Characterization of alpha-adrenoceptors in canine mesenteric vein

Interaction between alpha(1)- and alpha(2)-adrenoreceptors contributes to enhanced constrictor effects of norepinephrine in mesenteric veins compared to arteries

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.

Differential contributions of alpha-1 and alpha-2 adrenoceptors to vasoconstriction in mesenteric arteries and veins of normal and hypertensive mice

Mesenteric veins are more sensitive than arteries to the constrictor effects of sympathetic nerve stimulation and alpha-adrenergic receptor agonists. In the present study, we tested the hypothesis that alpha(2)-adrenergic receptors (alpha(2)-ARs) contribute to in vitro agonist-induced constriction in veins but not arteries and that alpha(2)-AR function is down-regulated in mesenteric arteries and veins in deoxycorticosterone acetate-salt (DOCA-salt) hypertension. Norepinephrine (NE) concentration-response curves were similar in SHAM and DOCA-salt arteries and veins indicating that adrenergic reactivity of mesenteric blood vessels is not altered in murine DOCA-salt hypertension in vitro. Veins were 30-fold more sensitive to NE than arteries. The alpha(1)-AR antagonist, prazosin (0.003-0.3 microM), produced concentration-dependent rightward shifts of the NE concentration-response curves in arteries but not veins. The alpha(2)-AR agonists, clonidine and UK-14,304, did not constrict arteries or veins in the absence or presence of indomethacin (10 microM) and nitro-L-arginine (NLA; 100 microM). The alpha(2)-AR antagonists, yohimbine (0.003-0.3 microM) and rauwolscine (0.1 microM) did not affect NE responses in SHAM or DOCA-salt arteries but antagonized NE responses in veins. These data indicate that there are different alpha-AR contractile mechanisms in murine mesenteric arteries and veins. Alpha(1)-ARs, but not alpha(2)-ARs, mediate direct contractile responses in arteries and veins while alpha(2)-ARs contribute indirectly to NE-induced constrictions in veins but not arteries in vitro. There may be direct protein-protein interactions between alpha(1)- and alpha(2)-ARs or between their signaling pathways in veins. This contribution of alpha(2)-ARs may account for the greater sensitivity of veins compared to arteries to the contractile effects of NE.

Heterogeneity of vascular smooth muscle alpha-adrenoceptors in canine blood vessels

1. For the past decade, using radioligand binding, contractility and immunohistochemical techniques, we have been characterizing vascular smooth muscle (VSM) adrenoceptors in four functionally different canine blood vessels, namely the dog aorta (DAO), dog mesenteric artery (DMA), dog mesenteric vein (DMV) and dog saphenous vein (DSV). 2. This communication briefly reviews our findings (with a special emphasis on alpha-adrenoceptor subtypes), which showed that none of the four canine vessels showed the same complement of alpha-adrenoceptor subtypes. 3. All four vessels elicited alpha 1-adrenoceptor contractile responses (antagonized by prazosin), but alpha 2-adrenoceptor responses (sensitive to rauwolscine), were found only in DMV and DSV. 4. Pharmacological characterization using alpha 1-adrenoceptor subtype-selective antagonists showed that DAO contains mainly alpha 1B-adrenoceptors (some alpha 1D-adrenoceptors) and DMA has primarily alpha 1A/1L-adrenoceptors, alpha 1-Adrenoceptors in DMV are primarily of the alpha 1D subtype, but either a new or unusual alpha 1-adrenoceptor subtype was revealed in the DSV.

Chloroethylclonidine and 2-aminoethyl methanethiosulfonate recognize two different conformations of the human alpha(2A)-adrenergic receptor

The substituted cysteine-accessibility method and two sulfhydryl-specific reagents, the methane-thiosulfonate derivative 2-aminoethyl methanethiosulfonate (MTSEA) and the alpha(2)-adrenergic receptor (alpha(2)-AR) agonist chloroethylclonidine (CEC), were used to determine the relative accessibility of engineered cysteines in the fifth transmembrane domain of the human alpha(2A)-AR (Halpha2A). The second-order rate constants for the reaction of the receptor with MTSEA and CEC were determined with the wild type Halpha2A (cysteine at position 201) and receptor mutants containing accessible cysteines at other positions within the binding-site crevice (positions 197, 200, and 204). The rate of reaction of CEC was similar to that of MTSEA at residues Cys-197, Cys-201, and Cys-204. The rate of reaction of CEC with Cys-200, however, was more than 5 times that of MTSEA, suggesting that these compounds may interact with two different receptor conformations. MTSEA, having no recognition specificity for the receptor, likely reacts with the predominant inactive receptor conformation (R), whereas the agonist CEC may stabilize and react preferentially with the active receptor conformation (R*). This hypothesis was consistent with three-dimensional receptor-ligand models, which further suggest that alpha(2A)-AR activation may involve the clockwise rotation of transmembrane domain 5.