| Abstract: | Prostaglandin E (PGE) receptor density in hepatic plasma membranes can be down-regulated by in vivo exposure to the 16,16-dimethyl analog of PGE2, and this is associated with desensitization of PGE-sensitive adenylate cyclase. These studies examined adenylate cyclase response to other agonists in membranes whose PGE receptor density was 51% decreased and whose maximal PGE-stimulated adenylate cyclase activity was 31% decreased. Down-regulated membranes had a 37% decrease in their maximal response to glucagon, indicating that treatment with the PGE analog had induced both homologous and heterologous desensitization. To determine whether adenylate cyclase had been affected, stimulation with NaF, guanyl 5'-yl imidodiphosphate (GppNHp), and forskolin was examined in both intact and solubilized membranes. Intact membranes had decreased adenylate cyclase responses to all three stimulators (NaF, -41%; GppNHp, -25%; forskolin, -41%) as did solubilized membranes (NaF, -51%; GppNHp, -50%; forskolin, -50%), suggesting alterations in adenylate cyclase rather than indirect membrane effects. Cholera toxin activation and labeling were examined to more directly assess whether the guanine nucleotide (G/F) regulatory component of adenylate cyclase had been affected. Cholera toxin activation was 42% less in down-regulated membranes, and these membranes incorporated less label when the incubation was performed in the presence of [32]NAD. Solubilized G/F subunit activity from down-regulated membranes was less effective in reconstitution of adenylate cyclase activity from cyc- cell membranes than G/F activity from control membranes. These data indicate that in vivo exposure to the PGE analog causes both homologous and heterologous desensitization of adenylate cyclase as well as an apparent quantitative decrease in G/F. |
