Changes in plasma steroid levels after single administration of hCG or LHRH agonist analogue in dog and rat.

The present study was designed to investigate the effect of acute administration of gonadotropin on testicular steroid secretion in dog and rat. Animals received a subcutaneous injection of 25 IU/kg of hCG or 1.5 microgram/kg of [D-Trp6, des-Gly-NH2(10)]LHRH ethylamide (LHRH-A). Testosterone is the predominant steroid measured, in dog plasma, under basal conditions. After LHRH-A injection, testosterone levels are not significantly changed while dehydroepiandrosterone and androst-5-ene-3 beta,17 beta-diol (delta 5-steroids) levels are stimulated by almost 20-fold (P less than 0.01). When dogs were injected with hCG, we also observed a marked stimulation of dehydroepiandrosterone levels (20-fold; P less than 0.01) accompanied by a small increase of plasma testosterone concentration (2-fold, P less than 0.01). In rats injected with either hCG or the LHRH analogue, an increment of plasma testosterone (7-fold, P less than 0.01) is detected in the first hour while plasma dehydroepiandrosterone levels are slightly stimulated. Moreover, in rats injected with hCG, low plasma steroid levels are present between 4-12 h after injection due to testicular desensitization. This marked decrease is then followed by a second peak of steroid secretion 24 h later. Acute testicular steroidogenic responsiveness to hCG on the dog is, however, different: after stimulation, the levels of plasma dehydroepiandrosterone are maintained at a plateau and slowly decline after 24-48 h. Our data indicate that in dogs, stimulation of testicular steroidogenesis leads to an increase of plasma delta 5-steroid levels while the same stimuli cause, in the rat, a stimulation of delta 4-androgen, particularly testosterone.