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

Ho Sook Choi and D. A. Eisner

The aim of this work was to investigate the role of sarcolemmal Ca²⁺-ATPase in rat ventricular myocytes. We have measured intracellular Ca²⁺ concentration ([Ca²⁺]_i) using indo-1. The actions of the ATPase inhibitor carboxyeosin were studied.

Carboxyeosin increased resting [Ca²⁺]_i and the magnitude of the systolic Ca²⁺ transient and slowed the rate of its relaxation by 5 %.

Carboxyeosin increased the magnitude of the caffeine-evoked increase in [Ca²⁺]_i and slowed its relaxation by 20 %. Removal of extracellular Na⁺ slowed the rate constant by 80 %. When Na⁺ was removed in a carboxyeosin-treated cell, the caffeine-evoked increase in [Ca²⁺]_i did not decay.

Carboxyeosin increased the integral of the Na⁺-Ca²⁺ exchange current activated by caffeine. This is, in part, due to an increase in sarcoplasmic reticulum Ca²⁺ content.

When extracellular Na⁺ was removed, there was a transient increase in [Ca²⁺]_i which then decayed. The rate of this decay was slowed by carboxyeosin by a factor of about four. The residual decay could be abolished with caffeine.

In the absence of extracellular Na⁺, increasing extracellular Ca²⁺ concentration ([Ca²⁺]_o) elevated [Ca²⁺]_i. In carboxyeosin-treated cells, [Ca²⁺]_i was much more sensitive to [Ca²⁺]_o.

These results demonstrate the role of a carboxyeosin-sensitive Ca²⁺-ATPase in the control of resting [Ca²⁺]_i and the reduction in [Ca²⁺]_i following an increase in [Ca²⁺]_i.

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