Passive solute and fluid transport in brook trout (Salvelinus fontinalis) urinary bladder epithelium

Abstract

The passive transport of solutes across the brook trout (Salvelinus fontinalis) urinary bladder epithelium was examined in vitro in Ussing-style membrane chambers. The low transepithelial conductance (average 0.14–0.20 mS∙cm−2) and low mannitol permeability (6.9 ± 1.4 × 10−11 cm∙s−1, mean ± SE) indicate that both the transcellular and paracellular pathways have limited solute permeability. Fluid transport measurements in in vitro bag preparations indicate low hydraulic conductivity (1.6 ± 0.4 × 10−7 cm∙s−1∙atm−1; 1 atm = 101.325 kPa) and suggest that the absorbate is hyperosmotic, 5-fold more concentrated than the bathing solutions. Voltage clamping experiments with unidirectional 22Na+ and 36Cl− fluxes indicated that Na+ passive diffusion occurs primarily via a transcellular pathway, whereas the epithelium behaves as a simple resistive barrier to Cl−; thus, a diffusional portion of the Cl− flux may be paracellular. The balance of the Cl− serosa-to-mucosa flux is nonconductive and apparently represents anion exchange. Current–voltage relations were nonlinear as is typical of some tight epithelia. Bladder urine is highly hypotonic, with sodium, potassium and chloride contents of 2.00 ± 0.36, 0.76 ± 0.19 and 1.31 ± 0.20 mM, respectively. In addition to the previously demonstrated absorptive neutral NaCl active transport, present results indicate a barrier function of the urinary bladder epithelium in which hydraulic conductivity and ion and uncharged solute permeabilities are low. These characterisitics are in turn consistent with the production in vivo of a very dilute urine.