1 After ganglionic blockade and bilateral vagotomy, vasopressor responses induced by activation of postsynaptic alpha 1- and alpha 2-adrenoreceptors were elicited in the intact circulatory system of rabbits. 2 The hypertensive effects of the selective stimulating agents methoxamine (alpha 1-agonist) and B-HT 920 (alpha 2-agonist) were effectively antagonized by the adrenoreceptor antagonists prazosin and yohimbine, respectively. These findings confirm the existence of two types of postsynaptic alpha-adrenoreceptors (alpha 1- and alpha 2-type) in vascular smooth muscle of rabbits. 3 The calcium antagonistic drug nifedipine did not affect the maximal increase in diastolic pressure brought about by methoxamine, whereas it strongly inhibited the hypertensive effects of B-HT 920. 4 It is concluded that this confirmation of the selective inhibition of postjunctional alpha 2-adrenoreceptor-mediated vasopressor responses by a calcium antagonistic drug, such as nifedipine, indicates that this activity constitutes a general phenomenon. This finding supports the hypothesis that an influx of extracellular calcium is necessary for the vasoconstriction mediated by postsynaptic alpha 2-adrenoreceptors.
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