The role of presynaptic alpha-adrenoceptors in the regulation of blood pressure in the conscious rabbit.

1 Changes in mean arterial pressure, heart rate and plasma noradrenaline after alpha-adrenoceptor blockade with several alpha-adrenoceptor antagonists have been studied in the conscious rabbit in order to investigate the possible role of presynaptic alpha-adrenoceptors in cardiovascular regulation. 2 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 alpha-adrenoceptor blockade determined by the pressor response to intravenous phenylephrine. 3 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 microgram kg-1 min-1). 4 After baroreceptor deafferentation by sinoaortic denervation the falls in mean arterial pressure were much greater and the rise in plasma noradrenaline was markedly attenuated. 5 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. 6 The magnitude of the baroreflex response to changes in pressure make it unlikely that the functional significance of the presynaptic alpha-adrenoceptor can be readily determined by measurement of plasma noradrenaline in intact animals.