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 of⁴⁵Ca²⁺ from⁴⁵Ca²⁺ equilibrated tissue into ice-cold (4°C) wash medium could be best fitted with a model containing a minimum of 3 compartments.⁴⁵Ca²⁺ uptake into and⁴⁵Ca²⁺ efflux from the most slowly exchanging compartment (compartment 3) at 37°C allowed it to be subdivided into two fractions; a rapidly exchanging fraction (t_1/21.25 min) and a slowly exchanging fraction (t_1/250 min). The total Ca²⁺ content of compartment 3 was enhanced by isoproterenol but little affected by caffeine. The slowt_1/2 for exchange of the Ca²⁺ in compartment 3 at 4°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°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 M/kg) indicating that this postulated intracellular compartment may play a significant role in the normal contraction-relaxation cycle.