How big is the electrochemical potential difference of Na+ across rat renal proximal tubular cell membranes in vivo?

The intracellular Na+ concentration of surface loops of proximal tubules was measured with double-barrelled Na+ -sensitive microelectrodes in micropuncture experiments on rat kidneys in situ and in vivo. With the help of a fast recording system, it was possible to select only valid measurements, by discarding all records in which the Na+ concentration rose after the impalement presumably as a result of sodium ion inflow via puncture leaks. Under free-flow conditions cell Na+ activity averaged 13.1, S.D. +/- 2.1 mmol/l which, assuming an activity coefficient of 0.75, corresponds to a Na+ concentration of 17.5 mmol/l. The simultaneously recorded cell membrane potential was -73.5 mV in agreement with best estimates derived previously. The comparison of cell sodium activity and cell pH (Yoshitomi and Frömter [17]) demonstrates that the electrochemical potential difference of Na+ is greater than that of H+, as required for the operation of Na+/H+ counter transport in the brushborder membrane but that bicarbonate efflux via the sodium bicarbonate cotransport system at the peritubular cell membrane requires a HCO3- to Na+ stoichiometry of greater than 2.0 or the involvement of other ions at least under free-flow conditions.