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

Apical ATP, ATP, UTP and UDP evoked transient increases in short circuit current (I SC , a direct measure of transepithelial ion transport) in confluent Caco‐2 cells grown on permeable supports. These responses were mediated by a population of at least three pharmacologically distinct receptors. Experiments using cells grown on glass coverslips showed that ATP and UTP consistently increased intracellular free calcium ([Ca 2+ ] i ) whilst sensitivity to UDP was variable. Cross desensitization experiments suggested that the responses to UTP and ATP were mediated by a common receptor population. 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 obtained for cells grown on glass. Experiments in which I SC and [Ca 2+ ] i were monitored simultaneously in cells on Transwell membranes, confirmed that apical ATP and UTP increased both parameters and showed that the UDP‐evoked increase in I SC was accompanied by a [Ca 2+ ] i ‐signal. Ionomycin consistently increased [Ca 2+ ] i in such polarized cells but caused no discernible change in I SC . However, subsequent application of apical ATP or UTP evoked a small rise in I SC but no rise in [Ca 2+ ] i . UDP evoked no such response. As well as evoking increases in [Ca 2+ ] i , the ATP/UTP‐sensitive receptors present in Caco‐2 cells thus allow direct control over ion channels in the apical membrane. The UDP‐sensitive receptors, however, appear to simply evoke a rise in [Ca 2+ ] i .British Journal of Pharmacology (2000) 131 , 1651–1658; doi: 10.1038/sj.bjp.0703743