The role of sarcolemmal Ca2+-ATPase in the regulation of resting calcium concentration in rat ventricular myocytes

1. The aim of this work was to investigate the role of sarcolemmal Ca2+-ATP ase in rat ventricular myocytes. We have measured intracellular Ca2+ concen tration ([Ca2+](i)) using indo-1. The actions of the ATPase inhibitor carbo xyeosin were studied. 2. Carboxyeosin increased resting [Ca2+](i) and the magnitude of the systol ic Ca2+ transient and slowed the rate of its relaxation by 5%. 3. Carboxyeosin increased the magnitude of the caffeine-evoked increase in [Ca2+], and slowed its relaxation by 20%. Removal of extracellular Na+ slow ed ed the rate constant by 80%. When Naf was removed in a carboxyeosin-trea ted cell, the caffeine-evoked increase in [Ca2+](i) did not decay. 4. Carboxyeosin increased the integral of the Na+-Ca2+ exchange current act ivated by caffeine. This is, in part, due to an increase in sarcoplasmic re ticulum Ca2+ content. 5. When extracellular Naf was removed, there was a transient increase in [C a2+](i) which then decayed. The rate of this decay was slowed by carboxyeos in by a factor of about four. The residual decay could be abolished with ca ffeine. 6. In the absence of extracellular Na+, increasing extracellular Ca2+ conce ntration ([Ca2+](i)) elevated [Ca2+](i). In carboxyeosin-treated cells, [Ca 2+](i) was much more sensitive to [Ca2+](O). 7. These results: demonstrate the role of a carboxyeosin-sensitive Ca2+-ATP ase in the control of resting [Ca2+](i) and the reduction in [Ca2+](i) foll owing an increase in [C2+](i).