Changes in cyclic nucleotide levels and contractile force in the isolated hypoxic rat heart during perfusion with glucagon.

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Abstract
Isolated rat hearts were perfused with hormonal concentrations of glucagon during a hypoxic perfusion to determine whether it would enhance recovery after reoxygenation. Rat hearts were divided into two groups: (1) those perfused with glucose-free Tyrode's solution and (2) those perfused with Tyrode's solution containing glucose. During 3 minutes of exposure to hypoxia both untreated hearts and hearts perfused with glucagon demonstrated a decrease in contractile force to 10-20% of control. When glucose was present in the perfusion medium, cardiac performance was better during both the periods of hypoxia and reoxygenation. During reoxygenation, recovery of contractile force was significantly better (P less than 0.05) in glucagon-perfused hearts than in untreated hearts; this improved recovery occurred regardless of whether glucose was included in the medium. The enhanced recovery of the glucagon-perfused hearts was associated with decreases in myocardial levels of guanosine, 3',5'-monophosphate (cyclic GMP) both during the periods of hypoxia and reoxygenation. At the end of the hypoxic period, cyclic GMP levels in the glucagon-perfused hearts were 20-64% of the levels in untreated hearts. Similarly, after 5 minutes of reoxygenation cyclic GMP levels in the glucagon-perfused hearts were 21% of the levels in the untreated hearts. The effect of glucagon on adenosine 3',5'-monophosphate (cyclic AMP) concentrations in untreated hearts and in hearts receiving glucagon was not significantly different either after 3 minutes of hypoxia or during reoxygenation. The rate of anaerobic glycolysis after 3 minutes of hypoxia was higher in untreated hearts than in glucagon-perfused hearts, as determined by the lactate content of coronary perfusates. These studies suggest that hormonal concentrations of glucagon exert a protective effect on the hypoxic rat heart which involves a modulation of cardiac cyclic GMP accumulation.