Effect of pH on chloride absorption in the flounder intestine.

Chloride absorption in the small intestine of the winter flounder, Pseudopleuronectes americanus, is reported to be sensitive to ambient pH. We studied this sensitivity in isolated stripped intestinal mucosa mounted in modified Ussing chambers. Unidirectional 36Cl fluxes (JClm----s, JCls----m) were measured under short-circuited conditions in bathing solutions containing various combinations of HCO3- (0-20 mM), partial pressure of CO2 (0-36 mmHg), and pH (6.77-7.85). We found that JClm----s, net 36Cl flux (JClnet), and short-circuit current (Isc) increased and JCls----m decreased predominately in response to increases in bathing solution pH. There was a linear relationship between pH and both JClnet (r = 0.92, P less than 0.01) and Isc (r = 0.96, P less than 0.005) between pH 6.77 and 7.74. The pH effect was completely reversible, did not require either CO2 or HCO3-, and was not affected by the presence of mucosal barium at 1 mM. Mucosal bumetanide (0.1 mM) completely inhibited the pH effect. These data suggest that the process by which Cl- is absorbed in the flounder intestine is sensitive to pH. The data do not indicate whether pH affects Na+-K+-2Cl- cotransport or a Cl- transport pathway in series with this process. The direction of Cl- absorption in response to pH contrasts with inverse relation of pH and Cl- absorption in mammalian small intestine.