An estimate of the salt concentration in the lateral intercellular spaces of rabbit gall-bladder during maximal fluid transport.

The ability of the gall-bladder to transport water between identical bathing solutions depends on active NaCl transport, which is thought to maintain the salt concentration in the lateral intercellular spaces above bathing solution levels and thus to create a local osmotic gradient. The mean value of this gradient has been estimated by an electrical procedure, based on measuring the small diffusion potential resulting from this gradient and from the preferential cation permeability of the gall-bladder. The electrical potential difference (p.d.) in maximally transporting rabbit gall-bladders is 1.4 mV, mucosal-solution positive to serosal solution. This p. d. is reversibly abolished or greatly reduced by six procedures which abolish or greatly reduce fluid transport (low temperature, replacement of Cl(-) by SO 4 (--) , replacement of Cl(-) and HCO 3 (-) by SO 4 (--) , replacement of Na(+) by choline, removal of HCO 3 (-) , and metabolic poisoning). The p. d. is increased by symmetrical partial replacement of NaCl by sucrose, which is expected to increase the salt concentration gradient between the lateral spaces and the bathing solutions. Since the transport mechanism of the gall-bladder is a neutral NaCl pump that cannot produce a p. d. directly, it is concluded that the observed p. d. is the expected diffusion potential. From this diffusion potential and from the measured value of a diffusion potential resulting from a known NaCl concentration gradient, the mean concentration of NaCl in the lateral spaces is calculated to be of the order of 10MM above the bathing solution value. Comparison of the external osmotic gradient required to stop water flow with the p. d. recorded under this condition of zero flow supports the validity of interpreting the p.d. in this fashion as a measure of the excess local salt concentration.