Halothane‐sparing effect of xylazine in dogs and subsequent reversal with tolazoline

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Abstract
Halothane MAC (the minimum alveolar concentration of halothane to produce anesthesia in 50% of the animals tested) was 0.92 .+-. 0.16 vol % in 8 English Pointer dogs. Alterations in halothane MAC induced by an i.v. bolus of xylazine (1.1 mg/kg) and then tolazoline (5 mg/kg) was determined in each dog following control (halothane MAC) measurement. Following xylazine administration, MAC significantly decreased to 0.57 .+-. 0.023%. Immediately following determination of the xylazine-halothane MAC value in each dog, tolazoline was administered and the halothane requirement (MAC) was again assessed. Halothane MAC significantly increased to 1.24 .+-. 0.036%. Tolazoline administration induced immediate arousal in the xylazine-halothane anesthetized dogs requiring a rapid increase in halothane concentration to maintain anesthesia. The administration of tolazoline, an .alpha. adrenergic antagonist, following xylazine administration significantly increased the anesthetic requirement (MAC) of halothane. Xylazine, an .alpha. 2 adrenergic agonist, decreased halothane anesthetic requirement (MAC) in the 8 dogs studied. Apparently, stimulation of central .alpha. 2 receptors is the mechanism by which xylazine produces sedation and inhibition of CNS excitatory neurotransmitter release decreases halothane anesthetic requirement. The increase in halothane requirement and arousal from xylazine-halothane anesthesia that occurred following i.v. tolazoline administration indicates an increase in CNS excitatory neurotransmitter activity.