Sequential patterns of circulating LH and FSH in female sheep during the early postnatal period: effect of gonadectomy.

To determine the basis for the marked daily variability in circulating luteinizing hormone (LH) in the female lamb shortly after birth and to determine whether circulating follicle-stimulating hormone (FSH) exhibits similar variability during this period of life, serum levels of both gonadotropins were monitored frequently (20-min intervals) over a 3- or 6-hr period each week for the first 9 weeks of life. Similar measurements of circulating LH and FSH were made in lambs ovariectomized at 2 weeks of age to assess whether the ovary influences the secretion of these gonadotropins. Pulsatile release of LH, but not FSH, was observed in both groups of lambs. Although mean concentrations of circulating FSH in intact female lambs were similar to basal levels of the cyclic adult, mean concentrations of serum LH were much higher. Because of the discontinuous release of LH in intact lambs after 4-5 weeks of age, patterns of LH were often indistinguishable from those of the long-term ovariectomized adult. At 9 weeks of age the frequency of episodic LH release in intact lambs ranged from 0.33-1.1 pulses/hr. Castration at 2 weeks of age produced a concomitant, but delayed, rise in mean serum LH and FSH beginning 4-5 weeks later. By 9 weeks of age serum LH, but not FSH, had attained levels comparable to those of the castrated adult, and the mean frequency of LH release (about hourly) was similar for both groups. The results indicate that by 9 weeks of age the ovary of the lamb influences the secretion of LH and FSH. Because in the 9-week-old intact female lamb episodic release of LH occurs producing high (castrate) levels of this gonadotropin while circulating concentrations of FSH remain stable and within basal levels of the adult cycling female, it is suggested that acute concomitant release of LH and FSH does not occur at this age and that separate negative feedback loops for the control of LH and FSH secretion may exist during early postnatal life.