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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