The simultaneous measurement of epithelial ion transport and intracellular free Ca2+ in cultured equine sweat gland secretory epithelium

We explored the relationship between nucleotide-evoked changes in intracellular free calcium ([Ca2+](i)) and anion secretion by measuring [Ca2+](i) and I-SC simultaneously in Fura-2-loaded, cultured equine sweat gland epithelia. Apical ATP, UTP or UDP elicited sustained increases in [Ca2+](i) that were initiated by the mobilization of cytoplasmic Ca2+ but maintained by Ca2+ influx. However, although these nucleotides also increased I-SC, this response was transient whereas the [Ca2+](i) signals were sustained. Experiments in which external Ca2+ was removed/replaced showed that Ca2+. entering nucleotide-stimulated cells elicited very little change in I-SC. Cross desensitization experiments showed that UTP-stimulated epithelia became insensitive to ATP but that UTP could increase both [Ca2+](i) and I-SC in ATP-stimulated cells by activating 'pyrimidinoceptors' essentially insensitive to ATP. Thapsigargin evoked a sustained rise in [Ca2+](i) that was accompanied by a maintained increase in I-SC. However, this increase in I-SC was dependent upon external Ca2+ and so the responses to nucleotides and thapsigargin have different properties. ATP increased I-SC in thapsigargin-treated cells without causing any rise in [Ca2+](i) while ionomycin increased both parameters. The data therefore show that apical P2Y receptors allow nucleotides to increase I-SC via two mechanisms, one of which appears to be [Ca2+](i)-independent control of anion channels.