Effect of norepinephrine on the release of progesterone and ovarian oxytocin in cattle

Abstract

Heifers had catheters inserted from the aorta abdominalis cranial to the origin of the ovarian artery on Day 10 of the oestrous cycle (in which oestrus is Day 0). On Day 11, four experiments were performed. In Experiment 1, norepinephrine (NE) was infused (0.3 μg kg −1 body weight min −1) for 30 min. This produced a rapid rise ( P < 0.01) in peripheral plasma oxytocin from the basal concentration (3–6 pg ml −1) to more than 100 pg ml −1 in 5 min. Oxytocin levels fell to the pretreatment values within 60 min of the end of infusion. Progesterone concentrations increased 3–5 ng ml −1 ( P < 0.01) above the pre-infusion level during drug infusion. The increase in levels of both hormones was either concomitant, or the oxytocin rise preceded that of progesterone. The changes in concentration and amounts of secreted oxytocin were similar ( P > 0.05) to those seen after injecting a luteolytic dose of an analogue of prostaglandin PGF 2α (Experiment 3). In Experiment 2, PGF 2α was given i.m. at the start of NE infusion. This did not alter the profile observed for oxytocin seen in Experiment 1 but the stimulatory effect of NE on progesterone secretion was reduced within 15 min. In Experiment 4, an i.m. injection of PGF 2α was given 1 h after the NE infusion. Each treatment produced a similar increase in oxytocin release, comparable with increases observed in Experiments 1 and 2 ( P > 0.05) respectively. In conclusion, NE stimulates the release of both ovarian oxytocin and progesterone, indicating some relationship between these hormones in the mid-luteal phase of the oestrous cycle in cattle. NE stimulated a release of oxytocin which was comparable with that produced by injected PGF 2α and did not deplete the corpus luteum (CL) of oxytocin. On this basis, it seems that NE can affect the components of the post-translational processing pathway of oxytocin synthesis in the bovine CL.