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

Abstract

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 P2Y₂ 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 P2Y₂ receptors, the monolayer cultures express at least one additional receptor population that allowed nucleotides to increase I_SC.

3. The UDP-evoked increase in I_SC 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.

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