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

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.