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  • 1
    ISSN: 1432-2013
    Keywords: SITS ; Probenecid ; Phloretin ; Acetazolamide ; Lactate ; Renal tubule
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract The transport ofd-lactate across the epithelium of the late proximal convolution was investigated by two methods: 1. by measuring the zero net flux transtubular concentration difference (Δc tt,45s) and the permeability (P) ofd-lactate and calculating from both the transtubular active transport rate (J lac act ). 2. By measuring the 3.5 s efflux ofd-lactate from the tubular lumen, while blood was flowing through the capillaries. The 3.5 s efflux comprises two components, one going through the brush border (J lac bb ) and one going the paracellular pathway (J lac paracell =P lac·c lac lumen). Both,J lac act andJ lac bb ofd-lactate gave the sameK m 1.9 and 1.7 mmol/l and the same maximal transport rate 3.2 and 2.9 pmol cm−1 s−1. TheK i ofl-lactate tested againstJ lac act andJ lac bb ofd-lactate was also the same: 1.1 and 1.0 mmol/l. These data indicate that under our experimental conditions only the flux through the brush border seems to be rate limiting and thatd-lactate uses the same transport system asl-lactate. When Na+ was omitted from the perfusatesJ lac act disappeared completely, whileJ lac bb was reduced by 64%. These data reflect the Na+ dependence of thed-lactate transport through the brush border. Variation of intra-and extracellular pH by raisingpCO2, omitting HCO 3 − from the perfusates or adding acetazolamide had no effect on the transport ofd-lactate when α-ketoglutarate was used as fuel. However, when acetate was used as fuel, intracellular acidosis brought the reducedJ lac act back to the values obtained with α-ketoglutarate as fuel. It is suggested that this is an effect on a contraluminal transport step. Probenecid (5 mmol/l) and phloretin (0.25 mmol/l) inhibitedJ lac act significantly.J lac bb , however, was only inhibited by probenecid when acetate was used as fuel. These data indicate that both compounds act on thed-lactate exit at the contraluminal cell side, but that probenecid acts in addition at the luminal cell side. SITS (1 mmol/l) augmentedJ lac bb when acetate was used as fuel and is similar to the effect of lowering intracellular pH as described above. The SH reagents mersalyl (1.0 mmol/l) and maleolylglycine (1 mmol/l) did not influenceJ lac bb .
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  • 2
    ISSN: 1432-2013
    Keywords: Renal tubule ; Phosphate transport ; Extracellular pH ; Intracellular pH ; Acetazolamide
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Abstract Inorganic phosphate (Pi) transport was evaluated using the standing droplet method with simultaneous microperfusion of the peritubular capillaries. To evaluate rather small differences in Pi 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 Pi transport. 2. If, however, bicarbonate was omitted from the perfusate and 2 mmol/l phosphate (pH 7.4) was the only buffer, Pi 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), Pi 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 Pi transport when both perfusates had a HCO 3 − -concentration of 25 mmol/l. It reduced, however, the Pi 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 Pi transport when the luminal perfusate contained 4 mmol/l bicarbonate. It lowered, however, the Pi transport significantly when the luminal perfusate had 25 mmol/l bicarbonate. 6. Acetazolamide, 10−4 M, lowered the Pi transport when the luminal perfusate contained 4 or 25 mmol/l bicarbonate. At 4 mmol/l luminal HCO 3 − , raising thepCO2 to 228 mmol/l depressed Pi transport even more. At 25 mmol/l luminal bicarbonate, raising thepCO2 from 38 to 114 mm Hg reversed the acetazolamide inhibition of the Pi transport almost completely. The data indicate that luminal acidosis and intracellular alkalosis inhibits the transtubular Pi transport. A shift of the intracellular pH to a more alkaline value seems to be responsible for the inhibition of Pi transport by acetazolamide, while omission of buffer from the perfusate inhibits Pi transport by effecting an acidic luminal pH.
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