The effects of intravenous 6-hydroxydopamine on peripheral alpha-adrenoreceptors.

1 Peripheral alpha-adrenoreceptor-mediated responses were studied in conscious intact rabbits and in animals pretreated with intravenous 6-hydroxydopamine (50 mg/kg). 2 Treatment caused a large decrease in noradrenaline in plasma and peripheral tissue but only small falls in blood pressure. 3. There was a shift to the left in pressor dose-response curves to the alpha 1-adrenoreceptor agonist phenylephrine and the mixed alpha 1/alpha 2-adrenoreceptor agonist noradrenaline in 6-hydroxydopamine-pretreated animals. The alpha 1-adrenoreceptor antagonist prazosin was less effective at blocking the phenylephrine pressor response in these animals. 4 There was no change in pressor responses to the alpha 2-adrenoreceptor selective agonists clonidine and guanabenz and alpha-adrenoreceptor antagonists were equally effective at blocking this pressor response in 6-hydroxydopamine-pretreated and in intact rabbits. 5 In conscious animals 6-hydroxydopamine pretreatment causes alteration in responses mediated by postsynaptic alpha 1-but not postsynaptic alpha 2-adrenoreceptors. These changes may help maintain blood pressure in the 6-hydroxydopamine treated rabbits.