Adipocyte G-proteins and adenylate cyclase. Effects of adrenalectomy

Steroid hormones modulate the ability of cells to respond to hormones that act via cyclic AMP. In adipocytes of adrenalectomized rats, cyclic AMP accumulation and lipolysis in response to adrenaline are attenuated. However, the mechanism(s) of these effects are poorly understood. The effects of altered glucocorticoid status in vivo on the steady-state amounts of components of the hormone-sensitive adenylate cyclase were analysed in rat adipocytes. β-Adrenergic receptors were analysed by using radioligand binding and immunoblotting with an anti-receptor antiserum. Neither the amount of radioligand binding nor the amount of β-adrenergic-receptor peptide (M(r) 67,000) was altered by adrenalectomy, whereas treatment of adrenalectomized rats with dexamethasone was found to increase both parameters by more than 25% with respect to the control. Forskolin-stimulated adenylate cyclase activity was unchanged in membranes isolated from adipocytes of adrenalectomized rats, but was decreased (50%) in those from dexamethasone-treated rats. The α-subunit of G(s) was probed by using cholera-toxin-catalysed ADP-ribosylation. Immunoblotting was used to analyse the steady-state amounts of G-protein β-subunits (β-G(35/36). Adrenalectomy was associated with decreases in the steady-state amounts of α-G(s) (30%) and β-G(35/36) (50%). Dexamethasone treatment of adrenalectomized animals partially restored the lipolytic response of adipocytes to adrenaline and the amounts of α-G(s), increased the amounts of β-G(35/36) subunits from 50% to 150% of control values, increased β-adrenergic receptors by more than 25% and decreased adenylate cyclase activity (50%). These results suggest that the steady-state amounts of components of hormone-sensitive adenylate cyclase are differentially regulated by glucocorticoids.