¹ From the Department of Pharmacology and Therapeutics, Faculty of Medicine, University of Manitoba, Winnipeg 3, Manitoba, Canada

The relationship between cellular calcium (Ca) stores and isometric contractile force was investigated in isolated, gas-perfused cat hearts. The hearts were preperfused for approximately 5 min with substrate-free Krebs solution modified to contain 0, 1.25, 2.5, 5.0 or 10.0 mEq/liter Ca, gas-perfused for up to 120 min, and then perfused with zero-Ca Krebs solution. Contractile force and [Ca] in the effluent fluid were measured. Our results indicate that: (a) The washout of Ca was characteristic of a three compartment system, (b) A Ca compartment directly associated with muscle contraction was identified by correlation of the rate of Ca washout with the rate of decay of contractility (r = 0.79). (c) Ca content in the compartment was correlated with contractile force (r = 0.77). (d) Contractile force and the Ca content of the compartment which was correlated with contractility approached a steady state during 120 min of gas perfusion.

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