Mesenteric veins are more sensitive than arteries to the constrictor effects of sympathetic nerve stimulation and alpha-adrenoceptor agonists. We tested the hypothesis that alpha(1)- and alpha(2)-adrenoceptors interact to enhance adrenergic reactivity of mesenteric veins. We studied neurogenic and agonist-induced constrictions of mesenteric veins and arteries in vitro. Norepinephrine concentration-response curves were left-shifted in veins compared to arteries. UK 14,304 (0.01-1 microM, alpha(2)-adrenoceptor receptor agonist) did not constrict arteries or veins but enhanced constrictions and Ca(2+) signals mediated by alpha(1)-adrenoceptor stimulation in veins. Yohimbine (alpha(2)-adrenoceptor receptor antagonist) and MK912 (alpha(2C)-adrenoceptor receptor antagonist), but not alpha(2A)- or alpha(2B)-adrenoceptor antagonists, produced rightward shifts in norepinephrine concentration-response curves in veins. Pharmacological studies revealed that alpha(1D)-adrenoceptors mediate venous constrictions. Norepinephrine responses in veins from alpha(2C)-adrenoceptor knock-out (KO) mice were not different from wild type veins. Yohimbine inhibited norepinephrine constrictions in alpha(2C)-adrenoceptor KO veins suggesting that there is upregulation of other alpha(2)-adrenoceptors in alpha(2C)-KO mice. These data indicate that alpha(1D)- and alpha(2C)-adrenoceptors interact in veins but not in arteries. This interaction enhances venous adrenergic reactivity. Mesenteric vein-specific alpha(2)-adrenoceptor linked Ca(2+) and perhaps other signaling pathways account for enhanced venous adrenergic reactivity.

Institute for Clinical and Experimental Medicine Prague Czech Republic

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