Abstract

In the accompanying article, we showed that AP5A displayed heterogenous vasoactive effects in rat resistance arteries. It induced a stable vasoconstriction in the superior epigastric artery (SEA) and a transient vasoconstriction in the mesenteric resistance artery (MrA). In the phenylephrine-precontracted MrA AP5A induced a marked vasorelaxation. In this study the noncompetitive inhibition of the AP5A-induced vasoconstriction with pyridoxal-phosphate-6-azophenyl-2′,4′-disulphonic acid was found to be significantly stronger in MrA than in SEA. The nonselective P2 purinoceptor antagonist suramin inhibited AP5A-induced vasoconstriction in MrA only. The vasoconstriction by the P2X purinoceptor agonist α,β-methylene ATP was inhibited by with pyridoxal-phosphate-6-azophenyl-2′,4′-disulfonic acid and suramin similarly to that induced by AP5A. Thus, the AP5A-induced vasoconstriction is due to P2X receptor activation, but two different P2X receptors seem to be operational in the two different vessels. The AP5A-induced vasorelaxation of phenylephrine-precontracted MrA was inhibited by the P2Y₁ receptor antagonist ADP3′5′. The vasorelaxation induced by ADPβS (P2Y₁ agonist) also was inhibited by ADP3′5′. These findings suggest that AP5A-induced vasorelaxation of MrA is caused by P2Y₁ receptor activation. The P1 (A₂) receptor antagonist 3,7-dimethyl-1-propargylxanthine only slightly inhibited AP5A-induced vasorelaxation at high concentrations. Adenosine and the A₂receptor agonist CGS21680 failed to produce significant vasorelaxation. Therefore, vasorelaxation in MrA does not involve A₂purinoceptor activation. AP5A-induced vasorelaxation was not inhibited by Ca²⁺- or ATP-dependent K⁺ channel blockade with clotrimazole, apamin, or glibenclamide. These data indicate that vasoconstriction in MrA and SEA by AP5A is due to different P2X receptors, and vasorelaxation in precontracted MrA is due to P2Y₁ receptor activation.