Sodium Extrusion and Potassium Uptake in Guinea Pig Kidney Cortex Slices

Abstract

Slices from the cortex corticis of the guinea pig kidney were immersed in a chilled solution without K and then reimmersed in warmer solutions. The Na and K concentrations and the membrane potential V,,, were then studied as a function of the Na and K concentrations of the reimmersion fluid. It was found that Na is extruded from the cells against a large electrochemical potential gradient. Q0 for net Na outflux was -2.5. At bath K concentrations larger than 8 mM the behavior of K was largely passive. At the outset of reimmersion (Vm > Ej) K influx seemed secondary to Na extrusion. Na extrusion would promote K entrance, being limited and requiring the presence of K in the bathing fluid. At bath K concentrations below 8 m, K influx was up an electrochemical potential gradient. Thus a parallel active K uptake is apparent. Qo0 for net K influx was -2.0. Dinitrophenol inhibited net Na outflux and net K influx, Q o became <1.1 for both fluxes. The ratio between these fluxes varied. Thus at the outset of reimmersion the net Na outflux to net K influx ratio was > 1. After 8 minutes it was < 1.

Effects of replacing medium sodium by choline, caesium, or rubidium, on water and ion contents of renal cortical slices

“…The steady-state volume is determined by the rate of ion transport by the pump and the rate of net ion leakage across the cells, the latter reflecting both the electrochemical potential gradient affecting the ions and the membrane permeability to them. As Stein (1967) has pointed out, so long as cellular potassium is held in the steady state at a concentration greater than that required for electrochemical equilibrium (and this seems to be true in renal cortical slices, Whittembury, 1965;Proverbio & Whittembury, 1975) the rate of change in cellular volume when metabolism is inhibited, or, as in the present experiments, when extracellular sodium is replaced by another cation, will reflect the difference between the rate at which cellular cations (mainly potassium but also sodium) and chloride (Leaf, 1956). …”

Energy barriers to sodium extrusion from sodium‐rich kidney cortex slices

“…This involved the use of outermost slices only, an incubation temperature of 250 C, and the use of acetate as an additional substrate in some experiments. Whittembury (1965) and by other authors. The energy requirements for Na excretion into various re-immersion media are given in Table 3, the effect of acetate in the medium being shown by comparing (A) and (B).…”

“…Two millilitres of 0-15 N-HN03 were then added to the dry residue, and after 24 hr at room temperature (McLean, 1963) Na and K were estimated in the supernatant fluid by flame photometry. Na and K were expressed as concentrations in the cell water, using a value of 26 % for the extracellular space (Robinson, 1950;Whittam, 1956;Whittembury, 1965 As stated in Methods, it was at first thought that the energy barrier could be calculated from the relation between [K] (1965) in which he showed by direct micro-electrode measurements, under conditions very similar to those used here, that in guinea-pig kidney cortex slices Em is equal to the K equilibrium potential (EK) at external K concentrations of 8 mm and over but as [K]o is decreased below 8 mm the membrane potential becomes HONOR SMYTH progressively lower than the K equilibrium potential. Most of these microelectrode measurements were taken at the end of a 50 min incubation period but the author showed, by plotting time courses, that with both 2 and 8 mm external K, the Em at the start of re-immersion is approximately 8 mV lower than the final figure.…”

“…Most of these microelectrode measurements were taken at the end of a 50 min incubation period but the author showed, by plotting time courses, that with both 2 and 8 mm external K, the Em at the start of re-immersion is approximately 8 mV lower than the final figure. Therefore, acting on the assumption that potentials of guinea-pig and rat kidneys are not likely to differ to any great extent, the Em measurements of Whittembury (1965), adjusted to apply to the start of re-immersion and extrapolated graphically when necessary for intermediate [K]o values, have been used in calculating energy requirements in Table 1 and subsequently in this paper. (Table 1B) also cuts out Na extrusion, the energy level here being in the region of 1.5 cal/m-equiv.…”

“…Since the potential measurements of Whittembury (1965) were to be used in the calculations, conditions were as close to his as possible. This involved the use of outermost slices only, an incubation temperature of 250 C, and the use of acetate as an additional substrate in some experiments.…”