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

1. Apical 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. 

2. Experiments using cells grown on glass coverslips showed that ATP and UTP consistently increased intracellular free calcium ([Ca²⁺]_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. 

3. Messenger RNA transcripts corresponding to the P2Y₂, P2Y₄ and P2Y₆ 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. 

4. Experiments in which I_SC and [Ca²⁺]_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 [Ca²⁺]_i-signal. 

5. Ionomycin consistently increased [Ca²⁺]_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 [Ca²⁺]_i. UDP evoked no such response. 

6. As well as evoking increases in [Ca²⁺]_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 [Ca²⁺]_i.