Concurrent and independent HCO3- and Cl- secretion in a human pancreatic duct cell line (CAPAN-1)

The present study investigated both HCO3- and Cl- secretions in a human pancreatic duct cell line, CA-PAN-1 using the short-circuit current (I-sc) technique. In Cl-/HCO3--containing solution, secretion (1 mu M) or forskolin (10 mu M) stimulated a biphasic rise in the I-sc which initially reached a peak level at about 3 min and then decayed to a plateau level after 7 min. Removal of external Cl- abolished the initial transient phase in the forskolin-induced I-sc while the plateau remained. In HCO3-/CO2-free solution, on the contrary, only the initial transient increase in I-sc was prominent. Summation of the current magnitudes observed in Cl--free and HCO3--free solutions over a time course of 10 min gave rise to a curve which was similar, both in magnitude and kinetics, to the current observed in Cl-/HCO3--containing solution. Removal of external Na+ greatly reduced the initial transient rise in the forskolin-induced I-sc response, and the plateau level observed under this condition was similar to that obtained in Cl--free solution, suggesting that Cl--dependent I-sc was also Na+-dependent. Bumetanide (50 mu M), an inhibitor of the Na+-K+-2Cl(-) cotransporter, and Ba2+ (1 mM), a K+ channel blocker, could reduce the forskolin-induced I-sc obtained in Cl-/HCO3--containing or HCO3--free solution. However, they were found to be ineffective when external Cl- was removed, indicating the involvement of these mechanisms in Cl- secretion. On the contrary, the HCO3--dependent (in the absence of external Cl-) forskolin-induced I-sc could be significantly reduced by carbonic anhydrase inhibitor, acetazolamide (45 mu M). Basolateral application of amiloride (100 mu M) inhibited the I-sc; however, a specific Na+-H+ exchanger blocker, 5-N-methyl-N-isobutylamiloride (MIA, 5-10 mu m) was found to be ineffective, excluding the involvement of the Na+-H+ exchanger. However, an inhibitor of H+-ATPase, N-ethylmaleimide did suppress the I-sc (IC50 = 22 mu M). Immunohistochemical studies also confirmed the presence of a vacuolar type of H+-ATPase in these cells. H2DIDS (100 mu M), an inhibitor of Na+- HCO3- cotransporter, was without effect. Apical addition of Cl- channel blocker, diphenylamine-2,2'-dicarboxylic acid (DPC, 1 mM), but not disulfonic acids, DLDS (100 mu m) or SITS (100 mu M), exerted an inhibitory effect on both CT and HCO3--dependent forskolin-induced I-sc responses. Histochemical studies showed discrete stainings of carbonic anhydrase in the monolayer of CA-PAN-1 cells, suggesting that HCO3- secretion may be specialized to a certain population of cells. The present results suggest that both HCO3- and Cl- secretion by the human pancreatic duct cells may occur concurrently and independently.