The actions of medetomidine may not be mediated exclusively by α2-adrenoceptors in the equine saphenous vein

Spirals of endothelially denuded equine saphenous vein were used to study the pre- and post-junctional effects of medetomidine in vitro . The pD₂ values were calculated for noradrenaline (6.7 /pm 0.1), phenylephrine (5.6 /pm 0.1), BHT 920 (6.2 /pm 0.2) and UK 14304 (5.7 /pm 0.2). Medetomidine produced a biphasic response, with a pD₂1 of 8.2 /pm 0.1 and a pD₂2 of 5.7 /pm 0.1 in the equine saphenous vein ( n = 6 ). Prazosin (10^?7 m) significantly shifted the second phase of the medetomidine concentration-response curve to the right (pD₂1 was 8.1 /pm 0.2 and pD₂2 was 5.0 /pm 0.2, P < 0.05). Rings of equine saphenous vein were electrically stimulated to investigate the pre-junctional effects of medetomidine. Increasing concentrations of the α₂-adrenoceptor agonist BHT 920 reduced the response to electrical stimulation in a concentration dependent manner to a maximum of 40 /pm 5%. whereas medetomidine (0.1-100 nm) caused a concentration dependent enhancement to a maximum of 490 /pm 150%. These results suggest α₁- and α₂-adrenoceptors are functional in the equine saphenous vein, but that medetomidine is not acting exclusively as an α₂-adrenoceptor agonist.