Contrasting Effects of Cyclic AMP Increase Caused by β-Adrenergic Stimulation or by Adenylate Cyclase Activation on Ventricular Fibrillation Threshold of Isolated Rat Heart

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Abstract
Increased myocardial tissue cyclic AMP has been associated with both a positive inotropic and a proarrhythmic effect. We wished to determine whether two agents that increase myocardial cyclic AMP levels by different mechanisms would induce comparable changes in vulnerability of the heart to ventricular fibrillation (VF) and in the inotropic status. Using an isolated perfused rat heart model, we studied the effects of α-adrenoceptor stimulation by isoproterenol (ISO) and direct activation of adenylate cyclase by forskolin. The ventricular fibrillation threshold (VFT) was taken as an index of the vulnerability to VF and peak left ventricular systolic pressure (LVSP) as a measure of the force of LV contraction. ISO resulted in a dose-related increase in tissue cyclic AMP with a corresponding decrease in VFT and a marked increase in LVSP. Forskolin produced a delayed but exponential increase in cyclic AMP at concentrations < 3 × 10–7 M with relatively small increases in LVSP. With forskolin, the VFT decreased only at extremely high cyclic AMP levels, suggesting that the drug had increased cyclic AMP in a compartmentalized manner. The discrepant effects of ISO and forskolin on VFT could not be explained by changes in heart rate (HR). These results show that an increase in tissue cyclic AMP can have markedly different arrhythmogenic effects depending on the mechanism by which cyclic AMP is increased.

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