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 ventricu lar cells from both adult rat and from neonatal rat cultures. The pres ent study further characterizes this compartment in the adult ventricu lar myocyte. The compartment's exchange is significantly decreased by 20 muM 3,4-dichlorobenzamil. Its content responds significantly to var iation of the extracellular Ca concentration ([Ca]o) and to reversal o f the Na-Ca exchanger by reduction of extracellular Na concentration t o 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 re duced by Ca-45 labeling in the presence of 10 mM caffeine (23%) and 1 mM ryanodine (22%). This compartment is not significantly affected by 1 muM thapsigargin. Ca-45 activity is retained in the compartment when Ca-45 washout is conducted without Na or Ca in the washout solution, i.e., Na-Ca exchange is not operative. Under these conditions, additio n of 1 mM Cd to the washout produces a significant release of Ca-45 fr om the compartment. The results are interpreted to indicate that the c ompartment in the adult cell has two components that are in equilibriu m: a relatively small sarcoplasmic reticulum (SR) component and a larg er ''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 represe nts Ca bound to the cytoplasmic leaflet of the sarcolemma at SR juncti on regions is discussed.