β-Endorphin in sows during late pregnancy: effects of cloprostenol and oxytocin on plasma concentrations of β-endorphin in the jugular and uterine veins

Abstract

ABSTRACT β-Endorphin was measured in the plasma of pigs during late pregnancy and at different stages of the oestrous cycle. In pregnant animals, β-endorphin secretion from uteroplacental tissues into the maternal circulation and the possible effects of oxytocin and the prostaglandin F2α (PGF2α) analogue cloprostenol on β-endorphin release were determined. Plasma β-endorphin concentrations in pregnant sows were significantly higher than in non-pregnant pigs. However, there were no significant changes in β-endorphin values throughout the oestrous cycle. Because the increase in plasma β-endorphin concentrations had occurred before luteolysis and onset of labour it could not be attributed to the stress of parturition. The surgical intervention of a laparotomy increased β-endorphin release into peripheral plasma. Cloprostenol but not oxytocin caused an immediate increase in plasma β-endorphin concentrations. At parturition, endogenous PGF2α may be involved in the regulation of β-endorphin secretion. Concentrations of β-endorphin in the jugular and uterine vein plasma were not significantly different, and so it would appear that β-endorphin in the plasma of pregnant sows is not of uteroplacental origin. In conclusion, changes in the concentration of β-endorphin in peripheral plasma, associated with pregnancy but not the oestrous cycle, exist in pigs. Hence a physiological function of peripheral opioid peptides in the periparturient sow is feasible. Journal of Endocrinology (1993) 136, 199–206