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

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% (weight/weight). 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 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) and 6.7 ± 0.44, respectively. The electrical resistance was an order of 10¹⁰ ohm and the response time was less than 10 sec. Using this microelectrode, a free flow micropuncture experiment was carried out in the bullfrog kidney and the intracellular Na⁺ activity as well as the membrane PD was 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⁺ was, the higher the cellular K⁺, and vice versa, the sum of these two being kept nearly constant. The above finding may be somehow related to the isosmosis in the reabsorptive process across the proximal tubular epithelium.

Key Words:

• Na⁺-microelectrode
• monensin
• intracellular Na⁺
• proximal tubule