Alpha-adrenergic responsiveness in rat isolated perfused heart after abdominal aortic coarctation

Chronic isoproterenol pre-treatment, a well-known model of compensatory hypertrophy associated with cardiac beta-adrenoceptor desensitization, enhances the inotropic response to phenylephrine in rat isolated perfused hearts, supporting the hypothesis that myocardial alpha-adrenoceptor stimulation contributes to the maintenance of myocardial performance in situations in which cardiac beta-adrenoceptor function is compromised. To further corroborate this hypothesis, the effects of abdominal aortic coarctation on cardiac alpha-adrenergic responsiveness were investigated in Langendorff heart preparations. Abdominal aortic coarctation causes cardiac hypertrophy (21%) as shown by a significant increase in the ratio of ventricular dry weight to bodyweight. In preparations from hypertrophied rats, both maximum increases in left ventricular systolic pressure and heart rate elicited by isoproterenol (10(-12) to 10(-4) M) were significantly reduced (the isoproterenol concentration producing 50% of the maximum positive inotropic and chronotropic responses was enhanced almost 21- and 2-fold, respectively). However, the positive inotropic response to phenylephrine (10(-12) to 10(-4) M) remained unaffected following abdominal aortic coarctation, when compared with sham-operated rats. In preparations from both groups, phenylephrine infusion did not induce significant changes in heart rate. These results show that although abdominal aortic stenosis induced desensitization of cardiac beta-adrenoceptors, it did not enhance cardiac alpha-adrenoceptor responsiveness. This suggests that such an enhancement depends on the experimental model used to induce cardiac hypertrophy associated with desensitization of cardiac beta-adrenoceptors.