Hormonal secretion in male rats chronically treated with [D-Trp6, Pro9-NEt]-LRF.

Chronic daily administration of the LRF agonist [DTrp⁶, Pro⁹NEt]LRF (A) was accompanied by a blunting of the LH, FSH and testosterone responses. However, plasma LH levels continued to be elevated with each injection, while plasma FSH values remained within control range. As a result, LH/FSH ratios of treated rats were higher than those of control animals. Preinjection plasma testosterone levels were significantly lowered. Repeated injections of A were usually accompanied by increases in plasma progesterone levels which were largest on the second day of qd administration schedules. The intermittent administration of A every 2nd, 3rd or 4th day resulted in continued large increases in plasma progesterone concentrations. These results suggest that the periodically elevated secretion of progesterone and/or LH in the presence of lowered levels of circulating testosterone, may play a role in the paradoxical antigonadal effects of LRF agonists.