Ca^2+ uptake into and the Ca^2+ content of both resting and stimulated neonatal canine left ventricular muscle strips were studied with 45Ca^2+. The characteristics of the total tissue Ca2+ exchange were defined by allowing 45Ca2+ to efflux from 45Ca^2+equilibrated muscle into ice-cold (4 °C) modified Tyrode’s solution. The resulting 45Ca^2+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 Ca^2+ 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 ty, for exchange of the slow compartment was shorter at 37 °C than at 4 “C, and that its Ca^2+ 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 Ca^2+ content of the slow compartment, consistent with the view that Ca^2+ sites within this compartment play a significant role in the excitation-contraction coupling process.

Keywords:
Neonatal canine ventricle, Calcium fluxes, Isoproterenol, Electrical stimulation
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1987
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