [Ca]tot than did stimulated ones. Non-stimulated guinea-pig myocytes had a significantly lower [Ca]tot when compared with stimulated ones. Veratrine increased [Ca]tot in both species in a concentration-dependent fashion. In addition, following veratrine the difference between [Ca]tot in non-stimulated and stimulated rat myocytes was no longer significant. These results support those of others who have demonstrated that stimulation is associated with a gain of cellular calcium in both rabbit and guinea-pig ventricle and a calcium loss in rat ventricle.