Adenylate cyclase activity in fetal rabbit myocardium

Adenylate cyclase activity [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] was determined in vitro in fetal rabbit myocardial membranes from individual fetal pups at 21 to 31 days gestation (term, 31 days). Basal and NaF-stimulated adenylate cyclase activities did not change during 21-31 days gestation. Significant stimulation of the enzyme by l-isoproterenol was observed only in the presence of guanosine triphosphate (100 microM). Under these conditions, maximal adenylate cyclase stimulation by l-isoproterenol (100 microM) was significantly higher at 25-31 than at 21 days gestation. Moreover, EC50 (Kact) for l-isoproterenol at 25-31 days was significantly lower than at 21 days gestation. We conclude that, in fetal rabbit myocardial membranes, there is an increase in the sensitivity of adenylate cyclase stimulation by l-isoproterenol from 21 to 25-31 days gestation.