Investigations into the effect of two sedatives on the stress response in cattle.

The effects of the sedatives acepromazine (an alpha-adrenergic antagonist) and xylazine (an alpha 2-adrenergic agonist) on plasma indicators of stress in cows were assessed after intramuscular injection and transport. After blood samples had been taken for baseline values, nine cows were given an intramuscular injection of saline (2.5 ml), acepromazine (0.05 mg/kg in 2.5 ml) or xylazine (0.05 mg/kg in 2.5 ml) on different occasions at least 1 week apart. The animals were then transported for 5 min by truck to a different environment and blood sampled for a further 1-3 h. There was a significant increase in plasma cortisol concentration (3.29 +/- 1.59 x baseline) after the injection of saline and transport. The injection of acepromazine also resulted in a significant increase in cortisol concentration (2.84 +/- 0.84 x baseline). There was no similar increase after injection of xylazine. This suggests that alpha 2-adrenergic receptors are involved in the response of plasma cortisol concentrations to stressors. An hyperglycaemic response occurred after xylazine (1.66 +/- 0.49 x baseline) and saline (1.20 +/- 0.1 x baseline) but not after acepromazine. Both sedatives produced a metabolic alkalosis (1.13 +/- 0.01 x baseline pH after xylazine and 1.034 +/- 0.02 x baseline pH after acepromazine). A greater decrease in haematocrit was seen after both sedatives (0.88 +/- 0.04 x baseline after xylazine, 0.81 +/- 0.08 x baseline after acepromazine) than after the injection of saline (0.97 +/- 0.06 x baseline).