Nucleotide-evoked calcium signals and anion secretion in equine cultured epithelia that express apical P2Y2 receptors and pyrimidine nucleotide receptors

1 Experiments with a spontaneously transformed equine epithelial cell line showed that certain nucleotides increased intracellular free calcium ([Ca2+](i)) in cells plated on glass coverslips. The rank order of potency was ATP = UTP > 5-Br-UTP, whilst UDP and ADP were ineffective. The response thus appears to be mediated by P2Y(2) receptors. 2 Nucleotides also increased short circuit current (I-sc) in cells grown into epithelial monolayers and the rank order of potency was UDP > UTP > 5-Br-UTP > ATP > ADP. The increase in [Ca2+](i) and the rise in I-sc thus have different pharmacological properties. Cross-desensitization experiments indicated that, as well as P2Y(2) receptors, the monolayer cultures express at least one additional receptor population that allowed nucleotides to increase I-sc. 3 The UDP-evoked increase in Ise was essentially abolished in BAPTA-loaded epithelia suggesting that this response is dependent upon increased [Ca2+](i). Moreover, experiments in which I-sc and [Ca2+](i) were measured simultaneously showed that the UDP- and ADP-evoked increases in I-sc were accompanied by increases in [Ca2+](i). 4 When grown under conditions which favour the development of a polarized phenotype, these epithelial cells thus appear to express [Ca2+](i)-mobilizing receptors sensitive to UDP and ADP that are not present in non-polarized cells on coverslips.