Influence of acidosis on inotropic effect of catecholamines in newborn rabbit hearts

The influence of acidosis on the inotropic effect of isoproterenol was studied in the isolated arterially perfused heart of the newborn rabbits. Baseline mechanical function during acidosis (pH 6.8) was not different from control (pH 7.4). However, the inotropic effect of isoproterenol was significantly suppressed in the acidotic muscles. The increment of myocardial adenosine 3',5'-cyclic monophosphate (cAMP) content during isoproterenol infusion was also reduced in acidosis. Inotropic effects of Ca and dibutyryl cAMP in the acidotic muscles were not significantly different from those in the control muscles. beta-Receptor number and affinity in the respiratory acidotic muscle was similar to those in the control muscle. Effects of pH on myocardial beta-receptor and adenylate cyclase activity were further determined in the membrane fraction by changing the pH of the reaction medium from 7.4 (control) to 6.8 or 6.0. beta-Receptor numbers were significantly decreased at pH 6.0 but not at pH 6.8. Adenylate cyclase activity was depressed at pH 6.8 and 6.0. These data suggest that the inotropic effect of isoproterenol is diminished in the acidotic muscle. This may be due to the decreased activation of cAMP production, which in turn most likely results from depressed adenylate cyclase activity.