In vivo and in vitro responses to gonadotropin releasing hormone in the turtle, Chrysemys picta, in relation to sex and reproductive stage.

In vivo and in vitro responsiveness to gonadotropin releasing hormone (GnRH) was studied in the turtle, Chrysemys picta, after manipulation of reproductive condition by temperature: Warm temperatures (28 degrees) induced testicular growth and ovarian regression compared to cold (17 degrees) treatment. Only males (and primarily from cold treatment) responded to GnRH injection (40 micrograms/100 g body wt intracardiac); correlated increases occurred in plasma LH and testosterone. Effects of GnRH (10 and 100 ng/ml) on LH and FSH secretion by hemipituitaries were studied in a superfusion system; tissues responded to between 0.1 and 1 ng/ml GnRH. Sex differences were evident in both acute and chronic effects of GnRH. Although both groups of females had significantly (sixfold) higher pituitary LH content, basal secretion rates of gonadotropins were similar, and LH and FSH secretion in males was more responsive to GnRH. Gonadotropin secretion rates by male glands showed high initial increments (approx four- to sevenfold) followed by an attenuation (especially LH) during 5 hr of GnRH superfusion. In contrast, tissues from warm-treated females showed a smaller initial response (approx twofold) followed by a progressive increase in output over time, and glands from cold-treated females did not respond to GnRH. Total LH secretion by superfused male hemiglands represented almost half the total LH recovered (secreted + stored); whereas, females secreted only 5% ("cold-treated") or 10% ("warm-treated") of total LH. Thus, the capacity of the pituitary to respond to GnRH is influenced by both sex and reproductive condition in the turtle. Secretion of both FSH and LH were similarly stimulated by GnRH, but thyrotropin (TSH) secretion was independent of GnRH.