The use of a potential-sensitive cyanine dye for studying ion-dependent electrogenic renal transport of organic solutes. Uptake of l-malate and d-malate by luminal-membrane vesicles

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Abstract
The mechanisms of uptake of dicarboxylic acids by rabbit renal luminal-membrane vesicles were studied by the use of filtration and spectrophotometric techniques as described in an accompanying paper [Kragh-Hansen, Jørgensen & Sheikh (1982) Biochem. J. 208, 359–368]. Addition of l- or d-malate to dye-membrane-vesicle suspensions in the presence of Na+ gradients (extravesicular>intravesicular) resulted in spectral curves indicative of depolarization events. The renal uptake of dicarboxylic acids was dependent on the type of Na+-salt anion present and could be correlated with the ability of the anions to penetrate biological membranes (i.e. Cl−>SO42−>gluconate). Identical results were obtained by a filtration technique with Sartorius membrane filters. The results indicate that the dicarboxylic acids are taken up by the membrane vesicles in an electrically positive form (i.e. Na+/substrate coupling ratio 3:1) by an Na+-dependent transport system. This proposal was further supported by spectrophotometric experiments with various ionophores such as valinomycin, gramicidin and nigericin. The absorbance changes associated with simultaneous addition of l- and d-malate and spectrophotometric competition studies revealed that the two isomers are taken up by a common transport system. Spectral changes of the dye induced by addition of increasing concentrations of l- or d-malate indicated that the transport system favours the unphysiological d-form rather than the l-form of malate. Furthermore, it was observed that the affinity of both isomers for the transport system was dependent on the concentration of Na+ in the medium.

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