Relationships of gonadotropins, testosterone, and cortisol in response to GnRH and GnRH antagonist in boars selected for high and low follicle-stimulating hormone levels

Considerable variation exists in the serum levels of gonadotropins in boars ; this results in differential testicular function. Boars (Chinese Meishan, European White composite, and crosses of the two breeds) selected for high and low circulating FSH concentrations were used to define possible differ ences in pituitary sensitivity to GnRH and CnRH antagonist and gonadal and adrenal responses. After a 2-h pretreatment sampling period, boars were inj ected with GnRH or GnRH antagonist and repetitively sampled via jugular can nula for changes in serum concentrations of FSH, LH, testosterone, and cort isol. In response to varying doses of GnRH or GnRH antagonist, FSH, LH, or testosterone changes were not different in high-or low-FSH boars. Declines in LH after GnRH stimulation were consistently faster in boars selected for high FSH. Chinese Meishan boars had considerably higher cortisol concentra tions than White composite boars (132.2 +/- 28.5 vs 67.4 +/- 26.8 ng/mL, re spectively; P < .01). When select high- and low-gonadotropin Meishan:White composite crossbreds were sampled, cortisol levels were elevated but compar able between the two groups (126.5 +/- 13.7 vs 131.4 +/- 13.4 ng/mL, respec tively). After GnRH antagonist lowered LH concentrations, administration of hCG resulted in increased testosterone and cortisol concentrations. Althou gh testosterone concentrations remained high for 30 h, cortisol concentrati ons returned to normal levels within 10 h after hCG injection. The mechanis m by which boars selected for high gonadotropins achieve increased levels o f LH and FSH may not be due to differences in pituitary sensitivity to GnRH but to differences in clearance from the circulation.