Cell pH of rat renal proximal tubule in vivo and the conductive nature of peritubular HCO ⁻₃ (OH⁻) exit

Abstract

Intracellular pH (pH_c) was measured on surface loops of rat kidney proximal tubules under free-flow conditions in vivo using fine tip double-barrelled pH microelectrodes based on a neutral H⁺ ligand. The microelectrodes had Nernstian slopes and a resistance of the order of 10¹² Ω. By using a driven shield feed back circuit the response time to pH jumps was lowered to around 1 s. At a peritubular pH of 7.42 and a luminal pH of 6.68 ± 0.13 (n=27), pH_c was 7.17 ± 0.08 (n=19). Perfusing the peritubular capillaries suddenly with bicarbonate Ringer solutions of plasma-like composition which were equilibrated with high or low CO₂ pressures, acidified or respectively alkalinized the cells rapidly as expected from the high CO₂ permeability of the cell membranes. Such data allowed us to calculate the cytoplasmic buffering power of the tubular cells. Sudden peritubular perfusion with Ringer solution containing only 3 mmol/l of HCO ⁻₃ at constant physiological CO₂ pressure led to a similar fast cell acidification which indicated that the peritubular cell membrane is also highly permeable for bicarbonate or OH⁻ (H⁺). The latter response was completely blocked by the stilbene derivative SITS at the concentration of 10⁻³ mol/l. The observations indicate first that pH_c of rat proximal tubule is more acidic than was previously thought on the basis of distribution studies of weak acids, second that intracellular bicarbonate concentration is around 13 mmol/l and third that bicarbonate exit across the peritubular cell membrane is a passive rheogenic process via a conductive pathway which can be inhibited by SITS. The latter point confirms the conclusion which we had derived previously from membrane potential measurements in response to changing peritubular bicarbonate concentrations.