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Biology of Reproduction, Vol 25, 244-252, Copyright © 1981 by Society for the Study of Reproduction
1 Department of Physiology,
University of Pittsburgh School of Medicine,
Pittsburgh, Pennsylvania 15261 Attempts were made to characterize the time courses of concentrations of circulating LH, FSH,
PRL, testosterone (T), and cortisol throughout the light-dark cycle (lights on 0600-1800 h) in
intact adult male rhesus monkeys. Blood samples were withdrawn from five animals every 20 min
with a remote sampling device that permitted continuous access to the venous circulation with
minimal restraint of the animal. Mean concentrations of plasma T were highest during darkness and
declined abruptly shortly after the lights came on to reach a nadir Plasma prolactin levels were lowest at
3 h later at 0900 h. Mean T
levels then increased progressively and in an apparently rhythmic fluctuating fashion throughout
the remainder of the light phase of the 24 h cycle. Individual animals exhibited very striking
moment-to-moment changes in plasma T concentration composed of 7-12 discrete peaks during
the 24 h cycle. The frequency of these episodes of testicular T secretion throughout the light phase
of the cycle, and particularly during the initial 6 h of illumination (approximately one plasma T
peak every 5 h), was noticeably slower than that during darkness (approximately one peak every 2 h).
The amplitude of plasma T peaks, however, was similar throughout the entire light-dark cycle.
Essentially every peak in concentration of circulating T was preceded by a brief, but distinct,
elevation in plasma LH, and most discharges of LH (89%) were followed by an episode of T secretion. These findings suggest that in the male rhesus monkey the characteristic diurnal variation in
circulating T is determined primarily by changes in the frequency of episodic testicular T secretion
which, in turn, appears to be occasioned by a corresponding pattern of intermittent release of LH.0800 h, and concentrations of circulating cortisol
exhibited, as expected, a diurnal variation typical of this plasma steroid. The time course of FSH
was not forthcoming because concentrations of this plasma gonadotropin in the circulation were
usually immeasureable by the RIA employed.
Note:
ACKNOWLEDGMENTS
The author expresses his deep appreciation to Dr.
Ernst Knobil (Department of Physiology, University
of Pittsburgh School of Medicine), whose interest and
support enabled these studies to be initiated. The
author also expresses his gratitude to Dr. Hans-Jurgen
Quabbe (Endokrinologische Abteilung, Freie Universität Berlin) for his help with some of the experiments,
to Debby Morelli for her expert technical assistance,
and to the staffs of the RIA laboratories and primate
colony of the Center for Research in Primate Reproduction, University of Pittsburgh School of Medicine,
for their technical support.
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