Progesterone, testosterone and estradiol-17β inhibit gonadotropin-releasing hormone stimulation of G protein GTPase activity in plasma membranes from rat anterior pituitary lobe

In order to understand the biochemical mechanisms underlying the rapid, non-genomic effects of gonadal steroids on gonadotropin secretion, we examined the effects of progesterone, testosterone and estradiol-17β on the low Km GTPase activity associated with transducer G proteins coupled to gonadotropin-releasing hormone (GnRH) receptors. Homogenates of anterior pituitary lobes from adult male rats were processed by discontinuous sucrose gradient centrifugation to isolate plasma membranes. The low Km GTPase activity (EC 3.6.1.-) was assayed in 5 μg membrane protein using [γ-32P]GTP at 37°C in an ATP-regenerating buffer containing 1 μmol/l unlabeled GTP. One hundred nmol/l each of progesterone, testosterone and estradiol-17β maximally stimulated low Km GTPase activity by 61%, 59% and 45%, respectively (p<0.05). Time course studies revealed that 100 nmol/l progesterone stimulated the enzyme activity by 93% and 62% at 5 and 30 min, respectively; 100 nmol/l testosterone stimulated GTPase activity by 100% and 72% at 5 and 30 min, respectively: 100 nmol/l estradiol-17β stimulated GTPase activity by 80% and 70% at 5 and 30 min, respectively. GnRH stimulated the low Km GTPase activity by about 60% in a concentration-dependent manner. In the presence of the gonadal steroids, the ability of GnRH to stimulate the GTPase activity was inhibited. For example, stimulation ranged from 36% to 60% with 0.1–100 nmol/l GnRH alone, but only from 7% to 20% in the presence of GnRH and 100 nmol/l progesterone (p<0.05). Similarly, in the presence of 100 nmol/l estradiol-17β, GnRH stimulation of the enzymatic activity ranged from 12% to 19%. It appeared that testosterone was less effective in inhibiting GnRH-stimulated GTPase activity; stimulation ranged from 15% to 32% in the presence of GnRH and 100 nmol/l testosterone (p<0.05). These results, while suggesting that the gonadal steroids disrupt GnRH receptor-G protein interactions, are consistent with the notion that steroids have a profound effect at the membrane level prior to their interaction with the cytosolic receptors.