The effect of α₂-adrenoceptor agonists and antagonist on oxytocin release and milk removal in cows

The effect of α₂-adrenoceptor agonist xylazine and detomidine and α₂-adrenoceptor antagonist yohimbine on oxytocin (OT) release and milk removal were studied in cows. Experiment 1, 6 Holstein cows were used. After the control milking the different doses of xylazine (4 mg, 8 mg and 16 mg) were intravenously injected before morning milking (-5 min) in a cross-design manner. Xylazine showed a dose dependent decrease of OT release. Experiment 2, during 3 consecutive days 5 Brown Swiss cows were intravenously pretreated before milking (-5 min) with saline; 24 mg xylazine; 200 mg yohimbine followed by 24 mg xylazine (-2 min); 200 mg yohimbine; detomidine 12 mg. Xylazine caused the total inhibition of OT release resulting in milk flow cessation. Yohimbine partially restored OT release and milk flow in 3 of 5 xylazine treated cows. In 4 of 5 cows the detomidine significantly increased the release of OT during milking, but disturbed or inhibited milk flow. In detomidine treated cows OT (0.1 i.u. i.v.) did not restore milk flow (only 12.9±4.5% of total milk yield was released). In conclusion, two different effects were found for the two α₂-adrenoceptor agonists xylazine and detomidine on OT release and milk removal.