Vectorial Bicarbonate Transport by Capan-1 Cells: a Model for Human Pancreatic Ductal Secretion
Human pancreatic ducts secrete a bicarbonate-rich fluid but our knowledge of the secretory process is based mainly on studies of animal models. Our aim was to determine whether the HCO<sub>3</sub><sup>-</sup> transport mechanisms in a human ductal cell line are similar to those previously identified in guinea-pig pancreatic ducts. Intracellular pH was measured by microfluorometry in Capan-1 cell monolayers grown on permeable filters and loaded with BCECF. Epithelial polarization was assessed by immunolocalization of occludin. Expression of mRNA for key electrolyte transporters and receptors was evaluated by RT-PCR. Capan-1 cells grown on permeable supports formed confluent, polarized monolayers with well developed tight junctions. The recovery of pH<sub>i</sub> from an acid load, induced by a short NH<sub>4</sub><sup>+</sup> pulse, was mediated by Na<sup>+</sup>-dependent transporters located exclusively at the basolateral membrane. One was independent of HCO<sub>3</sub><sup>-</sup> and blocked by EIPA (probably NHE1) while the other was HCO<sub>3</sub><sup>-</sup>-dependent and blocked by H<sub>2</sub>DIDS (probably pNBC1). Changes in pH<sub>i</sub> following blockade of basolateral HCO<sub>3</sub><sup>-</sup> accumulation confirmed that the cells achieve vectorial HCO<sub>3</sub><sup>-</sup> secretion. Dose-dependent increases in HCO<sub>3</sub><sup>-</sup> secretion were observed in response to stimulation of both secretin and VPAC receptors. ATP and UTP applied to the apical membrane stimulated HCO<sub>3</sub><sup>-</sup> secretion but were inhibitory when applied to the basolateral membrane. HCO<sub>3</sub><sup>-</sup> secretion in guinea-pig ducts and Capan-1 cell monolayers share many common features, suggesting that the latter is an excellent model for studies of human pancreatic HCO<sub>3</sub><sup>-</sup> secretion.