Effects of cholera toxin on the coupling of thyrotropin-releasing hormone to a guanine nucleotide-binding protein in cultured GH3 cells.

The effects of cholera toxin on the coupling of the thyrotropin-releasing hormone (TRH) receptor to a guanine nucleotide-binding (G) protein were examined in a GH3 clonal strain of rat pituitary tumor cells. Incubation of the cells with cholera toxin (50 ng/ml) for 16 hr caused a decrease in [3H]methyl-TRH binding to 59% of the control level and in TRH-stimulated low Km GTPase activity from 143 to 107% of the control level in the membrane-containing fraction. The effects of cholera toxin were time dependent; TRH-stimulated GTPase activity was reduced after a 3-hr incubation, whereas cholera toxin decreased [3H]methyl-TRH binding in the membrane-containing fraction after a 5-hr incubation. These results suggest that the inhibition of TRH-stimulated GTPase activity by cholera toxin treatment is not due to the decrease of receptor binding caused by this toxin. On the other hand, incubation of GH3 cell membranes with preactivated cholera toxin and NAD+ did not substantially alter the binding of [3H]methyl-TRH. In contrast, the cholera toxin-treated membranes demonstrated a partial reduction in the activity of TRH-induced low Km GTPase activity and a 10-fold increase in the concentration of guanine nucleotide required for a half-maximal effect in regulating the TRH receptor affinity for [3H]methyl-TRH. These data suggest that cholera toxin may act directly on a G protein that is associated with TRH-receptors.