A circadian rhythm of luteinizing hormone secretion in ovariectomized rats.

Physiol, 214(2): 213-217. 1968.-The possibility of a circadian pattern of luteinizing hormone (LH) synthesis and release was investigated by measuring LH in pituitary 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 or HCG before prepuberal ovariectomy; other rats which were not injected were ovariectomized either preor postpuberalyl. At autopsy 5 months after ovariectomy, pituitary and plasma LH levels in all groups were elevated significantly over levels in intact controls. Pituitary 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 four groups, being highest in the early part of the “light-on” period. The plasma levels of LH seen in the two untreated ovariectomized groups were similar, and significantly higher than in the gonadotropinpretreated rats. Thus gonadotropin pretreatment partially suppressed the increased LH release following steroid withdrawal, but had no effect upon the timing of this release.