Endocrinology, Vol 113, 997-1003, Copyright © 1983 by Endocrine Society

ARTICLES

Influence of light-dark cycle on antepartum serum relaxin and progesterone immunoactivity levels and on birth in the rat

OD Sherwood, SJ Downing, TG Golos, WL Gordon and MK Tarbell

Relaxin and progesterone are produced by the corpora lutea in the pregnant rat. Relaxin immunoactivity levels are elevated in peripheral sera during the last 12 days of pregnancy. In rats maintained under a conventional photoperiod of 14 h of light (0600-2000 h) and 10 h of darkness (2000-0600 h), there is an antepartum elevation in serum relaxin to maximal levels coincident with the rapid decline in serum progesterone to basal levels during the light phase of the photoperiod 1 day before birth. Therefore, we postulated that this maximal elevation in serum relaxin levels may be temporally associated with functional luteolysis and linked to the photoperiod. In the present study the photoperiod was advanced near midpregnancy in order to examine further the relationship of the antepartum elevation in serum relaxin levels with both functional luteolysis and the photoperiod. Three groups of rats were maintained under a conventional photoperiod of 14 h of light (0500-1900 h) and 10 h of darkness until days 7 and 8 of pregnancy when the photoperiod was advanced 8 h in group 2 (G2) and advanced 18 h in G3 relative to the conventional photoperiod that was maintained in G1. Serum relaxin and progesterone levels were determined in blood samples obtained at 4-h intervals from 2000 h on day 19 of pregnancy until birth. The times of occurrence of birth, maximal relaxin levels, and decline of progesterone to basal levels in G2 and G3 were generaly advanced 50-60% of the advancement of the photoperiod. There was a close temporal association between the attainment of maximal relaxin levels and basal progesterone levels; they both occurred during the light phase of the photoperiod, approximately 24 h before birth in all three groups. We conclude that the antepartum elevation of serum relaxin to maximal levels may be associated with functional luteolysis and that its time of occurrence is influenced by the photoperiod. This study also provides evidence that the antepartum elevation of relaxin levels consists of two phases which occur at a 24- h interval. It is proposed that these two phases in the elevation of relaxin levels may be indicative of an increasingly effective endogenous circadian luteolytic process whose time of occurrence is influenced by the light-dark schedule.

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