The alpha 2-adrenoceptor agonists clonidine, TL99 and DPI enhance vasoconstrictor responses to sympathetic nerve stimulation and noradrenaline in the rat tail artery preparation.

1. Clonidine (1-30 nmol/l) enhanced the vasoconstrictor responses to sympathetic nerve stimulation in isolated perfused artery preparations from the rat tail. Higher concentrations of clonidine produced vasoconstriction in most artery preparations and reduced the response to sympathetic nerve stimulation. 2. The enhancing effect of clonidine was not affected by prazosin but was blocked by idazoxan, indicating that it was due to activation of alpha 2-adrenoceptors. 3. The alpha 2-adrenoceptor agonists DPI and TL99, like clonidine, enhanced vasoconstrictor responses to sympathetic nerve stimulation. 4. Vasoconstrictor responses to noradrenaline were enhanced by clonidine and TL99. 5. The findings suggest that activation of postjunctional alpha 2-adrenoceptors results in facilitation of the contractile response elicited by activation of alpha 1-adrenoceptors.