Extracellular nucleotides, such as uridine 5'-triphosphate (UTP) and adenosine 5'- triphosphate (ATP), were previously reported to induce contraction of gastric smooth muscle (SM). A dinucleotide uridine adenosine tetraphosphate (Up4A) is a newly identified endothelium-derived contraction factor (EDCF), and induces contraction in vascular SM. It was not tested whether Up4A also induces contraction of gastric SM. Our hypothesis was that Up4A induces contractions of gastric SM, which may differ in circular and longitudinal muscle (CM and LM, respectively). CM and LM were isolated from rat gastric fundus for the measurement of isometric tension using tissue bioassay. Addition of Up4A to tissue bath induced transient contractile responses in both CM and LM, which were similar to those induced by ATP and UTP. In CM, the ranks of potencies and efficacies were UTP > Up4A > ATP. In LM, however, Up4A was much more potent than UTP and ATP. P2X1,2,4,5,7 and P2Y1,2,4,6 receptors were detected in gastric SM using RT-PCR. Up4A failed to induce any contraction in either LM or CM in the absence of extracellular Ca2+ or in the presence of nimodipine, an inhibitor of voltage-dependent Ca²⁺ channel. IP5I (5 μM), a P2X antagonist, did not attenuate Up4A-induced contractions in both LM and CM, but suramin (5 μM), a P2Y receptor antagonist, significantly inhibited Up4A contraction in CM, but not in LM. Up4A contraction in CM, but not in LM, was also inhibited by pretreatment with indomethacin (100 nM), a cyclooxygenase inhibitor, Y-27632 (500 nM), an inhibitor for Rho-activated kinase (ROCK). Therefore, we conclude that Up4A induces extracellular Ca²⁺-dependent contractions in rat gastric LM and CM, and Up4A contraction in CM is through suramin-sensitive P2Y receptor and subsequent activation of the cyclooxygenase and ROCK pathways. 

Effects of Y-27632 on Up4A-induced contractions of CM and LM preparations. (*p < 0.01, n=5).