Uridine adenosine tetraphosphate is a novel neurogenic P2Y1 receptor activator in the gut

Abstract

Enteric purinergic motor neurotransmission, acting through P2Y1 receptors (P2Y1R), mediates inhibitory neural control of the intestines. Recent studies have shown that NAD+and ADP ribose better meet criteria forenteric inhibitory neurotransmitters in colon than ATP or ADP.Here we report that human and murine colon muscles alsorelease uridine adenosine tetraphosphate(Up4A) spontaneouslyand upon stimulation of enteric neurons. Release of Up4A was reduced by tetrodotoxin,suggesting that at least a portion of Up4A is of neuralorigin. Up4A caused relaxation(human and murine colons) and hyperpolarization (murine colon) that was blocked by the P2Y1R antagonist, MRS 2500, and by apamin, an inhibitor of Ca2+-activatedsmall-conductance K+ (SK) channels. Up4A responseswere greatly reduced or absent in colonsof P2ry1-/- mice. Up4A induced P2Y1R-SK-channel-mediated hyperpolarization inisolated PDGFRα+cells, which are postjunctional targets for purinergic neurotransmission. Up4A caused MRS 2500-sensitive Ca2+ transients in human 1321N1 astrocytoma cells expressing human P2Y1R. Up4A was more potentthan ATP, ADP, NAD+, or ADP ribose in colonic muscles. In murine distal colon Up4A elicited transient P2Y1R-mediated relaxation followed by a suramin-sensitive contraction. HPLC analysis of Up4A degradation suggests that exogenous Up4A first forms UMP and ATP in thehuman colon and UDP and ADP in the murine colon. Adenosine then isgenerated by extracellular catabolism of ATP responses to Up4A are not mediated by its metabolites. This study shows that Up4A is a potent native agonist for P2Y1R and SKchannel activation in human and mouse colon.