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

Abstract

The rest- and excitation-dependent shifts of Ca and⁴⁵Ca 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 with⁴⁵Ca 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 tissue⁴⁵Ca content by 0.229 mmol/kg ww. In these preparations, the gain in⁴⁵Ca is equivalent to the net Ca uptake. Continued post-rest stimulation at the rate of 60/min resulted in recovery of prerest content of⁴⁵Ca and of total Ca. Gain of⁴⁵Ca was paralleled by recovery of contractile force.

Uptake of⁴⁵Ca 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. Thus⁴⁵Ca 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.