Effect of porcine relaxin on cervical dilatation, pelvic area and parturition in beef heifers.

Purified porcine relaxin (3,000 U/mg) was administered either into the cervical os or by intra-muscular injection to crossbred beef heifers beginning 4 days before expected parturition, in an attempt to elucidate the physiological roles of relaxin in cervical dilatation, the pelvic area, and parturition. Relaxin (3,000 U in a gel vehicle), when administered into the cervical os during late pregnancy, induced significant dilatation of the cervix 8 and 16 h later, as compared to vehicle-treated controls. This induced cervical dilatation did not cause premature parturition in relaxin-treated heifers and was similar to controls. Exogenous relaxin during late gestation elicited an increased growth rate of the pelvic area, as determined by sequential measurements of height and width of the pelvic canal. These results indicate that, before parturition, pelvic width increased more rapidly than pelvic height and that exogenous relaxin elicited a greater response in both parameters. Highly significant nocturnal elevations in concentrations of progesterone in peripheral blood serum occurred in vehicle-treated control heifers during late pregnancy. These nocturnal elevations in serum levels of progesterone were significantly reduced after 8 and 16 h of relaxin treatment in experimental animals as compared to control heifers. The mechanisms by which porcine relaxin reduces ovarian progesterone secretion in beef heifers remain undefined.