Changes in relaxin precursor messenger ribonucleic acid levels in ovaries of rats after hysterectomy and removal of conceptuses, and during the estrous cycle.

Preprorelaxin mRNA levels in the rat ovary were determined by dot hybridization of unfractionated RNA with 32P-labeled cDNA. The sharp rise in these levels occurring near midpregnancy could be prevented by hysterectomy. After the rise occurred in intact rats, hysterectomy sharply reduced the amount of messenger. Reduction of the number of conceptuses to one also reduced ovarian preprorelaxin mRNA levels. Thus, the uterus and/or its contents were necessary for both stimulation and maintenance of preprorelaxin mRNA levels. The sharp decline in ovarian preprorelaxin mRNA levels after hysterectomy was partially prevented by the administration of 17 beta-estradiol, but not by 5 alpha-dihydrotestosterone. However, 17 beta-estradiol did not stimulate the rise in ovarian preprorelaxin mRNA levels when given to pregnant rats that were hysterectomized near the time of the natural increase in preprorelaxin mRNA levels. The amount of preprorelaxin mRNA at estrus was 3.5 times greater than the proestrous value. The estrous level was reduced to about half by the time of metestrus. The amount of ovarian preprorelaxin mRNA at estrus was 200-400 times less than that from 20-day pregnant rats. The changes in ovarian preprorelaxin mRNA levels during the estrous cycle suggest a possible role for relaxin in nonpregnant rats. Because of the correlation between changes in preprorelaxin mRNA levels and the concentration of immunoreactive relaxin in the ovaries and blood, shown by others, the amount of precursor mRNA probably regulates the concentration of relaxin in the rat ovary. The agents and mechanisms involved in this regulation remain to be determined, but estrogen appears to be important for maintenance of the elevated preprorelaxin mRNA levels in the ovary.