Abstract
The actions of magnesium and calcium ions on the metabolic and contractile effects of epinephrine were studied in the isolated, perfused rat heart. The breakdown of high energy phosphate and increase in glycogenolysis produced by epinephrine were markedly inhibited in the presence of 20 mM magnesium ions in the perfusion fluid. This depressant effect of magnesium was overcome by raising the extracellular concentrations of calcium. Epinephrine caused a large increase in phosphorylase a activity in hearts perfused with control or a high magnesium medium, although glycogenolysis was severely depressed by excess magnesium. Epinephrine-induced glycogenolysis. as measured by lactate efflux, responded rapidly to alterations in the magnesium concentration in the perfusion fluid in the absence of corresponding changes in force of contraction. It is apparent that, in the heart stimulated by a catecholamine, carbohydrate metabolism is more readily affected by divalent cations than is myocardial contraction.
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