H-Secretion — A Driving Force in Nacl Absorption of the Guinea Pig Gallbladder
The absorption of Na and Cl in the gallbladder of the guinea pig is partly dependent on the HCO3 in the bathing medium (D.W.Martin, J.Membrane Biol., 18, 219, 1974; J. Wood et al., Pflügers Arch., 365, R15, 1976). To elucidate the nature of this HCO3-effect we measured net volume fluxes and the net fluxes of Na, Cl and H. The alkalination of the mucosal side in HCO3-buffered solutions, due to a Cl-HCO3 exchange (K. Heintze et al., Naunyn Schmiedebergs Arch. Pharm., 297, R38, 19 77), was reversed into an acidification, if HCO3 was omitted. In this HCO3-free solution luminal pH and the fluid absorption decreased within 40 min exponentially to a final pH of 6.5±0.05 and an absorption rate of 6.7±1.6 /ul·cm−2·hr−1 respectively. The time courses of luminal acidification and of fluid absorption in PO4-buffered and in HCO3-free salines were identical whereas the luminal pH after 40 min was 7.2±0.01 in the PO4-buffered group. Hence the decline of fluid absorption was probably not due to a limiting H-gradient. Neither Tris nor pyruvate nor glycodiazine could mimic the absorption stimulating effect of HCO3 but butyrate concentration dependently restored net fluid and Na absorption. 25 mM butyrate were equieffective to 25 mM HCO3. Formiate (no effect), acetate, propionate and butyrate were increasingly effective in stimulating fluid absorption. Both butyrate and HCO3 are likely to combine with a secreted H to form lipid soluble molecule in the lumen of the gallbladder.