The steady-state relationship between sodium and chloride transmembrane electrochemical potential differences inNecturus gallbladder

Summary

Intracellular C1, K and Na activities (a ⁱ_Cl ,a ⁱ_k anda ⁱ_Na ) and transmucosal membrane potential (E _m) in epithelial cells ofNecturus gallbladder were measured at different external Na concentrations ([Na]_o), with liquid ion-exchanger and conventional microelectrodes. Bladders were mounted in a divided chamber at 23°C between identical HCO₃-free Ringer solutions containing 5mm K. The pH was 7.2. Tris was substituted for Na. Measurements were made under steady-state conditions as determined by the constancy of the transepithelial potential difference. Both,a ⁱ_Cl anda ⁱ_Na increased in a saturable fashion with [Na]_o.E _m did not change significantly. Average values (±sem) under normal conditions ([Na]_o=100mm) fora ⁱ_Cl ,a ⁱ_Na andE _m were 16.8±0.8mm (n=9), 9.7±0.6mm (n=10) and −52.6±0.6 mV (n=26), respectively. In Na-free mediaa ⁱ_Cl declined to its equilibrium value.a ⁱ_K (96±2mm;n=7) did not change when [Na]_o was varied between 100 and 10mm but decreased to 80±3mm (n=4) in Na-free media.

Transmembrane electrochemical potential differences,$\Delta \bar \mu _j$, for Cl and Na were calculated at four different [Na]_o levels. A highly significant linear relation between$\Delta \bar \mu _{Cl}$ and$\Delta \bar \mu _{Na}$ was found, indicating that Cl and Na transport are energetically linked. The results support the view that the energy necessary for intracellular Cl accumulation is derived from the simultaneous dissipation of the chemical potential gradient of Na across the apical membrane and that the coupled entry mechanism is electroneutral.