Effects of progesterone and estradiol on uterine secretion of prostaglandin F-2 alpha in response to oxytocin in ovariectomized sows

Thirty ovariectomized sows were used in an experiment designed to determine whether the ability of the porcine uterus to release prostaglandin (PC) F- 2 alpha in response to oxytocin is regulated by progesterone (P-4) and estr adiol (E-2). Sows were assigned to one of four treatment groups: 1) no ster oids (ovariectomized controls; n = 8), 2) E-2 (n = 8), 3) P-4 (n = 7), or 4 ) E-2 + P-4 (n = 7). P-4 and E-2 were administered so as to mimic the norma l temporal changes that occur in these hormones during the estrous cycle. A group of intact sows (n = 9) was included for comparison. All sows receive d an injection of oxytocin (30 IU, i.v.) on Days 12, 15, and 18 postestrus. Jugular venous blood samples were collected from 60 min before through 120 min after injection of oxytocin for quantification of 13,14-dihydro-15-ket o-PGF(2 alpha) (PGFM). Preinjection baseline concentrations of PGFM, the ma gnitude of the PGFM response above baseline, and area under the PGFM respon se curve (AUC) were calculated for each sow on each day and compared among, treatment groups by ANOVA. Among the ovariectomized sows receiving steroid replacement, baseline concentrations of PGFM were low on Day 12 postestrus in all four groups. On Days 15 and 18, baseline concentrations remained lo w in the two groups that did not receive P-4 but increased in those that di d. Both the magnitude of the response to oxytocin and AUC were small on Day 12 postestrus in all 4 groups. By Day 15, the magnitude of the response an d AUC increased in the group that received both P-4 and E-2 but remained lo w in the other three groups. By Day 18, responses to oxytocin were greater in both groups that received P-4 than in those that did not. Baseline conce ntrations were similar in intact sows and in those that received both P-4 a nd E-2 on all three days examined. The magnitude of the response and the AU C were greater in the ovariectomized sows receiving P-4 and E-2 replacement than in the intact control sows on Days 15 and 18 postestrus. From these r esults, we conclude that P-4 and E-2 interact to control the time when the uterus begins to secrete PGF(2 alpha) in response to oxytocin and the amoun t of PGF(2 alpha) secreted.