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Abstract

Inorganic phosphate (P_i) transport was evaluated using the standing droplet method with simultaneous microperfusion of the peritubular capillaries. To evaluate rather small differences in P_i transport and to eliminate the influence of tubular heterogeneity, the technique of crossed paired samples was applied. (1.) In chronic PTX rat changing the luminal or both luminal and peritubular pH by varying the HCO⁻₃ -concentration between 4 and 50 mmol/l at constant 5% CO₂ had no influence on P_i transport. (2.) If, however, bicarbonate was omitted from the perfusate and 2 mmol/l phosphate (pH 7.4) was the only buffer, P_i transport was decreased from the control. It was, however, further reduced when the perfusates were gased with 5% CO₂ i. e. the starting pH was 5.6. (3.) When the solutions contained HEPES buffer (25 mmol/l), P_i transport at pH 8 was much larger than at pH 6.0. (4.) Raising the CO₂ pressure from 35 to 70 mm Hg did not change the P_i transport when both perfusates had a HCO⁻₃ -concentration of 25 mmol/l. It reduced, however, the P_i transport, when the luminal perfusate had only 4 mmol/l bicarbonate. (5.) Lowering the CO₂ pressure from 38 to 7.6 mm Hg did hardly change the P_i transport when the luminal perfusate contained 4 mmol/l bicarbonate. It lowered, however, the P_i transport significantly when the luminal perfusate had 25 mmol/l bicarbonate. (6.) Acetazolamide, 10⁻⁴ M, lowered the P_i transport when the luminal perfusate contained 4 or 25 mmol/l bicarbonate. At 4 mmol/l luminal HCO ⁻₃, raising the pCO₂ to 228 mmol/l depressed P_i transport even more. At 25 mmol/l luminal bicarbonate, raising thepCO2 from 38 to 114 mm Hg reversed the acetazolamide inhibition of the P_i transport almost completely. The data indicate that luminal acidosis and intracellular alkalosis inhibits the transtubular P_i transport. A shift of the intracellular pH to a more alkaline value seems to be responsible for the inhibition of P_i transport by acetazolamide, while omission of buffer from the perfusate inhibits P_i transport by effecting an acidic luminal pH.