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

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 P2Y2 receptors. Nucleotides also increased short circuit current (ISC) 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 ISC thus have different pharmacological properties. Cross-desensitization experiments indicated that, as well as P2Y2 receptors, the monolayer cultures express at least one additional receptor population that allowed nucleotides to increase ISC. The UDP-evoked increase in ISC was essentially abolished in BAPTA-loaded epithelia suggesting that this response is dependent upon increased [Ca2+]i. Moreover, experiments in which ISC and [Ca2+]i were measured simultaneously showed that the UDP- and ADP-evoked increases in ISC were accompanied by increases in [Ca2+]i. 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.