Circhoral Oscillations of Plasma LH Levels in the Ovariectomized Rhesus Monkey ¹ , ²

Abstract

Blood samples were withdrawn at 10-, 20- or 30-min intervals for 6–12 hr from 13 ovariectomized and 7 intact female rhesus monkeys bearing indwelling cardiac catheters and restrained in primate chairs. Plasma LH and GH concentrations were determined by radioimmunoassay. Twenty-four experiments in ovariectomized animals revealed a striking rhythmic pattern in plasma LH levels having a period of approximately 1 hr, which was asynchronous with variations in plasma GH concentrations when these occurred. The LH oscillations, which had an excursion of 40–92 % of the mean hormone concentration, occurred with a mean period of 75 min and a mode of 60 min. In a typical rhythmic sequence, LH levels increased from nadir to peak during one 10-min sampling interval and decayed exponentially for the next 60 min. The t½ calculated from 19 such decay curves based on 10-min sampling intervals was 72 min, which is similar to the estimated t½ of exogenous LH in intact females. Moreover, there was a significant positive correlation between the duration of the decay and the magnitude of the next peak. Such oscillations did not occur in intact females. It is suggested that these circhoral (about an hr) oscillations in plasma LH concentrations, occasioned by ovariectomy, represent pulsatile discharges of LH from the pituitary and probably account for the greater part of the elevated LH plasma levels seen in ovariectomized animals. (Endocrinology87: 850, 1970)