Pharmacological characterization of Uracil nucleotide-sensitive P2Y receptors in mouse ileum

Since uracil nucleotide-preferring receptors, belonging to the P2Y receptor family and responding to either uridine triphosphate (UTP) or uridine diphosphate (UDP), have been proposed to be present at different cellular level in the gut, regulating various functions, we aimed to investigate whether their activation by uracil nucleotides may modulate the contractility of the intestinal muscle. Experiments were carried out in vitro, and the contractility of the longitudinal muscle from mouse ileum was recorded as changes of the isometric tension. UDP or UTP evoked a concentrationdependent, tetrodotoxin insensitive, contractile response. UDP effect was antagonized by suramin and by PPADS, P2 receptor antagonists, whilst UTP was antagonized only by PPADS. The responses of both nucleotides do not show cross-tachyphylaxis with responses to ATP. MRS 2758, P2Y6 receptor antagonist, antagonized only the effects induced by UDP. Thus, UDP and UTP would act via selective, non ATP-sensitive, receptor subtypes. UDP activates P2Y6 receptors whilst UTP, due to the pharmacological profiles, likely would activate P2Y4 receptors. U73122, phospholipase C (PLC) inhibitor, and ciclopiazonic acid, sarcoplasmic reticulum Ca2+-ATPase inhibitor, markedly reduced both UDP and UTP effects. In conclusion, selective muscular receptors for uracil nucleotides are present in mouse ileum longitudinal muscle subserving contraction via release of Ca2+ from intracellular stores by a PLC-dependent pathway.