Molecular Pharmacology, Vol 6, 659-666, Copyright © 1970 by the American Society for Pharmacology and Experimental Therapeutics

Excitation-Contraction Coupling in Heart

III. Evidence against the Involvement of Adenosine Cyclic 3',5'-Monophosphate in Calcium Transport by Sarcotubular Vesicles of Canine Myocardium

¹ Department of Physiology, Faculty of Medicine, University of Manitoba, Winnipeg 3, Manitoba, Canada

Sarcotubular vesicles isolated from dog myocardium accumulated 2.51 ± 0.48 µmoles of calcium per milligram of protein in the presence of oxalate and hydrolyzed 18.39 ± 1.96 µmoles of ATP per milligram of protein in 30 min at 37°. Cyclic 3', 5'-AMP (10^-6-10^-5 M) influenced neither the rate nor the extent of calcium uptake and ATP hydrolysis. However, at higher concentrations (4 x 10^-3 M), cyclic AMP reduced the ability of these membranes to accumulate calcium and hydrolyze ATP. Heart sarcotubular vesicles bound 66.54 ± 4.53 mµmoles of calcium per milligram of protein in the absence of oxalate and exchanged 15.82 mµmoles of calcium per milligram of protein in 2 min at 25°. Neither calcium binding nor the exchangeability of membrane-bound calcium was affected appreciably by cyclic AMP. About 60% of the bound calcium of the heart sarcotubular vesicles was released in the incubation medium in 10 min at 25° after the addition of EDTA. Cyclic AMP had no effect on the rate or the extent of calciunm efflux in this system. These results do not indicate any action of cyclic AMP on calcium transport across the sarcotubular membranes of myocardium. Neither epinephrine nor glucagon influenced the ability of these vesicles to accumulate calcium in the absence or presence of oxalate. It is suggested that the postulated mechanism of increased calcium movement across cardiac sarcoplasmic reticulum due to an increased level of cyclic AMP under the influence of different inotropic interventions be considered with due caution.