Stimulation of the release of [32P]guanosine 5'-diphosphate from G proteins in the rat anterior pituitary lobe by gonadotrophin-releasing and thyrotrophin-releasing hormones.

Plasma membranes (1-2 mg protein) purified from the anterior pituitary lobes of adult male rats were incubated with 0.6 mumol [alpha-32P]guanosine 5'-triphosphate (GTP) l-1 in an ATP-regenerating buffer at 37 degrees C for 60 min; during this incubation the [32P]GTP was hydrolysed and the nucleotide that was predominantly bound to the membranes was [32P]GDP. The release of [32P]GDP from the membranes was monitored at 37 degrees C; the amount released was proportional to the protein concentration and increased as a function of time. 5'-Guanylylimidodiphosphate (Gpp(NH)p) increased [32P]GDP release by up to 30% at 0.1 mumol l-1. Although 10 nmol Gpp(NH)p l-1 had no effect on GDP release, it appeared to stabilize the hormonal effect by blocking further GDP-GTP exchange. Gonadotrophin-releasing hormone (GnRH) agonist and thyrotrophin-releasing hormone (TRH), at 0.1 mumol l-1 caused a maximum increase in the release of [32P]GDP of 31-38%. The GnRH agonist (0.1 mumol l-1) stimulated GDP release by 21%, 24%, 17% and 14% at 30 s, 1, 2 and 5 min, respectively. TRH (0.1 mumol l-1) stimulated GDP release by 38%, 30%, 17% and 16% at 30 s, 1, 2 and 5 min, respectively. A GnRH antagonist also stimulated [32P]GDP release, albeit less effectively than GnRH agonist; the antagonist did not inhibit agonist stimulation of GDP release. These results indicate that ligand binding to the GnRH and TRH receptors results in interaction of the receptor with a guanine-nucleotide-dependent transducer protein (G protein) and activation of GTP-GDP exchange.(ABSTRACT TRUNCATED AT 250 WORDS)