Intracellular measurement of Na activity using neutral carrier Na ion-selective microelectrode

To measure intracellular Na+ activities, a double-barreled Na ion-selective microelectrode was constructed with a neutral carrier Na+ ligand (ETH 227). The slope constant and detection limit were 56 mV and 3-5 mM, respectively, the selectivity of Na+/K+ being 40-50. In the bullfrog, the cell Na+ was 14.4 mEq/liter for the proximal tubule and 12.6 mEq/liter for the sartorius muscle.