α2-Adrenoceptors exist postsynaptically to subserve vasoconstriction and presynaptically to modulate norepinephrine release into the synaptic cleft. Because adrenoceptors may downregulate in response to chronic stimulation, we investigated the activity of α2-receptor-mediated vasoconstriction in patients with congestive heart failure, who had increased levels of plasma norepinephrine. We used the isolated forearm model and intra-arterial infusion of subsystemic doses of yohimbine, a specific α2-blocker, in 11 patients with heart failure and in 15 normal subjects. Yohimbine produced a dose-related increase in forearm blood flow and decrease in forearm vascular resistance. These findings were consistent with a direct vasodilator effect mediated by blockade of the postsynaptic α2-vascular receptor. Furthermore, the vasodilator responses in patients with heart failure were similar to the normal subjects in terms of the percent increase in forearm blood flow, the dose-response relation, and the fractional response to hyperemia and phentolamine; thus, α2-receptor-mediated vasoconstriction is neither enhanced nor down-regulated in heart failure. In addition, in patients with heart failure and in normal subjects, yohimbine produced an increase in the forearm venous norepinephrine concentration, consistent with an inhibition of the presynaptic α2-receptor resulting in an augmented release of norepinephrine into the synaptic cleft. Thus, these data suggest that the postsynaptic α2-receptor is an important mediator of vasoconstriction in patients with heart failure. Despite chronic elevations in plasma norepinephrine in patients with heart failure, α2-receptor mechanisms subserving vasoconstriction and inhibition of norepinephrine release into the synaptic cleft are still functional in heart failure.
- nervous system, sympathetic
- receptors, α-adrenergic