Steady-state levels of G-proteins and β-adrenergic receptors in rat fat cells. Permissive effects of thyroid hormones

Thyroid hormones exert a permissive influence on the ability of cells to respond to other hormones. In hypothyroidism, stimulation of adenylate cyclase by β-adrenergic agonists is impaired in rat fat cells, whereas inhibition by adenosine is potentiated. The effects of thyroid status on steady-state levels of the G-protein subunits α-G(o), α-G(i), and β-G(35/36) were investigated using specific antibodies and quantitative immunoblotting of rat fat cell membranes. The amount of α-G(o) (M(r) 39,000, α-G₃₉) detected in fat cell membranes of euthyroid rats was 44 ± 5 pmol/mg of membrane protein (n = 5). In the hypothyroid state, the amount of the α-subunits of G(i) (M(r) 41,000, α-G₄₁) and G(o) were found to be markedly increased in comparison to the control. The steady-state level of α-G₄₁ and α-G₃₉ increased more than 50 and 70%, respectively, in the hypothyroid state. The β-subunit of G-proteins of rat fat cells appears as a doublet of proteins with M(r) = 35,000/36,000 on sodium dodecyl sulfate-polyacrylamide gels. The amount of β-G(35/36) detected in fat cell membranes of euthyroid rats was 0.20 ± 0.03 nmol/mg of protein (n = 5) and was found to increase by about 60% in the hypothyroid state. Administration of triiodothyronine in vivo (short term hyperthyroidism) resulted in a decrease in the amounts of α-G₄₁ and α-G₃₉ subunits (25 and 20%, respectively). In contrast to these effects of thyroid hormones on G(o) and G(i), the steady-state level of β-adrenergic receptors was not significantly altered by changes in thyroid status. Thus, thyroid status in vivo can modulate the steady-state levels of specific G-proteins.