Myocardial adenylate cyclase activity in ritodrine-treated pregnant rabbits

To determine whether prolonged exposure to ritodrine results in down-regulation of myocardial adenylate cyclase activity during pregnancy, New Zealand White rabbits with time-dated pregnancies were randomly assigned to a sham-operated control group and to a ritodrine-treated group. Starting at 22 days' gestation (term 31 days), animals received either intravenous ritodrine intermittently or the equivalent volume of vehicle for 3 days. Adenylate cyclase activity was determined in a plasma membrane fraction obtained from ventricular tissue. There was no difference in the basal activity or the basal and guanosine triphosphate activity between the two groups. l-Isoproterenol-dependent stimulation required the presence of guanosine triphosphate. Both maximal and 50% enzymatic stimulation were equivalent in both groups. Furthermore, sodium fluoride-dependent stimulation was similar in both groups. Finally, adenylate cyclase activity in the presence of forskolin and manganese chloride was similar in both groups. From the above, we conclude that intermittent systemic infusions of ritodrine to pregnant rabbits did not result in down-regulation of myocardial adenylate cyclase activity.