Effects of alpha- and beta-adrenergic agonists on intramammary pressure and milk flow in dairy cows

alpha-Adrenergic mimetics inhibit milk removal whereas beta-adrenergic mimetics have a favourable effect upon milk removal in dairy cows. This work aimed to relate these effects to the milk ejection reflex mechanism and/or milk flow from the cisternal cavities only. The alpha-adrenergic agonist phenylephrine and the beta-adrenergic agonist isoproterenol were injected prior to and after milk ejection. Applied after milk ejection, the phenylephrine treatment reduced intramammary pressure (IMP) and milk yield markedly. Phenylephrine application before milk ejection prevented IMP from rising during teat stimulation or after oxytocin injection. Milking produced small amounts of milk at minimal milk flow rates. The very different responses to phenylephrine application prior to and after milk ejection indicate separate adrenergic inhibitory mechanisms for milk ejection and milk flow. The application of the beta-adrenergic agonist isoproterenol did not alter the IMP. Nevertheless, this compound improved milk removal by increasing milk flow. The observation of bimodal milk flow at milkings without preceding teat stimulation indicates a delayed milk ejection. Isoproterenol injection without teat stimulation enhanced milk flow rates, but the bimodality remained unchanged. We conclude that isoproterenol enhances milk flow without affecting milk ejection.