Summary. Plasma concentrations of β-endorphin and met-enkephalin were measured, with appropriate radioimmunoassays, in cows during gestation and at parturition and in newborn calves. During pregnancy β-endorphin immunoreactivity (IR) concentration increased, but values during the last month of gestation were not different from those at parturition. Highest met-enkephalin IR levels were obtained in cows during calving. A term Caesarean section caused an increase in plasma β-endorphin and metenkephalin IR concentrations, but no such increase occurred in cases of a preterm Caesarean section. In calves β-endorphin IR values were lower before umbilical cord rupture than immediately after birth. Values decreased continuously thereafter. This was also the case for met-enkephalin IR concentrations in calves born at term. In preterm calves met-enkephalin IR values were low immediately after delivery and increased during the first hour of life. A significant correlation existed between the degree of acidosis and plasma levels of both opioid peptides in the calves. We conclude that a direct stimulation of peripheral β-endorphin release by the pain or stress associated with calving does not seem to exist in cattle, whereas met-enkephalin seems to be more directly related to parturition. In calves the change to the extrauterine environment causes an immediate, increased release of both opioids.
Keywords: β-endorphin; met-enkephalin; cattle; parturition; neonate