Pituitary and gonadal responses to continuous infusion of gonadotropin releasing hormone in the male bullfrog, Rana catesbeiana.

Abstract

Results of continuous constant-rate intravenous infusions of synthetic mammalian gonadotropin releasing hormone (GnRH) for 1–4 days into intact adult male bullfrogs, Rana catesbeiana, indicate that the control and dynamics of gonadotropin [follicle-stimulating hormone (FSH) and luteinizing hormone (LH)] secretion in the frog differs from that in mammals. In the bullfrog, there was no temporal dissociation between the secretory profiles of the two gonadotropins (Gn), and the pituitary did not develop a clear refractoriness to GnRH even after 3–4 days of continuous infusion. A dose-response relationship was observed both for the temporal pattern and magnitude of increase in plasma Gn during infusion. For the two frogs at the lowest dose of GnRH (0.25 μg/h), increases in plasma FSH and LH were gradual. At all higher doses, a biphasic pattern of increase in both plasma FSH and LH was apparent; this consisted of an initial peak in plasma Gn at 0.5–1.0 h and a second higher peak at about 24 h (n=5) or 48 h (n=1) of infusion. Thereafter, levels of plasma Gn remained relatively stable and elevated well above baseline (by at least 50× and 20× for LH and FSH, respectively) for the duration of infusion. The highest levels of plasma Gn were attained during the course of infusion with 8.0 μg/h GnRH; the responses to 1.0 and 4.0 μg/h were lower and indistinguishable from one another and 0.25 μg/h GnRH was only marginally effective in elevating plasma Gn. Associated with changes in plasma Gn were concomitant increments in plasma androgen that usually peaked at 2–5 h (n=4) or about 24 h (n=4), and, like plasma Gn, remained elevated above pretreatment levels for the duration of each infusion. Thus, the intact male bullfrog is unlike most male or female mammals tested in lacking a pronounced pituitary (or gonadal) desensitization for up to 4 days of continuous GnRH infusion.