Calcium exchange in the resting and electrically stimulated canine myocardium

45Ca2+ exchange was studied in small pieces of canine left ventricular free wall. The loss of 45Ca2+ from 45Ca2+ equilibrated tissue into ice-cold (4 degrees C) "wash" medium could be best fitted with a model containing a minimum of 3 compartments. 45Ca2+ uptake into and 45Ca2+ efflux from the most slowly exchanging compartment (compartment 3) at 37 degrees C allowed it to be subdivided into two fractions; a rapidly exchanging fraction (t1/2 approximately 1.25 min) and a slowly exchanging fraction (t1/2 approximately 50 min). The total Ca2+ content of compartment 3 was enhanced by isoproterenol but little affected by caffeine. The slow t1/2 for exchange of the Ca2+ in compartment 3 at 4 degrees C and its increased Ca2+ content following isoproterenol treatment suggest that this compartment contains some Ca2+ of intracellular origin. In addition, the finding that the rapidly exchanging part of compartment 3 could be preserved by cooling the tissue to 4 degrees C shows that rapidly exchanging Ca2+ compartments can be studied in superfused cardiac preparations using this technique. Action potentials, elicited by electrical stimulation of the tissue, caused large changes in the Ca2+ content of compartment 3 (up to 170 muM/kg) indicating that this postulated intracellular compartment may play a significant role in the normal contraction-relaxation cycle.