Uracil nucleotides are stored in platelets and all other cells, and are released into the extracellular space upon stimulation. They show various biological responses but their actions and mechanism are not well understood. This study was conducted to investigate the effects of uridine 5'-triphosphate(UTP) on vascular tone and to identify the characteristics of their receptors.

Aortic ring preparation were made from the rat descending thoracic aorta. Endo-thelial cells were preserved or removed by gentle rubbing, The basal tension of aortic ring was lgm and isometric contraction were recorded on polygraph using force transducer.

In aortic ring Precontracted by 100nM norepinephrine, UTP induced dual effect with various concentrations. UTP elicited endothelium-dependent relaxation at low concentrations(100nM-10µM), and endothelium-independent contraction at high concentrations(more than 30µM). Among uracil nucleotides, UDP was as much effective as UTP in vascular tone, but UMP and uridine were not. UTP(pA₅₀ 6.15) was more potent than ATP(5.17), ITP(4.75) and other nucleotides(TTP, GTP, CTP). At basal tension, UTP induced relaxation at low concentrations and contraction at hige concentrations in endothelium-intact ring. But in endothelium-removed ring, UTP elicited only contraction.

Prior treatment of aortic ring with suramin, a non-selective P₂-purinoceptor blocker, inhibited UTP-Induced relaxation and contraction. Reactive blue-2, a P2γ purinoceptor blocker, inhibited relaxation only, but α, β-methylene ATP, a P_2x Purinoceptor blocker, enhanced contractile response. ATP inhibited the UPT-induced relaxation, but 2-methylthio ATP did not alter the effects of UTP. It means that UTP and ATP act at the same receptor but 2-methylthio ATP does not.

These results suggest that UTP-induced relaxation is mediated by nucleotide receptors on endothelium and the contraction is mediated by pyrimidinoceptors on vascular smooth muscle.