THE ROLE OF PRESYNAPTIC α‐ADRENOCEPTORS IN THE REGULATION OF BLOOD PRESSURE IN THE CONSCIOUS RABBIT

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Abstract
Changes in mean arterial pressure, heart rate and plasma noradrenaline after α‐adrenoceptor blockade with several α‐adrenoceptor antagonists have been studied in the conscious rabbit in order to investigate the possible role of presynaptic α‐adrenoceptors in cardiovascular regulation. Prazosin (0.05–2 mg/kg) and phentolamine (0.5–20 mg/kg) produced dose‐dependent falls in mean arterial pressure and rises in plasma noradrenaline. These changes were related to the degree of postsynaptic α‐adrenoceptor blockade determined by the pressor response to intravenous phenylephrine. Similar changes in mean arterial pressure and plasma noradrenaline were observed after administration of the direct vasodilators hydralazine (1–10 mg/kg) and nitroprusside (2.5–55 μg kg−1 min−1). After baroreceptor deafferentation by sinoaortic denervation the falls in mean arterial pressure were much greater and the rise in plasma noradrenaline was markedly attenuated. Yohimbine (1 mg/kg) increased mean arterial pressure and plasma noradrenaline but it was not possible to exclude the possibility that central nervous effects of yohimbine underlay the increased sympathetic activity. The magnitude of the baroreflex response to changes in pressure make it unlikely that the functional significance of the presynaptic α‐adrenoceptor can be readily determined by measurement of plasma noradrenaline in intact animals.