Extracellular nucleotides, such as ATP, UDP, and UTP, regulate pulmonary vascular tone through P2X and P2Y receptors. Recently, uridine adenosine tetraphosphate (Up(4)A) was reported as a novel endothelium-derived vasoconstrictive factor. Up(4)A contains both purine and pyrimidine moieties, which potentially activate P2X and P2Y receptors. The present study examined the effect of Up(4)A on contractility of isolated rat pulmonary artery. Up(4)A at 1-100 microM stimulated contraction in a concentration-dependent manner. Up(4)A was equipotent as UTP and UDP in the endothelium-denuded artery while much more effective than UTP and UDP in endothelium-intact preparations. The vasoconstrictor effect of Up(4)A was inhibited by suramin but not IP(5)I or desensitization of P2X receptors with alpha,beta-methylene-ATP (alpha,beta-Me-ATP). Up(4)A-induced contraction was also inhibited by pretreatment with thapsigargin, nitrendipine, or EGTA but unaffected by H1152. Furthermore, unlike ATP and UTP, Up(4)A did not induce relaxation of endothelium-intact preparations precontracted with phenylephrine. These results suggest that Up(4)A is a potent vasoconstrictor, but not a vasodilator, of the rat pulmonary artery. Up(4)A likely acts through a suramin-sensitive P2Y receptor. The contractile effect of Up(4)A involves the entry of extracellular Ca(2+) and release of Ca(2+) from intracellular stores but not Ca(2+) sensitization via the RhoA/Rho kinase pathway. Up(4)A, therefore, potentially plays an important role in the regulation of pulmonary vascular tone.