Passive NaCl transport in the flounder urinary bladder: predominance of a cellular pathway

The urinary bladder of the winter flounder is a high-resistance epithelium that can absorb Na and Cl in an electrically silent manner. This active absorption (mucosa-to-serosa) of NaCl is, apparently uniquely, inhibited by mucosal hydrochlorothiazide (HCTZ). These experiments evaluated the notion that virtually all of the cellular Na and Cl permeation could be inhibited by mucosal HCTZ. Mucosal Ba2+ reduced the transepithelial conductance from 0.74 +/- 0.08 to 0.60 +/- 0.06 mS/cm2. Mucosal HCTZ reduced the serosa-to-mucosa flux (backflux) of Na from 0.70 +/- 0.08 to 0.29 +/- 0.03 mueq.cm-2.h-1 and the backflux of Cl from 1.92 +/- 0.22 to 0.38 +/- 0.03 mueq.cm-2.h-1. The treatment with these two agents caused the sum of the partial ionic conductances for Na and Cl to approximate the measured transepithelial conductance. In response to the imposition of a transepithelial voltage, the HCTZ-insensitive Na and Cl backfluxes behaved largely as predicted by the laws of simple ionic diffusion, although there was still a detectable cellular backflux. As judged from dilution voltages and tracer fluxes, the diffusional (paracellular) pathway(s) is nonselective for Na and Cl. The HCTZ-sensitive cellular Na and Cl backfluxes are dependent on the presence of mucosal Na and Cl. Neither backflux is significantly inhibited by serosal application of commonly used inhibitors of Na or Cl transport. The results demonstrate that the majority of passive Na and Cl flux is via a cellular pathway. The translocation across the apical membrane probably involves the same NaCl cotransport process responsible for NaCl absorption.