Prostagland in F2α -Induced Release of Oxytocin from Bovine Corpora Lutea in vitro1

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Abstract
Two experiments were conducted to study the in vitro effects of prostaglandins F2.alpha. (PGF2.alpha.), E2 (PGE2), and luteinizing hormone (LH) on oxytocin (OT) release form bovine luteal tissue. Luteal concentration of OT at different stages of the estrous cycle was also determined. In Experiment 1, sixteen beef heifers were assigned randomly in equal numbers (N=4) to be killed on Days 4, 8,12, and 16 of the estrous cycle (Day 0 = day of estrus). Corpora lutea were collected, an aliquot of each was removed for determination of initial OT concentration, and the remainder was sliced and incubated with vehicle (control) or with PGF2.alpha. (10 ng/ml), PGE2 (10 ng/ml), or LH (5 ng/ml). Luteal tissue from heifers on Day 4 was sufficient only for determination of initial OT levels. Luteal OT concentrations (ng/g) increased from 414 .+-. 84 on Day 4 to 2019 .+-. 330 on Day 8 and then declined to 589 .+-. 101 on Day 12 and 81 .+-. 5 on Day 16. Prostaglandin F2.alpha. induced a significant in vitro release of luteal OT (ng .cntdot. g-1 .cntdot. 2h-1) on Day 8 (2257 .+-. 167 vs. control 1702 .+-. 126) but on Days 12 or 16 of the cycle. Prostaglandin E2 and LH did not affect OT release at any stage of the cycle studied. In Experiment 2, six heifers were used to investigate the in vitro dose-response relationship of 10, 20 and 40 ng PGF2.alpha./ml of medium on OT release from Day 8 luteal tissue. A significant linear dose response was obtained, indicating OT secretion (ng .cntdot. g-1 .cntdot. 2h-1) was dose-dependent (control, 1824; 10 ng PGF2.alpha./ml, 2116; 20 ng PGF2.alpha./ml, 2332 and 40 ng PGF2.alpha./ml, 2436; common estimate of SE = 66). Oxytocin concentrations of luteal slices after incubation did not differ between treatment groups. These data substantiate previous reports that OT is synthesized by the bovine corpus luteum. Moreover, the increased luteal concentration of OT during the early part of the estrous cycle can be released in vitro by the action of PGF2.alpha.