The alpha adrenoceptor antagonist properties of idazoxan in normal subjects

The imidazoline derivative idazoxan, which has been shown to be a potent, selective alpha 2-adrenoceptor antagonist, was injected intravenously to eight men with normotension. There was a transient small increase in blood pressure and a decrease in heart rate within 20 min of injection, with a slight increase in plasma norepinephrine levels. These effects are consistent with antagonism of prejunctional alpha 2-adrenoceptors. In response to infusions of the relatively selective alpha 2-adrenoceptor agonist alpha-methylnorepinephrine, the pressor dose-response curve shifted to the right with idazoxan. These data provide evidence for receptors with alpha 2-adrenoceptor characteristics on resistance vessels in man. In vitro platelet aggregation studies provide further evidence of selective alpha 2-adrenoceptor antagonism by idazoxan, with greater potency and affinity than alpha-yohimbine. These observations are consistent with both pre- and postjunctional peripheral alpha 2-adrenoceptors in man and provide further support that idazoxan is a selective alpha 2-adrenoceptor antagonist.