Thyroid hormone regulation of β-adrenergic receptors and catecholamine sensitive adenylate cyclase in foetal heart

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Abstract
The effect of thyroid hormone on the .beta.-adrenergic receptor in rabbit fetal cardiac membranes was analyzed by measuring the binding of (-)[3H]dihydroalprenolol [DHA]. The specific activities (per mg protein) of .beta.-adrenergic receptors decreased with advancing gestational age, whereas the total activities (per heart) increased under the similar conditions. The change in the binding affinities was not statistically significant. 3,5,3''-L-triiodothyronine (T3) stimulated the (-)[3H]DHA binding capacities of the cardiac membranes of fetuses of all age groups. The enhancement in the receptor activity was completely inhibited by actinomycin D or cycloheximide. The contents of epinephrine, norepinephrine and cAMP increased with advancing gestational age, but, T3 had no significant effect on the catecholamines or cAMP. The activities of the basal NaF stimulated and guanylyl-5''-imidodiphosphate [Gpp(NH)p] stimulated adenylate cyclase remained unaltered by T3, but the activities increased progressively with fetal maturity. The absolute values of catecholamine stimulated adenylate cyclase activities in the hearts of T3 treated fetuses were higher compared to those in the untreated fetuses. The enhancement of the activities was totally blocked by the action of actinomycin D, cycloheximide or propranolol. Thyroid hormone enhances the number of .beta.-adrenergic receptor binding sites by synthesizing new receptor proteins resulting in increased catecholamine sensitivity.