Alpha-adrenergic and serotonergic receptors and forearm venous compliance in normal human subjects

The effect of alpha-adrenergic and serotonergic receptor agonists and antagonists on forearm venous capacitance and vascular resistance was studied in normal subjects. Venous volumes were significantly decreased by phenylephrine, an alpha 1-adrenoceptor agonist, and norepinephrine, which stimulates both alpha 1- and alpha 2-adrenoceptors, but not by the alpha 2-adrenoceptor agonist clonidine. Vascular resistance was increased significantly by all three agonists. Both the alpha 2-adrenoceptor antagonist yohimbine and the alpha 1-adrenoceptor antagonist prazosin attenuated the venoconstrictor and vasoconstrictor response to norepinephrine, indicating that both adrenoceptors are present. 5-Hydroxytryptamine significantly decreased venous volumes, while vascular resistance was not affected. Ketanserin, a 5-HT2 and alpha 1-adrenoceptor antagonist, attenuated the venous volume responses to 5-hydroxytryptamine and phenylephrine. It is concluded that there are alpha 1- and alpha 2-adrenoceptors and 5-HT2 receptors in the forearm venous system. However, vasoactive doses of clonidine are not venoconstricting. Forearm veins are sensitive to doses of 5-hydroxytryptamine, which do not change vascular resistance compared to alpha-adrenoceptor agonists, which affect both.