Calcium fluxes in the neonatal canine myocardium: The effects of isoproterenol and electrical stimulation

Ca²⁺ uptake into and the Ca²⁺ content of both resting and stimulated neonatal canine left ventricular muscle strips were studied with ⁴⁵Ca²⁺. The characteristics of the total tissue Ca²⁺ exchange were defined by allowing ⁴⁵Ca²⁺ to efflux from ⁴⁵Ca²⁺ equilibrated muscle into ice-cold (4°C) modified Tyrode's solution. The resulting ⁴⁵Ca²⁺ efflux curve could be described using a three-compartment model. These compartments were designated as rapid, intermediate and slow on the basis of their half-times (t( 1/2 )) for exchange. The t( 1/2 ) for the exchange of Ca²⁺ within the slow compartment at 4° C was 103.1 ± 7.0 min. The physiological characteristics of the slow compartment were investigated by performing experiments at 37°C and then allowing tissues to efflux at 4°C as described above. The uptake of Ca²⁺ into the slow compartment at 37°C consisted of an initial rapid uptake (t( 1/2 ) ~ 1.6 min) followed by a slower prolonged uptake (t( 1/2 ) ~ 54 min), indicating the presence of a nonhomogeneous compartment. The observations that the t( 1/2 ) for exchange of the slow compartment was shorter at 37°C than at 4°C, and that its Ca²⁺ content was significantly increased by isoproterenol (10^-6 M), suggests that the slow compartment is of intracellular origin. A single suprathreshold electrical stimulus, to elicit an action potential, caused a significant decrease (~350 μmol/kg; p < 0.01) in the Ca²⁺ content of the slow compartment, consistent with the view that Ca²⁺ sites within this compartment play a significant role in the excitation-contraction coupling process.