Intracellular bicarbonate in single cells of Necturus kidney proximal tubule

Summary

The intracellular bicarbonate concentration in the cytoplasmic water of kidney cells of Necturus was determined by means of double-barreled HCO₃ ⁻-selective liquid ion-exchange microelectrodes. These microelectrodes permit the simultaneous determination of intracellular HCO₃ ⁻ and membrane PD of single cells. The fact that these double-barreled microelectrodes yielded a normal peritubular cell membrane electrical PD (70 m V) may be taken as evidence against any significant cellular damage by the electrodes. This electrometric study yielded a mean intracellular [HCO₃ ⁻] in single proximal tubule cells of Necturus of 11.1±0.6 mM, a value which is more than an order of magnitude higher than that predicted for a Donnan-type electrochemical equilibrium distribution of HCO₃ ⁻ ions. Thus there is a net electrochemical gradient favoring the passive efflux of HCO₃ ⁻ ions across both individual cell membranes. The movement of HCO₃ ⁻ ions from cell-to-interstitium would contribute to the renal acidification function. Across the luminal cell membrane the two possible mechanisms are either active reabsorption (lumen-to-cell) of HCO₃ ⁻ ions as such or active H⁺ secretion (cell-to-lumen). Our directly measured relatively high intracellular [HCO₃ ⁻] and the associated calculated relatively alkaline kidney cell pH of 7.44 are both more consistent with the H⁺ secretion hypothesis.