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Schlagwörter:
Rat proximal tubule; Neutral amino acid transport; Electrophysiology; Epithelial transport kinetics; Competition experiments
Zusammenfassung:
We have used electrophysiological techniques in rat kidney proximal tubule to study the transport of the followingl-α-amino acids: alanine, phenylalanine, glutamine, methionine, cysteine, cystine, proline, and hydroxyproline as well as the transport of glycine and β-alanine. When applied in millimolar concentrations in the tubular lumen in the presence of Na+, all amino acids tested were found to depolarize the tubular cell membranes partially. This depolarization arises from current flow that is associated with the cotransport of Na+ and amino acids across the brushborder membrane (Frömter 1982 [9]). Peritubular application did not alter the membrane potentials in a conclusive way. The magnitude of the depolarization in response to luminal perfusion increased with increasing amino acid concentration and obeyed simple Michaelis-Menten kinetics, except for proline, hydroxyproline, and glycine, which exhibited double-site saturation kinetics. By analyzing the time course of the potential changes and distinguishing between initial and steady state depolarizations, it was possible to separate kinetic properties of the brushborder transport mechanisms from the lumped kinetic properties of the overall epithelium. Knowing the concentration dependence of the depolarizations, the competition of different amino acids for the same transport site was investigated: Two amino acids were applied in saturating concentrations in the tubular lumen, either singly or jointly, to determine whether the depolarizations were additive and whether the additivity exceeded the predictions from the kinetic experiments. From such studies the presence of three separate rheogenic transport systems is postulated for neutral amino acids: System I transports all neutrall-α-amino acids with the exception of cystine, system II transports proline, hydroxyproline and glycine, and system III transports β-alanine or under physiological conditions taurine. Studies with the oxidant diamide suggest that cystine may be mostly reduced to cysteine and transported as cysteine, however it is likely that an extra transport system exists which transports cystine possibly in electroneutral fashion together with dibasic amino acids.
