Nucleotide‐evoked calcium signals and anion secretion in equine cultured epithelia that express apical P2Y 2 receptors and pyrimidine nucleotide receptors

Experiments with a spontaneously transformed equine epithelial cell line showed that certain nucleotides increased intracellular free calcium ([Ca 2+ ] 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. 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 [Ca 2+ ] 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 . The UDP‐evoked increase in I SC was essentially abolished in BAPTA‐loaded epithelia suggesting that this response is dependent upon increased [Ca 2+ ] i . Moreover, experiments in which I SC and [Ca 2+ ] i were measured simultaneously showed that the UDP‐ and ADP‐evoked increases in I SC were accompanied by increases in [Ca 2+ ] i . When grown under conditions which favour the development of a polarized phenotype, these epithelial cells thus appear to express [Ca 2+ ] i ‐mobilizing receptors sensitive to UDP and ADP that are not present in non‐polarized cells on coverslips.