Evaluation of presynaptic alpha-receptor function in the canine renal vascular bed

The functional significance of presynaptic alpha-receptor modulation of sympathetic nerves was examined in vivo in the canine renal vascular bed. In pentobarbital-anesthetized dogs, the vasoconstrictor response to renal nerve stimulation and exogenous norepinephrine was compared before and during intra-arterial infusions of epinephrine, oxymetazoline, clonidine, and norepinephrine. Only epinephrine produced a modest decrease in stimulation-induced vasoconstriction at 1 Hz. After pretreatment with desipramine, intra-arterial infusions of epinephrine or norepinephrine did not alter stimulation-induced vasoconstrictor responses relative to exogenous norepinephrine. Further, neither yohimbine nor phentolamine (10(-9) to 10(-3) g, intra-arterial) produced a distinctly increased vasoconstrictor response to nerve stimulation relative to exogenous norepinephrine. Thus, studies using alpha-receptor agonists, antagonists, and inhibition of neuronal uptake failed to reveal a physiologically significant alpha-receptor-mediated negative feedback mechanism for stimulation-induced vasoconstriction in the canine renal vascular bed.