Determination of intracellular K⁺ activity in rat kidney proximal tubular cells

The intracellular K+ activity of rat kidney proximal tubular cells was determined in vivo, using intracellular microelectrodes. In order to minimize damage from the impaling electrodes, separate measurements on separate cells, were performed with single-barrelled KCl-filled non-selective electrodes and single-barrelled, K+-sensitive microelectrodes, which were filled with a liquid K+-exchanger resin that has also a small sensitivity to Na+. Both electrodes had tip diameters of 0.2 μm or below. The proper intracellular localization of the electrodes was ascertained by recording the cell potential response to intermittent luminal perfusions with glucose. The membrane potential measured with the non-selective microelectrodes was −76.3±8.1 mV (n=81) and the potential difference measured with the K+-sensitive microelectrode was −7.2±5.8 mV (n=32). Based on the activity of K+ in the extracellular fluid of ∼3 mmol/l the intracellular K+ activity was estimated to be ∼82 mmol/l. Assuming equal K+-activity coefficients to prevail inside and outside the cell, this figure suggests that the intracellular K+ concentration is ∼113 mmol/l which must be considered as a lower estimate, however. The data indicate that the K+-ion distribution between cytoplasm and extracellular fluid is not in equilibrium with the membrane potential, but that K+ is actively accumulated inside the cell. This result provides direct evidence for the presence of an active K+ pump in the tubular cell membranes, which in view of other observations, must be envisaged as a (not necessarily electroneutral) Na+/K+-exchange pump which operates in the peritubular cell membrane and is eventually responsible for the major part of the tubular solute and water absorption