Small increases in peripheral noradrenaline inhibit the milk-ejection response by means of a peripheral mechanism

Abstract

The effects of exogenous noradrenaline on the milk-ejection response were determined for nine Holstein cows. Noradrenaline was injected (0·95 nmol/kg) 15 s after the start of teat stimulation (preparation) or infused (0·13 nmol/kg per min, after bolus injection of 0·47 nmol/kg) starting 10 min before milking for 20 min. Cows were prepared (udder wash and dry) for 1 min before milking. Both injection and infusion resulted in approximately a 3·5-fold increase in peripheral noradrenaline at 1·75 min after the start of milking (baseline noradrenaline 0·83 and 0·89 nmol/l plasma; at 4 min, 2·00 and 3·00 nmol/l). Prolactin release was delayed and oxytocin release enhanced, while milk yield was decreased by 8·6% for both treatments. The maximum rate of milk flow was also depressed by treatment. In contrast, milking time increased for injection and decreased for infusion. In addition, a milk-yield-dependent change in the pattern of milk flow was seen in response to treatment. In medium-yield animals, two distinct milk-flow peaks were apparent and injection delayed the time to the second peak. We conclude that physiologically meaningful increases in peripheral noradrenaline can inhibit milk-ejection response by means of a peripheral mechanism not involving inhibition of release of oxytocin. J. Endocr. (1984) 100, 337–344