Sympathetic vasoconstriction sensitive to alpha 2-adrenergic receptor blockade. No evidence for preferential innervation of alpha 1-adrenergic receptors in the canine femoral bed.

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Abstract
In the canine femoral bed, we studied the involvement of vascular alpha 2-adrenergic receptors in vasoconstriction by stimulating the sympathetic nerve during different degrees of activation of metabolic counterregulation (constant pressure and constant flow perfusion). In chloralose-anesthetized, despinalized dogs under beta-blockade (2 mg/kg nadolol) and under a constant femoral perfusion pressure, cumulative doses of rauwolscine (0.03, 0.3, and 3.0 mg/kg i.v., n = 8) or of prazosin (0.012, 0.12, and 1.2 mg/kg i.v., n = 8) caused dose-dependent shifts to the right of the dose-response curve of intraarterial norepinephrine (NE) infusions. These cumulative doses also caused attenuations of the vasoconstriction initiated by lumbar sympathetic stimulation (0.1, 0.3, 1.0, and 3.0 Hz). Sham treatment (n = 8) had no such results. In experiments with constant flow perfusion (n = 6 for each antagonist), rauwolscine shifted the NE dose-response curve significantly more compared to the experiments with constant pressure perfusion, while prazosin was similarly effective under both conditions. Under constant flow perfusion, both antagonists dose-dependently attenuated the vasoconstrictions caused by sympathetic stimulation. While prazosin and sham treatment did not modify the overflow of NE from the femoral bed during stimulation, this overflow was augmented by rauwolscine during stimulation at 3 Hz, which indicated presynaptic modulation of NE release. Under both experimental conditions, no significant difference could be observed in the attenuation of low-frequency sympathetic vasoconstriction by the two antagonists, when dosages were compared on the basis of their action against infused NE. It is concluded that both a rauwolscine-sensitive component and a prazosin-sensitive component are involved in the competition between sympathetic vasoconstriction and metabolic dilation. The vascular alpha-adrenergic receptors activated by these two components have a similar postsynaptic localization relative to the nerve endings.