Mechanism of noradrenaline influence on the secretion of ovarian oxytocin and progesterone in conscious cattle

Abstract

Noradrenaline stimulates the concomitant release of ovarian oxytocin and progesterone in cattle within a few minutes, but the mechanism of its action is unknown. Changes in α- and β-receptors and blood pressure were considered as possible mechanisms of the noradrenaline effect. Heifers in group 1 (n = 4) were infused with noradrenaline (0.16 μg kg-1 min-1) for 30 min into the aorta abdominalis (cranial to the origin of the ovarian artery) on day 10. On days 11 and 12 before noradrenaline, phentolamine (α-blocker; 30 μg kg-1 min-1) or propranolol (β-blocker; 5 μg kg-1 min-1) were infused for 30 min. Four other heifers were infused with noradrenaline only as controls. Only propranolol inhibited the stimulatory effect of noradrenaline on the secretion of progesterone and oxytocin. In group 2, heifers (n = 4) were infused, making use of the latin square design, with vasoconstrictive (angiotensin; 0.042 μg kg-1 min-1) or vasodilatory (xanthinol-theophyline nicotinate; 250 μg kg-1 min-1) drugs that do not act through the adrenoceptors. Noradrenaline (0.3 μg kg-1 min-1) was given 1 h later as in group 1. Blood pressure changes were measured in the posterior aorta abdominalis and oxytocin and progesterone concentrations were determined in the blood samples collected from the jugular vein. Noradrenaline and angiotensin increased (P < 0.01), whereas xanthinol decreased (P < 0.01), blood pressure during their infusion. However, the rise of oxytocin and progesterone concentrations was observed only after noradrenaline infusion. We suggest that (i) noradrenaline enhances the secretion of progesterone and oxytocin from the corpus luteum acting through β-receptors; and (ii) increase of vascular blood pressure which does not occur concomitantly with β-receptor stimulation does not appear to be involved in the corpus luteum secretion.