Control of Luteal Relaxin Release by Prostaglandin F 2 α : Differences in the Sow Cycle and Pregnancy

Abstract

The effect of an in vivo prostaglandin F_2α (PGF_2α ) challenge in pregnant and cyclic sows was compared to determine whether PGF_2α -induced release of relaxin (RLX) from the corpus luteum (CL) in late pregnancy is also effective during the cycle. Ovarian venous RLX and progesterone were monitored by radioimmunoassay and RLX localized in the CL by immunohistochemistry. In Day 108 pregnant sows, infusion of PGF_2α (100 μ g) into the ovarian artery resulted in an immediate and sustained rise in ovarian venous RLX with an initial decline in progesterone levels by 30 min which then returned to pretreatment levels. In Day 13 or 15 cyclic sows with functional corpora lutea (i.e., elevated progesterone), RLX was undetectable in ovarian venous blood after 100 μg of PGF_2α . Administration of PGF_2α , via either the jugular vein or intramuscular route was also ineffective in releasing RLX from the CL of the cycle. The intensity of RLX immunostaining of the CL was similar in saline and PGF_2α -treated sows. These studies indicate that the control of RLX release from the sow CL differs in the estrous cycle and pregnancy.