Evidence that Luteinizing Hormone from the Maternal Pituitary Gland May Promote Antepartum Release of Relaxin, Luteolysis, and Birth in Rats*

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Abstract
The role of the maternal pituitary gland and LH in the regulation of the release of relaxin from the onset of the light phase on day 21 of pregnancy (day 21) until delivery (herein designated the antepartum period) was examined in rats. Serum relaxin levels were determined in rats hypophysectomized (HYPOX) or sham hypophysectomized (SHAM) on day 14. In SHAM rats, the elevated serum relaxin levels that occurred during the light phase of days 21 and 22 coincided with a rapid decline in serum progesterone to basal levels, and relaxin levels then declined markedly throughout the 18- to 24-h before delivery. During the antepartum period HYPOX rats differed from SHAM rats in the following ways: 1) mean serum relaxin levels were higher immediately before the antepartum period (day 20) and remained at the maximal levels the sham-operated animals experienced throughout the ensuing antepartum period; 2) elevations in relaxin levels generally occurred in individual rats; however, they usually occurred later than day 22 and, in many cases, during the dark phase of the photoperiod; 3) mean relaxin levels remained elevated throughout days 22 and 23; and 4) the antepartum decline in progesterone levels was protracted throughout and beyond day 22. Subsequent experiments determined the effects of LH on birth, relaxin levels, and functional luteolysis. Some Sprague-Dawley-derived rats give birth during the light phase of day 22 (day 22 rats), but generally, most give birth during the light phase of day 23 when maintained under the housing conditions employed for these studies. When LH was injected to both intact and HYPOX rats in the delay-vehicle polyvinylpyrrolidone on the morning of day 20, birth and antepartum ovarian events were advanced. All intact rats and all but one HYPOX rat gave birth on day 22, and the time of occurrence of the rapid decline in both serum relaxin and progesterone levels coincided with these events in day 22 rats. In contrast, a reduction of endogenous serum LH levels in intact rats by means of the injection of a LH antiserum from days 19 through 22 delayed birth as well as the antepartum decline in both serum relaxin and progesterone levels. The results of this study indicate that maternal pituitary LH may play a role in promoting the normal progression of birth, the antepartum elevation and decline in serum relaxin levels, and functional luteolysis in the pregnant rat.