Mechanisms of Activation of Cardiac Glycogen Phosphorylase in Ischemia and Anoxia

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Abstract
The effects of ischemia and anoxia on cardiac adenosine 3', 5'-monophosphate (cyclic AMP) concentration, glycogen phosphorylase activity ratio (-5'-AMP: +5'-AMP), phosphorylase kinase activity ratio (pH 6.8:8.2), and myocardial contractility (left ventricular dP/dt) were studied in an open-chest rat heart preparation. Ischemia produced by termination of coronary blood flow increased cyclic AMP from 0.55 to 0.77 µmoles/kg in 5 seconds and phosphorylase from 0.14 to 0.57 in 20 seconds. Anoxia induced by breathing N2 increased cyclic AMP from 0.50 to 0.62 µmoles/kg in 10 seconds and phosphorylase from 0.14 to 0.65 in 30 seconds. Phosphorylase kinase increased with ischemia but did not change with anoxia. Beta-receptor blockade with practolol prevented the rise in cyclic AMP and phosphorylase kinase but blocked the increase in phosphorylase only in ischemia. Myocardial contractility declined precipitously during the first 20 seconds of anoxia. Epinephrine (0.1 µg/kg) caused an increase in cyclic AMP comparable to that elicited by anoxia, and it produced an increase in dP/dt during N2 breathing. These results suggest that in the intact working heart ischemia induces phosphorylase a formation through a cyclic AMP-dependent transformation of phosphorylase kinase; however, in anoxia phosphorylase a formation depends only on the regulation of the catalytic activity of phosphorylase kinase without conversion of this enzyme to its activated form. An increase in cyclic AMP during anoxia is not associated with a positive inotropic response even though such a response is obtained with epinephrine. Factors other than the elevation of myocardial cyclic AMP may be limiting in the control of both cardiac glycogenolysis and inotropic state.