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

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Abstract
Intracellular pH (pHc) 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), pHc 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 pHc 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.