The Measurement of Intracellular Sodium Activities in the Bullfrog by Means of Double-Barreled Sodium Liquid Ion-Exchanger Microelectrodes

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Abstract
A double-barreled Na+-selective microelectrode was constructed with monensin as a liquid ion exchanger. The HCl-treated monensin was dissolved in a solvent (Corning 477317) at 10% (wt/wt). Internal reference solution of its ionic barrel was mixture of 0.49 M NaCl and 0.01 M KCl, the pH being adjusted to 3 with 0.1 M citrate-HCl buffer, whereas that of the PD [potential difference] barrel was 0.5 M KCl. Average slope and selectivity ratio (Na+/K+) tested on 10 different microelectrodes were -57.5 .+-. 1.87 mV/P(Na) (SEM [standard error of the mean]) and 6.7 .+-. 0.44, respectively. The electrical resistance was an order of 1010 .OMEGA. and the response time was less than 10 s. Using this microelectrode, a free flow micropuncture experiment was carried out in the bullfrog kidney and the intracellular Na+ activity and the membrane PD were determined on the proximal tubular cell. Average value (.+-. SEM, n = 15) for the intracellular Na+ and K+ was 20.7 .+-. 1.56 meq/l and 61.2 .+-. 1.16 meq/l, respectively, and -68.7 .+-. 0.88 mV for the peritubular membrane PD. There was a significant negative correlation between Na+ and K+ activities within the cell, i.e., the lower the ionic activity of cellular Na+, the higher the cellular K+, and vice versa, the sum of these 2 being kept nearly constant. This may be related to the isosmosis in the reabsorptive process across the proximal tubular epithelium.
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