K+ secretion activated by luminal P2Y2 and P2Y4 receptors in mouse colon

Extracellular nucleotides are important regulators of epithelial ion transport, frequently exerting their action from the luminal side. Luminal P2Y receptors have previously been identified in rat distal colonic mucosa. Their activation by UTP and ATP stimulates K+ secretion. The aim of this study was to clarify which of the P2Y receptor subtypes are responsible for the stimulated K+ secretion. To this end P2Y(2) and P2Y(4) knock-out mice were used to measure distal colonic ion transport in an Ussing chamber. In mouse (NMRI) distal colonic mucosa, luminal UTP and ATP with similar potency induced a rapid and transient increase of the transepithelial voltage (V-te) (UTP: from -0.81 +/- 0.23 to 3.11 +/- 0.61 mV, n = 24), an increase of equivalent short circuit current (I-sc) by 166.9 +/- 22.8 mu A cm(-2) and a decrease of transepithelial resistance (R-te) from 29.4 +/- 2.4 to 23.5 +/- 2.0 Omega cm(2). This effect was completely inhibited by luminal Ba2+ (5 mm, n = 5) and iberiotoxin (240 nm, n = 6), indicating UTP/ATP-stimulated K+ secretion. RT-PCR analysis of isolated colonic crypts revealed P2Y(2), P2Y(4) and P2Y(6) specific transcripts. The luminal UTP-stimulated K+ secretion was still present in P2Y(2) receptor knock-out mice, but significantly reduced (Delta V-te: 0.83 +/- 0.26 mV) compared to wild-type littermates (Delta V-te: 2.08 +/- 0.52 mV, n = 9). In P2Y(4) receptor knock-out mice the UTP-induced K+ secretion was similarly reduced. Luminal UTP-stimulated K+ secretion was completely absent in P2Y(2)/P2Y(4) double receptor KO mice. Basolateral UTP showed no effect. In summary, these results indicate that both the P2Y(2) and P2Y(4) receptors are present in the luminal membrane of mouse distal colonic mucosa, and stimulation of these receptors leads to K+ secretion.