Pharmacotherapeutic group: hypnotics and sedatives xylazine.
ATCvet code: QN05CM92
Pharmacodynamic properties
Xylazine belongs to the α2-adrenoceptor agonists.
Xylazine is a α2-adrenoceptor agonist, that acts by stimulation of central and peripheral α2-adrenoceptors. Through its central stimulation of α2-adrenoceptors, xylazine has potent antinociceptive activity. In addition α2-adrenergic activity, xylazine has α1-adrenergic effects.
Xylazine also produces skeletal muscle relaxation by inhibition of intraneuronal transmission of impulses at the central level of the central nervous system. The analgesic and skeletal muscle relaxation properties of xylazine show considerable interspecies variations. Sufficient analgesia generally will be attained in combination with other products only.
In many species, administration of xylazine produces a short-lived arterial pressor effect followed by a longer period of hypotension and bradycardia. These contrasting actions upon the arterial pressure apparently are related to the α2- and α1-adrenergic actions of xylazine.
Xylazine has several endocrine effects. Insulin (mediated by α2-receptors in pancreatic β-cells which inhibit insulin release), ADH (decreased production of ADH, causing polyuria) and FSH (decreased) are reported to be influenced by xylazine.
Pharmacokinetic properties
Absorption (and action) is rapid following intramuscular injection. Levels of drug peak rapidly (usually within 15 minutes) and then decline exponentially. Xylazine is a highly lipid soluble organic base and diffuses extensively and rapidly (Vd 1.9-2.7). Within minutes after an intravenous injection, it can be found in a high concentration in the kidneys, the liver, the CNS, the hypophyses, and the diaphragm. So there is a very rapid transfer from the blood vessels to the tissues. Intramuscular bioavailability is incomplete and variable ranging from 52-90% in the dog to 40-48% in the horse. Xylazine is metabolised extensively and eliminated rapidly (±70% via the urine, while the enteric elimination is ±30%). The rapid elimination of xylazine is probably related to an extensive metabolism rather than to a rapid renal excretion of unchanged xylazine.