The influence of intracellular sodium activity on the transport of glucose in proximal tubule of frog kidney

Inhibition of basolateral Na⁺/K⁺ ATPase by ouabain eventually abolishes transport of glucose. The present study was performed to test, if this effect is due to a dissipation of the electrochemical gradient for sodium or due to a regulatory inhibition of sodium-coupled glucose entry across the luminal membrane at increasing intracellular sodium activity. To this end, proximal convoluted tubules of the doubly perfused isolated frog kidney were perfused alternatively with solutions containing either 5 mmol/l glucose or raffinose. The potential difference across the peritubular cell membrane (PD_pt) and across the epithelium (PD_pt) has been recorded with conventional and across the peritubular cell membrane with ion selective microelectrodes (PD_pt). In the absence of luminal glucose PD_pt is (±SEM) -54.0±2.4 mV, PD_te=-1.2±2.0 mV and PD_pt^Na=-96±5 mV. The electrochemical gradient for sodium (μNa⁺) amounts to 95 mV and intracellular sodium activity to 14 mmol/l (extracellular sodium activity is 74 mmol/l). Luminal application of glucose leads to a rapid depolarisation of PD_pt (ΔPD_pt=8.6±0.9 mV and PD_pt^Na (ΔPD_pt^Na=11.1±3.0 mV) and to hyperpolarisation of PD_te (ΔPD_te=-0.8±0.2 mV). The peritubular application of ouabain leads to a gradual, reversible and proportional decline of PD_pt, PD_pt^Na and μNa⁺. Glucose induced ΔPD_pt and ΔPD_pt^Na decrease in parallel to PD_pt and PD_pt^Na, resp. In a separate series, the lumped conductance (G_m) of the luminal and basolateral cell membrane has been determined, which amounts to 2.4±0.3 μS/mm (tubule length). G_m decreases 23±4%, when PD_pt is decreased to half. ΔPD_pt and G_m allow the calculation of an apparent transport rate (T_Glu). Following the application of ouabain, T_Glu decreases in linear proportion to PD_pt and PD_pt^Na. There is no evidence for a significant regulatory inhibition of T_Glu. Rather, glucose transport operates in linear proportion to the potential difference across the luminal membrane.