Testicular steroid secretion in response to GnRH-mediated LH and FSH release in bulls.

Assay of frequently collected blood samples in four mature Hereford bulls indicated the existence of a tonic mechanism for the secretion of luteinizing hormone (LH) and follicle stimulating hormone (FSH), i.e., their episodic release was not observed. Increased plasma concentrations of LH and FSH, however, were obtained in response to an intravenous injection of 500 vg of gonadotropin releasing hormone (GnRH). These gonadotropins showed similar secretory profiles after GnRH, but the relative magnitude of the LH response (30-fold) was considerably greater than that of the FSH response (sevenfold). Concentrations of testosterone in jugular and spermatic vein blood were increased sevenfold after the administration of GnRH; whereas, concentrations of estrogen increased only twofold. Both steroids reached maximum concentrations 3 hr after GnRH (approximately 1 hr after the LH and FSH peak) then returned to preinjection levels. Estrogen profiles showed considerable between animal variation as compared to the testosterone secretory profiles.