NA CA EXCHANGER - ROLE IN EXCITATION-CONTRACTION COUPLING IN HEART-MUSCLE AND PHYSIOLOGICAL INSIGHTS FROM THE GENE STRUCTURE/

By examining rat and guinea pig heart cells loaded with the calcium-se nsitive indicator fluo-3 with a confocal microscope, we were able to i mage the release of calcium from the sarcoplasmic reticulum (SR). The spontaneous openings of SR Ca-release channels can be seen and have be en called ''calcium sparks''. In rat heart cells, a depolarization-act ivated calcium transient is largely abolished by the application of th e calcium channel blocker cadmium (10 mu M) and the few unblocked chan nels appear to be able to activate calcium sparks. Since the sodium cu rrent and the Na+/Ca2+ exchanger are largely unaffected by that amount of cadmium, the nearly complete abolition of the intracellular calciu m ([Ca2+](i)) transient suggests that the Na+/Ca2+ exchanger plays onl y a minor role in activating SR Ca release under the conditions of our experiments in rat heart cells. These findings are discussed with res pect to guinea pig heart cells and the absence of differences in the c DNAs for the cardiac isoform in these species.