A discrete Na+/Ca2+ exchange dependent, Ca2+ compartment in cultured neonatal rat heart cells. Characteristics, localization and possible physiological function

A pool of calcium has been identified in cultured neonatal rat heart cells whose exchange is dependent on extracellular sodium and calcium. In the absence of extracellular sodium and calcium this pool retains at least 760 μmol calcium/kg dry weight, which is released upon the introduction of sodium and calcium, with a t 1/2 of 4.4s. Video microscopy experiments of non-stimulated cells showed that the cells, despite the retention of 760 μmol calcium/kg dry weight, do not develop contracture, indicating that the majority of the pool is not retained in the general cytoplasm. Neither is the pool displaced by lanthanum. Several probes for the sarcoplasmic reticulum were used. Ryanodine (1 μM) had no significant effect on the size of the pool. Caffeine reduced the size of the pool by 60% and thapsigargin, an inhibitor of the sarcoplasmic reticulum Ca-ATPase reduced the content of the pool by 70%. High concentrations of ryanodine (1 mM) reduced the pool even further. The experimental data, in association with recently developed concepts of the diadic region, indicate that the pool may reside in the sub-sarcolemmal space between the sarcoplasmic reticulum and the sub-sarcolemma. The physiological significance of this pool is discussed.