Comparison of the milk leakage potencies of adrenergic agonists in lactating cows.

The activities of epinephrine (2-6 micrograms) and of norepinephrine (20-60 micrograms) on the relaxation of teat sphincter muscles were compared by measuring milk leakage from the full udder of 5 lactating cows. Substances were injected into the external pudic artery after pretreatment with an alpha-adrenoceptor blocking agent (prazosin or phentolamine). Milk leakage was induced by creating a vacuum (-30 cm of water) in a plethysmographic chamber enclosing one teat and volumes of milk loss were measured every min. In a second series of experiments the activities of the beta 2-adrenoceptor selective agonist zinterol (0.5-2.5 micrograms) and of the nonselective beta-agonist isoproterenol (3-22.5 micrograms) on sphincter muscle relaxation were compared. Epinephrine was found to be more potent than norepinephrine in inducing relaxation of sphincter muscles. Zinterol was more active than isoproterenol. It is concluded that there is a predominance of beta 2-adrenoceptors in the teat sphincters.