Ovarian effect of periodic episodes of elevated serum follicle stimulating hormone (FSH)

Periodic increases (episodes) of serum follicle stimulating hormone (FSH) were induced for various lengths of time (epochs) by the intraperitoneal injection of synthetic porcine luteinizing hormone releasing hormone (LH-RH) into immature female rats. The effect of the FSH on ovarian weight was evaluated with augmentation by human chorionic gonadotropin (HCG). Eight injections of LH-RH, at hourly intervals, produced increased ovarian weight in all animals; with 6 episodes 67% and with 4 only 33% responded. Increasing the length of the epoch of elevated serum FSH to 10 hours was without added effect. The minimally effective serum FSH level was estimated to be about 1000 ng/ml (RP-1). This concentration was produced by injecting LH-RH at 30 minute intervals over a period of 2 hours and it proved to be effective in increasing ovarian weight 48 hours later. Multiple 3 hour epochs, separated by at least 3 hours, were no more effective than a single epoch. Non augmented ovarian and uterine weights were significantly raised by injection of LH-RH on three consecutive days. The results suggest that a circadian rhythm in gonadotropin output could effectively cause normal ovarian development. Periods of increased pulsatile activity by the pituitary would need to be relatively brief to produce threshold concentration of gonadotropin for a threshold period of time.