Analysis of the α‐adrenoceptor‐mediated, and other, components in the sympathetic vasopressor responses of the pithed rat

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Abstract
1 The vascular receptors activated following sympatho-adrenal stimulation were determined by analysing the effects of ‘selective’ antagonists on the vasopressor response to spinal sympathetic nerve activation in the pithed rat. 2 The net vascular response to adrenal stimulation was a balance between α-adrenoceptor-mediated vasoconstriction and β-adrenoceptor-mediated vasodepression. Part of the α-adrenoceptor-mediated response was ‘prazosin-sensitive’ (α1) and the remainder was abolished by rauwolscine (α2). 3 As with adrenal stimulation, direct sympathetic nerve stimulation of the vasculature evoked pressor responses which were partly resistant to prazosin. Rauwolscine only partly blocked the prazosin-sensitive component. Reserpine pretreatment led to smaller responses than prazosin plus rauwolscine. Thus, the response resistant to α-adrenoceptor antagonists could be mediated, in part, by adrenoceptors distinct from α-adrenoceptors, as currently defined. 4 α, β-Methylene ATP reduced the nerve-mediated pressor response after α-adrenoceptor blockade or reserpine pretreatment but not in drug-free controls. 5 The results suggest that stimulation of the adrenal medulla can produce a vasopressor response which consists of summating α1 and α2-adrenoceptor-mediated components, and is identical to the effect of injected adrenaline. In contrast, the response to vasopressor nerve stimulation appears to be essentially mediated by α1-adrenoceptors, with a facilitatory influence from α2-adrenoceptors. A further response obtained after α-adrenoceptor blockade may contain a purinergic component and another which is adrenergic but not mediated by stimulation of α-adrenoceptors.