Multiple P2Y receptor subtypes in the apical membranes of polarized epithelial cells

1 Apical ATP, UTP and UDP evoked transient increases in short circuit curre nt (I-SC, a direct measure of transepithelial ion transport) in confluent C aco-2 cells grown on permeable supports. These responses were mediated by a population of at least three pharmacologically distinct receptors. 2 Experiments using cells grown on glass coverslips showed that ATP and UTP consistently increased intracellular free calcium ([Ca2+](i)) whilst sensi tivity to UDP was variable. Cross desensitization experiments suggested tha t the responses to UTP and ATP were mediated by a common receptor populatio n. 3 Messenger RNA transcripts corresponding to the P2Y(2), P2Y(4) and P2Y(6) receptors genes were detected in cells grown on Transwell membranes by the reverse transcriptase- polymerase chain reaction. Identical results were ob tained for cells grown on glass. 4 Experiments in which I-SC and [Ca2+](i) were monitored simultaneously in cells on Transwell membranes, confirmed that apical ATP and UTP increased b oth parameters and showed that the UDP-evoked increase in I-SC was accompan ied by a [Ca2+](i)-signal. 5 Ionomycin consistently increased [Ca2+](i) in such polarized cells but ca used no discernible change in I-SC However, subsequent application of apica l ATP or UTP evoked a small rise in I-SC but no rise in [Ca2+](i). UDP evok ed no such response. 6 As well as evoking increases in [Ca2+](i), the ATP/UTP-sensitive receptor s present in Caco-2 cells thus allow direct control over ion channels in th e apical membrane. The UDP-sensitive receptors, however, appear to simply e voke a rise in [Ca2+](i).