338 EFFECT OF ISOXSUPRINE ON β-ADRENERGIC RECEPTORS AND ADENYLATE CYCLASE IN FETAL RABBIT CARDIAC MEMBRANES

Abstract

Isoxsuprine (ISX) has been used in mothers with premature labor. To examine the effect of chronic exposure of ISX on fetal myocardium, eight doses of ISX (3 mg/kg) were injected intramuscularly into pregnant rabbits between 24-27 days gestation. At 28 days, fetal cardiac membranes were examined for β-adrenergic receptors (βAR) by (-)[3H]dihydroalprenolol (DHA) binding and assayed for adenylate cyclase (AC) activity. There was no difference in mean fetal heart weights in saline and ISX treated groups. (-)[3H]DHA binding sites were rapid (t½<30 sec.), reversible (t½<90 sec.) and saturable with high affinity and low capacity. β-adrenergic agonists and antagonists competed for these sites and had characteristics of β1-subtype of βAR. Chronic ISX treatment led to a decrease in the mean ± SEM of maximal (-)[3H]DHA bound from 126±7 fmoles/mg (n=6) to 64% of the control value, 81±8 fmoles/mg (n=6 (p<.01), with no change in affinity (Kd=1.8±0.2 vs. 2.4±0.3 nM). In addition, AC stimulation by (-) isoproterenol (10 μM) was reduced from 66.1±4.2 pmoles cAMP/mg/15 min. (n=5) to 62% of the control response, 41.2±4.1 pmoles cAMP/mg/15 min. (n=5)(p<.01) with no change in the basal activity in fetal cardiac membranes (24.5±1.9 vs. 24 ±0.9 pmoles cAMP/mg/15 min.). Thus, chronic ISX exposure of rabbit fetuses downregulated the βAR density in fetal cardiac membranes at 28 days and was accompanied by diminution of the response of AC to isoproterenol stimulation.