Epinephrine inhibiting milk ejection in lactating cows.

Blood oxytocin was determined on 10 Holstein cows during normal milking and during milking when epinephrine was injected intravenously before or after udder stimulation. The average peak oxytocin concentration during normal milking was 399.7 muU/ml plasma and was reached at 1 min after teat cup application. Oxytocin concentration in blood plasma declined rapidly after peak concentration and dropped to 30.5 muU/ml plasma within 4 min after the start of milking. By 5 min after removal of the teat cups, it had declined to less than 4 muU/ml plasma. The administration of epinephrine, either before or after udder washing, inhibited milk ejection as indicated by milk production and oxytocin concentrations. Peak oxytocin concentrations were 1.05 muU/ml plasma 1 min after teat cup application when epinephrine was injected before udder washing and 8.6 muU/ml plasma at teat cup application when epinephrine was injected after udder washing. These results and the use of a beta receptor blocker to inhibit the effect of epinephrine at the myoepithelial cell level indicated that epinephrine inhibited release of oxytocin from the neurohypophysis.