Forskolin-induced anion currents and depolarization were investigated to clarify the mechanism of HCO3 − secretion in the intralobular duct cells of rat parotid glands. Anion currents of the cells were measured at the equilibrium potential of K+, using a gramicidin-perforated patch technique that negligibly affects intracellular anion concentration. The forskolin-induced anion current was sustained and significantly (54%) suppressed by glibenclamide (200 μm), a blocker of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl− channel. The anion current was markedly suppressed by addition of 1 mm methazolamide, a carbonic anhydrase inhibitor, and removal of external HCO3 −. Forskolin depolarized the cells in the current-clamp mode. Addition of methazolamide and removal of external HCO3 − significantly decreased the depolarizing level. These results suggest that activation of anion channels (mainly the CFTR Cl− channel located in luminal membranes) and production of cytosolic HCO3 − induce the inward anion current and resulting depolarization. Inhibition of the Na+-K+-2Cl− cotransporter and the Cl−-HCO3 − exchanger had no significant effect on the current or depolarization, indicating that the uptake of Cl− via the Na+-K+-2Cl− cotransporter or the Cl−-HCO3 − exchanger is not involved in the responses. Taken together, we conclude that forskolin activates the outward movement (probably secretion) of HCO3 − produced intracellularly, but not of Cl− due to lack of active Cl− transport in parotid duct cells, and that the gramicidin-perforated patch method is very useful to analyze anion transport.
The Journal of Membrane Biology – Springer Journals
Published: Mar 1, 2001
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