Release of relaxin by a nonluteolytic dose of prostaglandin F2 alpha in pregnant swine.

Ten gilts were infused intravenously for 5 min on Days 105 and 108 of gestation with 50 micrograms prostaglandin F2 alpha (PGF2 alpha) in 10 ml of saline or saline alone. Plasma concentrations of relaxin and progesterone were determined in samples taken 1) immediately before and after infusion, 2) at 10-min intervals throughout the first h postinfusion, 3) at 30-min intervals throughout the next 5 h, and 4) at 12-h intervals from Day 103 of gestation through Day 1 postpartum. Plasma concentrations of relaxin were increased immediately after infusion with PGF2 alpha and peaked within 10 min postinfusion. The peak was followed by a rapid decline in plasma relaxin throughout the remainder of the first hour following infusion. Progesterone values did not change from preinfusion levels for either treatment during this time period and did not differ between treatments for the remainder of the gestation period. When compared to controls, PGF2 alpha-treated animals were not significantly different with respect to length of gestation, duration of parturition or frequency of live births. These results indicate that exogenous PGF2 alpha can stimulate relaxin release from the corpus luteum without inducing luteolysis.