Suppression of luteal progesterone secretion in the stumptailed macaque by an antagonist analogue of luteinizing hormone releasing hormone.

The dependence of progesterone secretion from the corpus luteum on pituitary gonadotrophin was examined in the cyclic stumptaile macaque by studying the effects of a single s.c. injection of a potent LH releasing hormone (LHRH) antagonist, [N-Ac-D-Nal(2)1, D-pCl-Phe2, D-Trp3, D-hArg(Et2)6, D-Ala10] LHRH. A dose of 100 micrograms antagonist/kg administered on days 9/10 of the luteal phase in three monkeys caused a marked temporary suppression of serum concentrations of LH and progesterone during the following 32h but levels still remained detectable and after 2 days serum hormone concentrations returned to the normal luteal-phase range. When the same animals were treated with 300 micrograms antagonist/kg at the same period during a subsequent cycle, serum LH levels were close to or at the limits of detection of the bioassay for the next 48h and progesterone concentrations declined steadily, reaching non-detectable values by 48h. In two monkeys the progesterone levels remained suppressed and they menstruated prematurely; in the third monkey the progesterone concentration rose to just above baseline and menstruation occurred at the expected time. Administration of 300 micrograms antagonist/kg on days 6/7 of the luteal phase in a further three monkeys also suppressed progesterone concentrations but not to baseline values, and after 2 days a normal progesterone profile was regained. These results suggest that the corpus luteum of the stumptailed macaque is largely dependent on pituitary gonadotrophin support during the mid to late luteal phase.