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Department of Anatomy and Developmental Biology, University College London, UK.

1. The isolated perfused rat mesenteric arterial bed was used to examine the activity of the adenine dinucleotides: beta-nicotinamide adenine dinucleotide (NAD); beta-nicotinamide adenine dinucleotide phosphate (NADP); flavin adenine dinucleotide (FAD); and of the alpha,omega-diadenosine polyphosphates: adenylyl adenosine (AP1A); P1,P2-diadenosine pyrophosphate (AP2A); P1,P3-diadenosine triphosphate (AP3A); P1,P4-diadenosine tetraphosphate (AP4A); P1,P5-diadenosine pentaphosphate (AP5A); P1,P6-diadenosine hexaphosphate (AP6A). Responses were compared with those of ADP, ATP, 2-methylthio-ATP (2-meSATP) and alpha,beta-methylene ATP (alpha,beta-meATP). 2. In basal tone preparations mono- and dinucleotides elicited vasoconstriction with the order of potency: alpha,beta-meATP > or = AP5A > or = AP6A > or = AP4A > or = 2-meSATP >> ATP >> ADP. The dinucleotides NAD, NADP, FAD, AP1A, AP2A and AP3A had no effect. 3. The P2X-purinoceptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (30 microM) virtually abolished vasoconstrictor responses to AP4A, AP5A and AP6A. 4. Auto- and cross-desensitization of vasoconstrictor responses to AP4A, AP5A, AP6A, ATP and alpha,beta-meATP were observed. 5. In raised tone preparations nucleotides elicited endothelium-dependent vasodilatation with the order of potency: 2-meSATP = ADP > ATP > AP3A > AP2A > AP1A = NADP = FAD > NAD. The nucleotides AP4A, AP5A, AP6A and alpha,beta-meATP had no vasodilator effects. 6. It is concluded that the alpha,omega-adenine dinucleotides AP4A, AP5A and AP6A elicit vasoconstriction, but not vasodilatation, in the rat mesenteric arterial bed via P2x-purinoceptors. In contrast, the dinucleotides NADP, FAD, AP1A, AP2A and AP3A elicit vasodilatation, but not vasoconstriction, via endothelial P2Y-purinoceptors. 7. It is suggested that there is a crucial relationship between the structure of the alpha,omega-diadenosine polyphosphates and their activity at P2X- and P2Y-purinoceptors with a pivotal role played by the polyphosphate chain. Molecules with four or more phosphates are vasoconstrictors, while those with three or less phosphates are vasodilators.

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