Summary. Plasma membranes (1–2 mg protein) purified from the anterior pituitary lobes of adult male rats were incubated with 0·6 μmol [α-32P]guanosine 5′-triphosphate (GTP) l−1 in an ATP-regenerating buffer at 37°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°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 μmol 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 μmol l−1 caused a maximum increase in the release of [32P]GDP of 31–38%. The GnRH agonist (0·1 μmol l−1) stimulated GDP release by 21%, 24%, 17% and 14% at 30 s, 1, 2 and 5 min, respectively. TRH (0·1 μmol 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. A receptor antagonist (at the GnRH receptor) has partial efficacy at eliciting this response. This GDP release represents an early biochemical marker of GnRH and TRH receptor stimulation.
Keywords: G proteins; pituitary; gonadotrophin-releasing hormone; thyrotrophin-releasing hormone; rat