Pharmacological characteristics of spinal alpha-adrenoreceptors in rats

1 Spinal alpha-adrenoreceptors involved in cardiovascular control have been investigated using selective alpha-adrenoreceptor agonists and antagonists in urethane-anaesthetized rats. 2 Intrathecal injections of clonidine, alpha-methylnoradrenaline, guanfacine and M7 at the C7-T1 level reduced blood pressure and heart rate. In contrast, phenylephrine, 5-hydroxytryptamine and procaine had little or no effect. These results suggest the involvement of spinal alpha 2-adrenoreceptors. 3 The fall in blood pressure produced by clonidine appeared to be attributable to a reduction in heart rate and stroke volume. Lower body vascular resistance was unchanged. 4 The clonidine-induced bradycardia was antagonised by prazosin, WB4101, piperoxan or yohimbine. Their relative potencies suggest that alpha 1-rather then alpha 2-adrenoreceptors mediate this response. 5 piperoxan and yohimbine clearly prevented the clonidine-induced fall in blood pressure; prazosin and WB4101 also appeared to antagonise clonidine but these results were complicated by the fact that these antagonists themselves reduced blood pressure. 6 It was difficult to interpret these results simply in terms of alpha 1- or alpha 2-adrenoreceptors. Thus spinal alpha-adrenoreceptors may be different from peripheral alpha 1- or alpha 2-adrenoreceptors.