Effect of thyroid status on β-adrenergic receptor, adenylate cyclase activity and guanine nucleotide regulatory unit in rat cardiac and erythrocyte membranes

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The effect of thyroid hormone on the β-receptor coupled adenylate cyclase in rat crude cardiac membranes was analysed by measuring the number of DHA-binding sites, adenylate cyclase activity and the amount of cholera toxin catalysed ADP-ribosylation of a protein with a molecular weight of 42,000 in cardiac and erythrocyte membranes. In crude rat cardiac membranes, the number of DHA-binding sites (78 ± 15 fmole/mg protein in the euthyroid state) is increased to 158 ± 20 fmole/mg protein in the hyperthyroid state and decreased to 51 ± 6 fmole/mg protein in the hypothyroid state; the affinity of the binding sites remained unchanged ( K D 2.9−4.3 nM). l-Isoprenaline (10 −4 M)-stimulated adenylate cyclase activity varied in parallel to the number of DHA-binding sites in hyper- and euthyroidism. Thyroid hormone, however, did not influence GppNHp (10 −4 M)-stimulated adenylate cyclase activity. Cholera toxin catalysed ADP-ribosylation of normal crude cardiac membranes resulted in a 1.8 fold increase in adenylate cyclase activity in the presence of GTP (10 −4) and l-isoprenaline (10 −4M), presumably as a result of inhibition of GTPase. In crude cardiac membranes cholera toxin catalyses the ADP-ribosylation of one major protein, which comigrates on sodium dodecylsulfate-polyacrylamide gel electrophoresis with the putative regulatory component of adenylate cyclase (mol. wt 42,000). In different thyroid states the amount of the regulatory component (as determined by cholera toxin dependent labelling) was equal (112 fmole/mg protein in euthyroid crude cardiac membranes). Basal activity of adenylate cyclase showed a significant difference between activity in euthyroid (3.7 ± 0.2 pmole cAMP/mg protein/min) and hypothyroid (5.4 ± 0.2 pmole cAMP/mg protein/min), but not in hyperthyroid crude cardiac membranes (3.4 ± 0.2 pmole cAMP/mg protein/min). Our results indicate, that thyroid hormone regulates the number of DHA-binding sites and basal activity (in hypothyroidism) in crude cardiac membranes and thereby causes different results in l-isoprenaline-induced adenylate cyclase activity.