[Old and new alpha 2-adrenoceptor agonists. 1. Xylazine and medetomidine]

The "discovery" of alpha 2-receptors permits the explanation of the mechanism of xylazine that was hitherto poorly understood. The substance can be classified as an alpha 2-adrenoceptor agonist. In the near future two new alpha 2-adrenoceptor agonists will be available in Germany: medetomidine for small animals and detomidine for large animals. Also a new alpha 2-adrenoceptor antagonist will be introduced: atipamezole. alpha 2-adrenoceptor agonists cause sedation, analgesia and muscle relaxation. Notable side effects are bradycardia, heart block and a dose dependent respiratory depression. A comprehensive survey on the pharmacology of the alpha 2-adrenoceptor agonists is given. The previous literature is summarized and discussed, including own studies and clinical experience. An attempt is made to assess the clinical importance of the new drugs. For certain indications medetomidine alone may not be satisfying and combination with other drugs becomes necessary. If sedation is inadequate, the combination with benzodiazepines or propofol is advised; if analgesia is poor, the combination with ketamine or an opioid is recommended.