Effect of prazosin on the function of the teat sphincter in lactating cows

Abstract

Summary Oxytocin was infused for 60 min into the jugular vein of 8 lactating cows with full udders before milking. Under these circumstances milk loss occurred spontaneously in 3 cows and was evoked in 5 cows by reducing pressure around one teat (–30 cm water) using a plethysmographic chamber. Milk loss was measured continuously. The postsynaptic α-blocking agent prazosin (PRZS) injected either into the udder artery (2 or 5 mg) or into the jugular vein (200 mg) consistently induced a significant increase in milk leakage. It is suggested that PRZS effectively inhibits α-adrenoceptors in the teat sphincter muscles and thus promotes the action of β-adrenoceptors, in this way inducing relaxation of the teat sphincter.