Further characterization of the Na-Ca exchange-dependent Ca compartment in rat ventricular cells.

A discrete subcellular Ca compartment, absolutely dependent upon Na-Ca exchange for its exchange, has been previously identified in ventricular cells from both adult rat and from neonatal rat cultures. The present study further characterizes this compartment in the adult ventricular myocyte. The compartment's exchange is significantly decreased by 20 microM 3,4-dichlorobenzamil. Its content responds significantly to variation of the extracellular Ca concentration ([Ca]o) and to reversal of the Na-Ca exchanger by reduction of extracellular Na concentration to zero. Its content decreases to 155 +/- 24 mumol/kg dry cell wt in 0.2 mM [Ca]o and increases to 623 +/- 48 mumol/kg at 6.5 mM [Ca]o, where it appears to be near saturation. The compartment is significantly reduced by 45Ca labeling in the presence of 10 mM caffeine (23%) and 1 mM ryanodine (22%). This compartment is not significantly affected by 1 microM thapsigargin. 45Ca activity is retained in the compartment when 45Ca washout is conducted without Na or Ca in the washout solution, i.e., Na-Ca exchange is not operative. Under these conditions, addition of 1 mM Cd to the washout produces a significant release of 45Ca from the compartment. The results are interpreted to indicate that the compartment in the adult cell has two components that are in equilibrium: a relatively small sarcoplasmic reticulum (SR) component and a larger "non-SR" component. This is in contrast to the compartment in the cultured cell in which the compartment is 2.6 times larger and has a larger SR component. The possibility that the non-SR component represents Ca bound to the cytoplasmic leaflet of the sarcolemma at SR junction regions is discussed.