INHIBITION OF MILK EJECTION IN COWS BY OXYTOCIN RECEPTOR BLOCKADE, ALPHA-ADRENERGIC RECEPTOR STIMULATION AND IN UNFAMILIAR SURROUNDINGS

Inhibition of milk ejection in cows by oxytocin receptor blockade (Ato siban) and alpha-adrenergic receptor stimulation (phenylephrine) prior to prestimulation was compared with inhibition of milk ejection in un familiar surroundings. In addition, Atosiban and phenylephrine mere ad ministered after a 1 min prestimulation or 1 min after the start of mi lking. Oxytocin concentrations increased during milking in all treatme nts. The spontaneously removed milk fraction (before oxytocin was inje cted) was similar for Atosiban and phenylephrine treatments and in unf amiliar surroundings, but lower than in controls. Peak flow rates were similar in all treatments, but reduced as compared with controls when phenylephrine and Atosiban were administered before prestimulation. P eripheral (Atosiban, phenylephrine) and central (unfamiliar surroundin gs) inhibition of milk ejection reduced the amount of available milk s imilarly. Drug treatments resulted in similar peak flow rates; however , teats were contracted after phenylephrine administration but not aft er Atosiban. The inhibition induced by Atosiban could be abolished by oxytocin injection, but not that induced by phenylephrine, which was a ntagonized by alpha-adrenergic receptor blockade. These results indica te that inhibition of milk ejection through activation of alpha-adrene rgic receptors is based on blockade of milk flow into the cistern, but not through the teats.