To investigate the possible role of melatonin in the regulation of the human menstrual cycle, the circadian pattern of melatonin was determined in the follicular and luteal phases of 10 normal women. Four-hourly sampling was used to derive a melatonin index which described the total exposure to melatonin for 24 h. This sampling procedure adequately represented the circadian melatonin output and demonstrated that pulses of melatonin secretion, inconsistent with a measured half-life of 47 min, did not exist. A significant increase (P less than 0.001) in the melatonin index was found in the luteal phase compared to that in the follicular phase. To investigate the influence of exogenous progestins on the melatonin pattern, repeated 24-h profiles were measured in 8 women taking the 3-phase contraceptive pill. There was a significant increase (P less than 0.01) in the melatonin index associated with an increase in the dose of progestin. These results are consistent with a positive relationship between melatonin and progesterone and suggest that changes in the circadian pattern of melatonin secretion, rising during the luteal phase with a fall before ovulation, may act as a modulator of cyclicity.

No abstract

The effects of different atrial natriuretic peptides on cyclic GMP formation and steroidogenesis have been studied in Percoll-purified mouse Leydig cells. Rat atrial peptides rANP (rat atrial natriuretic peptide), rAP-I (rat atriopeptin I) and rAP-II (rat atriopeptin II), in the presence of a phosphodiesterase inhibitor, stimulated cyclic GMP formation in a concentration-dependent manner. In the presence of saturating concentrations of the peptides, a 400-600 fold stimulation of cyclic GMP accumulation was observed. Among the peptides, rAP-II appeared to be the most potent. ED50 values (concentration causing half-maximal effect) for rAP-II, rANP and rAP-I were 1 X 10(-9) M, 2 X 10(-9) M and 2 X 10(-8) M respectively. A parallel stimulation of cyclic GMP formation and testosterone production by the cells was observed after incubation of the cells with various concentrations of rAP-II. In the presence of a saturating concentration of rAP-II (2 X 10(-8) M), maximum stimulation of intracellular cyclic GMP content was obtained within 5 min of incubation. Testosterone production by mouse Leydig cells could be stimulated by 8-bromo cyclic GMP in a concentration-related manner. At a 10 mM concentration of the cyclic nucleotide, steroidogenesis was stimulated to a similar extent as that obtained with a saturating concentration of human chorionic gonadotrophin (5 ng/ml). On the basis of these results we conclude that cyclic GMP acts as a second messenger in atrial-peptide-stimulated steroidogenesis in mouse Leydig cells. The steroidogenic effect of atrial peptides appears to be species-specific, since none of these peptides stimulated testosterone production by purified Leydig cells of rats, though in these cells a 40-60-fold stimulation of cyclic GMP formation in response to each of the three peptides was observed. However, 8-bromo cyclic GMP could stimulate testosterone production in rat Leydig cells. Therefore we conclude that the lack of steroidogenic response in rat Leydig cells to atrial-natriuretic-factor-stimulation results from an insufficient formation of cyclic GMP in these cells. This species difference would appear to result from a lower guanylate cyclase activity in rat Leydig cells.

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Bovine follicles having a higher concentration of progesterone than estradiol in the follicular fluid can be considered as atretic. Since we observed previously that there was an inverse relationship between the follicular fluid estradiol to progesterone (E/P) ratio and the prorenin level, we have proposed that a high prorenin level may be associated with follicular atresia. The aim of the present study was to corroborate this hypothesis by including additional indices to distinguish unambiguously between atretic and nonatretic follicles and to compare the prorenin levels in these two groups of follicles. The present study included examination of more than 200 follicles in the follicular fluid of which we have measured steroid and prorenin levels. The results obtained show a highly significant negative correlation between the prorenin level on the one hand and the E/P ratio, estrogen to total androgen ratio, or estradiol concentration on the other hand. As a further criterion for atresia, we have examined the histological characteristics of the follicles by light and electron microscopy and have found that 90% of histologically characterized atretic follicles had an E/P ratio less than 1 and an average prorenin level four to five times higher than nonatretic follicles. Finally, when we determined the FSH-stimulated cAMP response and the aromatase activity, in terms of the ability to convert exogenous androgen to estrogen in granulosa cells isolated from individual follicles, we observed a markedly higher prorenin level in the fluid of follicles whose granulosa cells responded poorly to FSH and showed a low aromatase activity, compared to follicles whose granulosa cells responded strongly to FSH and contained high aromatase activity. In summary, follicles that were classified as atretic on the basis of a number of biochemical and histological parameters contained significantly higher prorenin levels in their follicular fluid than nonatretic ones. Thus, a high follicular fluid prorenin level is a valid indicator for follicular atresia in bovine ovaries. However, the reason for this increase in follicular fluid prorenin level and whether this increase is a cause or a consequence of atresia remains to be determined.