Evidence for medetomidine as a selective and potent agonist at alpha 2-adrenoreceptors.

The activity on alpha-adrenoreceptors of medetomidine ((+/-)-4-(alpha,2,3-trimethylbenzyl)imidazole), an alpha-methyl derivative of detomidine, has been characterized in vivo and in vitro using detomidine, MPV 207, MPV 295, azepexole, clonidine and xylazine for reference purposes. Medetomidine (1-100 micrograms/kg i.v.) was a hypotensive and bradycardic compound in anaesthetized rats. Furthermore, it induced vasopressor (PD50 1.7 microgram/kg) and sympatho-inhibitory (ID50 1.6 microgram/kg) actions in pithed rats, the effects being antagonized by idazoxan (0.3 mg/kg i.v.) but not by prazosin (0.1 mg/kg i.v.). Medetomidine (30-300 micrograms/kg i.m.) had an alpha 2-adrenoreceptor mediated sedative effect on chicks. Medetomidine was, overall, more potent than detomidine, MPV 207, clonidine, xylazine, MPV 295 or azepexole in central (sedation in the chick) and peripheral (cardiac presynaptic in the pithed rat) actions on alpha 2-adrenoreceptors. Clonidine had, however, about an equal potency to medetomidine in the vascular smooth muscle of the pithed rat. Like detomidine and MPV 295, medetomidine had no agonistic activity in the rat aortic ring, but high concentrations antagonized methoxamine-induced contractions, giving a pA2 value of 5.68 for alpha 1-adrenoreceptor antagonism. The overall lipophilicity (log P') of medetomidine in the octanol/buffer (pH 7.4, 24-26 degrees C, HPLC technique) was 2.80. In summary, the experimental data suggest that medetomidine is a lipophilic compound with selective alpha 2-adrenoreceptor-stimulating properties and high potency. It may, therefore, prove to be a suitable pharmacologic tool for interventions in alpha 2-adrenoreceptor mediated effects in the autonomic nervous system.