Distal colonic Na(+) absorption inhibited by luminal P2Y(2) receptors

Luminal P2 receptors are ubiquitously expressed in transporting epithelia. In steroid-sensitive epithelia (e.g., lung, distal nephron) epithelial Na(+) channel (ENaC)-mediated Na(+) absorption is inhibited via luminal P2 receptors. In distal mouse colon, we have identified that both, a luminal P2Y(2) and a luminal P2Y(4) receptor, stimulate K(+) secretion. In this study, we investigate the effect of luminal adenosine triphosphate/uridine triphosphate (ATP/UTP) on electrogenic Na(+) absorption in distal colonic mucosa of mice treated on a low Na(+) diet for more than 2 weeks. Transepithelial electrical parameters were recorded in an Ussing chamber. Baseline parameters: transepithelial voltage (V (te)): -13.7 +/- 1.9 mV (lumen negative), transepithelial resistance (R (te)): 24.1 +/- 1.8 Omega cm(2), equivalent short circuit current (I (sc)): -563.9 +/- 63.8 microA/cm(2) (n = 21). Amiloride completely inhibited I (sc) to -0.5 +/- 8.5 microA/cm(2). Luminal ATP induced a slowly on-setting and persistent inhibition of the amiloride-sensitive I (sc) by 160.7 +/- 29.7 microA/cm(2) (n = 12, NMRI mice). Luminal ATP and UTP were almost equipotent with IC(50) values of 10 microM and 3 microM respectively. In P2Y(2) knock-out (KO) mice, the effect of luminal UTP on amiloride-sensitve Na(+) absorption was absent. In contrast, in P2Y(4) KO mice the inhibitory effect of luminal UTP on Na(+) absorption remained present. Semiquantitative polymerase chain reaction did not indicate regulation of the P2Y receptors under low Na(+) diet, but it revealed a pronounced axial expression of both receptors with highest abundance in surface epithelia. Thus, luminal P2Y(2) and P2Y(4) receptors and ENaC channels co-localize in surface epithelium. Intriguingly, only the stimulation of the P2Y(2) receptor mediates inhibition of electrogenic Na(+) absorption.