The effect of prolonged rest on calcium exchange and contractions in rat and guinea-pig ventricular myocardium.

The exchange of calcium in isolated, perfused rat ventricular myocardium was measured by means of 45Ca2+ and compared with that in guinea-pig. In the rat the total exchangeable calcium pool containing 0.84 +/- 0.03 mmol/kg of wet weight (w.w.) consists of at least two fractions: excitation-dependent, beat-to-beat exchanging fraction of about 0.1 mmol/kg w.w. which is lost at rest and re-gained during post-rest contractions; a fraction containing 0.74 +/- 0.04 mmol/kg w.w. of Ca2+ exchanging both at rest and during stimulation. During prolonged rest the rat ventricular myocardium loses only 13% (0.1 mmol/kg w.w.) of its exchangeable calcium whereas in the ventricle of guinea-pig heart 72% (0.94 mmol/kg w.w.) Ca2+ is lost at rest. These differences conform to the differences in the response of the contractile force to prolonged rest: the first post-rest contraction of the rat ventricle is stronger than the steady-state beats whereas the guinea-pig ventricular muscle loses its contractility. These results are compatible with the hypothesis that the calcium fraction released from resting guinea-pig ventricle is an important component of the mechanism of the slow force-frequency relationship.