Temporal Interrelationships Among Circulating Levels of Estradiol, Progesterone and LH During the Rat Estrous Cycle: Effects of Exogenous Progesterone¹

Abstract

Serum progesterone (P) levels display a circadian rhythm of varying amplitude during the estrous cycle. This rhythm is characterized by lowest levels of the day around noon and maximal levels at 1600 or 2300 hr. Amplitude was smallest on estrus and progressively greater on diestrus II, diestrus I and proestrus. Serum estradiol (E2) concentrations declined gradually through estrus till 0800 hr of diestrus I. Thereafter, levels increased rapidly to peak at noon, and returned later to near nadir at 2300 hr of diestrus I. During diestrus II and morning of proestrus serum E2 concentrations increased in a biphasic pattern; a moderate increase till 1800 hr of diestrus II was followed by relatively rapid rise to peak levels at 0800 hr of proestrus. A reciprocal relationship between circulating levels of E2 and P during the estrous cycle was also detected. The elevations in serum P levels during estrus, diestrus I and proestrus appeared to coincide with a gradual reduction in serum E2 levels. In addition to the surge of serum LH in the afternoon of proestrus small increases in serum concentration of LH over the base line value were noted at 0800 hr of diestrus I and at 1600 hr of estrus. The influence of experimentally altering the circulating levels of P on endogenous levels of E2 was next examined. Administration of P at 0800 hr of diestrus I blocked the afternoon rise in serum E2; the suppressive effects persisted till 1800 hr of diestrus II. Similarly, injection of P at 0800, 1800 or 2300 hr of diestrus II suppressed the rise in E2 normally observed at proestrus. These observations suggest that circadian variations in peripheral serum P concentrations during the estrous cycle may constitute a regulatory mechanism for the observed daily fluctuations in serum E2. (Endocrinology95: 1711, 1974)