Rat Liver Adenyl Cyclase Activity in Various Thyroid States

were injected with three doses of triiodothyronine (Ts) or of the diluent over a 6 day period, and liver ho-mogenates were assayed for basal, epinephrine-stimu-lated, and NaF-stimulated adenyl cyclase activity. Based on NaF-stimulated levels, total adenyl cyclase activity, ex-pressed per milligram of liver protein, was increased after thyroidectomy. Administration of T3 to either hypo-thyroid or euthyroid rats, however, had no effect on the NaF-stimulated levels. Basal and epinephrine-stimulated enzyme activities were the same in hypothyroid, euthy-roid, and hyperthyroid (euthyroid + T3) liver homoge-nates. In contrast, injections of T3 in hypothyroid rats increased the activities of basal and epinephrine-stimu-lated adenyl cyclase. In view of the findings in euthyroid and hyperthyroid liver, it is possible that this effect is transient. In general, no correlation was found between the effects of thyroid hormone on respiration and on adenyl cyclase activity of the rat liver. These results im-ply that the hepatic thermogenic response to thyroid hormone is not mediated by stimulation of adenyl cyclase activity with the possible exception of the early effects of T3 in the athyroid rat