Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) as a tool for differentiation of P2Y2-receptor-mediated vasorelaxation in guinea pig aorta.

The action of the putative P2Y1-receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) was studied in guinea pig aorta for potential use in the differentiation between P2Y1 and P2Y2 purine receptors. Concentration-effect curves to 2-methylthioadenosine triphosphate (2MeSATP) and uridine triphosphate (UTP), agonists at P2Y1- and P2Y2-receptor sites, respectively, and the common agonist adenosine triphosphate (ATP) were constructed in guinea pig aortic ring preparations with the tone raised by 5 microM of noradrenaline. A ranked order of agonist potency of 2MeSATP > ATP > UTP resulted from the construction of concentration response curves to vasorelaxation of the agonists. Deendothelialization virtually abolished vasorelaxation to UTP but made no significant difference to 2MeSATP-induced responses. PPADS exhibited noncompetitive inhibition at P2Y1-receptor sites by reducing the maximal response to 2MeSATP, although a trend towards a right shift of the concentration-effect curves was observed. In total contrast, PPADS enhanced P2Y2-mediated vasorelaxation to UTP by shifting the concentration-effect curves to the left and increasing maximal responses. Thus, PPADS is a noncompetitive antagonist at P2Y1 receptors but enhances responses, at least to UTP, at P2Y2 receptors in guinea pig aorta via a hitherto unknown mechanism. Thus, PPADS is a potentially useful substance that may be used for differentiation between P2Y1 and P2Y2 receptors in the guinea pig aorta.