Pituitary responsiveness to superfused GnRH in two species of ranid frogs

Dynamics of pituitary responsiveness to gonadotropin-releasing hormone (GnRH) as measured by LH and FSH secretion rates were examined for two species of ranid frogs in an in vitro superfusion system. The influence of sex was studied with juvenile bullfrogs, Rana catesbeiana, and responsiveness to long-term continuous superfusion with GnRH was examined in R. catesbeiana and R. pipiens. Female juvenile bullfrogs showed virtually no response to doses of GnRH up to 1000 ng/ml, whereas males responded to doses of GnRH as low as 10 ng/ml, and very large increments in LH and FSH were observed with 100 and 1000 ng/ml GnRH (responsiveness in the males was dependent on body size). Adult R. pipiens were more sensitive than bullfrogs to GnRH; 0.2 ng/ml GnRH was an effective dose. These data indicate that, relative to mammals, sensitivity to GnRH in frogs is higher than suggested by previous in vivo studies. As in mammals, "self-priming" in response to GnRH was evident; but unlike mammals, the "self-priming" occurred only after 7-12 hr of continuous superfusion with GnRH, and the R. pipiens glands were more resistant to desensitization during 48 hr of GnRH treatment. This study confirms that sex- and age-dependent differences in responsiveness to GnRH as well as "self-priming" found in previous in vivo studies are evidently dependent on properties of the anterior pituitary per se.