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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 ^act_lac ). 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^bb_lac ) and one going the paracellular pathway (J ^paracell_lac =P _lac^·c _{lac lumen}). Both, J ^act_lac and J ^bb_lac of d-lactate gave the same K_m 1.9 and 1.7 mmol/l and the same maximal transport rate 3.2 and 2.9 pmol cm⁻¹ s⁻¹. The K_i of l-lactate tested against J ^act_lac and J ^bb_lac of d-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 that d-lactate uses the same transport system as l-lactate.

When Na⁺ was omitted from the perfusates J ^act_lac disappeared completely, while J ^bb_lac was reduced by 64%. These data reflect the Na⁺ dependence of the d-lactate transport through the brush border. Variation of intra-and extracellular pH by raising p CO₂, omitting HCO⁻₃ from the perfusates or adding acetazolamide had no effect on the transport of d-lactate when α-keto glutarate was used as fuel. However, when acetate was used as fuel, intracellular acidosis brought the reduced J ^act_lac 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) inhibited J ^act_lac significantly. J ^bb_lac, 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) augmented J ^bb_lac 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 influence J ^bb_lac .