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 degrees 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 (t1/2) for exchange. The t1/2 for the exchange of Ca2+ within the slow compartment at 4 degrees C was 103.1 +/- 7.0 min. The physiological characteristics of the slow compartment were investigated by performing experiments at 37 degrees C and then allowing tissues to efflux at 4 degrees C as described above. The uptake of Ca2+ into the slow compartment at 37 degrees C consisted of an initial rapid uptake (t1/2 approximately 1.6 min) followed by a slower prolonged uptake (t1/2 approximately 54 min), indicating the presence of a nonhomogeneous compartment. The observations that the t1/2 for exchange of the slow compartment was shorter at 37 degrees C than at 4 degrees 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 (approximately 350 mumol/kg; p less than 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.