LUTEINIZING HORMONE LEVELS DURING THE MENSTRUAL CYCLE OF THE BABOON (PAPIO HAMADRYAS)

Abstract

ABSTRACT The peripheral plasma levels of luteinizing hormone (LH) as measured by an in vitro bioassay method were determined in daily plasma samples collected throughout one menstrual cycle in 8 normally menstruating baboons (Papio hamadryas). In addition LH was measured in plasma at three hourly intervals throughout the day in the follicular, peri-ovulatory and luteal phases of the cycle in 7, 3 and 6 animals respectively. The plasma levels of progesterone and oestradiol were also determined in the same samples throughout the menstrual cycle and during the period of the midcycle LH surge. The circulating LH profile measured throughout the cycle was characterized by a sharp mid-cycle surge (completed within one day) which was followed by a series of LH surges of varying intensity during the luteal phase of the cycle. The initial surge was considered to be pre-ovulatory as indicated by its relationship to the peak of plasma oestradiol and to the first significant increase in the levels of plasma progesterone above values found earlier in the follicular phase. A circadian rhythm of LH was observed during the luteal phase of the cycle; a 3 fold rise in LH was noted during the hours 15.00 to 24.00. No differences were observed throughout the day in the follicular phase of the cycle. The LH profile in three animals studied during the mid-cycle LH surge showed pronounced circadian changes with a major peak at 24.00 h. Plasma progesterone levels during this period rose sharply to values normally found in the mid-luteal phase of the cycle. A comparison of plasma levels of biologically active LH during the menstrual cycle of the baboon with those found in normally menstruating women reveals that in the baboon the LH peak is of much shorter duration and the levels in the follicular and peri-menstrual phases are significantly lower than in the human.