Excitation- and rest-dependent shifts of Ca in guinea-pig ventricular myocardium.

The rest- and excitation-dependent shifts of Ca and 45Ca in the isolated, perfused ventricles of guinea-pig hearts were investigated. As much as 50% of the total Ca content (2.2 mmol/kg ww) found in the ventricular muscle stimulated at a steady rate of 60/min, was released into perfusate during 4 min of rest. In the preparations perfused with 45Ca containing solution during the 4 min of rest or during the last 20 s of rest only, a single beat resulted in extra uptake of 0.359 and 0.287 mmol of labelled calcium (45Ca) per kg ww, respectively. Single post-rest excitation evoked in the ventricles which were previously perfused with radioactive solution for 64 min, resulted in increase in tissue 45Ca content by 0.229 mmol/kg ww. In these preparations, the gain in 45Ca is equivalent to the net Ca uptake. Continued post-rest stimulation at the rate of 60/min resulted in recovery of pre-rest content of 45Ca and of total Ca. Gain of 45Ca was paralleled by recovery of contractile force. Uptake of 45Ca in the preparations stimulated at the steady rate of 60/min was 0.137 mmol/kg ww and its value did not depend on the number of beats during exposure to the isotope. Thus 45Ca uptake over a number of steady-state beats may be regarded as equal to the uptake in a single beat. This uptake is by orders of magnitude larger than reported previously by other authors. It is proposed that contraction is triggered by Ca influx into the excited cells (Ca1), and that the response of contractile proteins to this trigger is controlled by a large intracellular Ca2 fraction whose volume is rate-dependent.