A circadian rhythm of luteinizing hormone secretion in ovariectomized rats

Abstract

The possibility of a circadian pattern of luteinizing hormone (LH) synthesis and release was investigated by measuring LH in pltuitary and plasma samples collected at 4-hr.intervals over a 24-hr. period from ovariectomized rats. Some of the rats had been pretreated with PMSG [pregnant mare''s serum gonado- tropln] or HCG [human chorionic gonadotropln] before prepuberalovariec-tomy; other rats which were not injected were ovariectomized either pre- or postpuberally. At autopsy 5 months after ovariectomy, pltuitary and plasma LH levels in all groups were elevated significantly over levels in intact controls. pltuitary LH content did not show a significant 24-hr.periodicity, and there were no differences among the groups in absolute level. Plasma LH concentrations fluctuated significantly with time in all 4 groups, being highest in the early part of the "light-on" period. The plasma levels of LH seen in the 2 untreated ovariectomized groups were similar, and significantly higher than in the gonadotropin-pretreated rats. Thus gonadotropln pretreatment partially suppressed the increased LH release following steroid withdrawal, but had no effect upon the timing of this release.