Sodium-alanine cotransport in renal proximal tubule cells investigated by whole-cell current recording.
Sodium-alanine cotransport was investigated in single isolated proximal tubule cells from rabbit kidney with the whole-cell current recording technique. Addition of L-alanine at the extracellular side induced an inward-directed sodium current and a cell depolarization. The sodium-alanine cotransport current was stereospecific and sodium dependent. Competition experiments suggested a common cotransport system for L-alanine and L-phenylalanine. Sodium-alanine cotransport current followed simple Michaelis-Menten kinetics, with an apparent Km of 6.6 mM alanine and 11.6 mM sodium and a maximal cotransport current of 0.98 pA/pF at -60 mV clamp potential. Hill plots of cotransport current suggested a potential-independent coupling ratio of one sodium and one alanine. The apparent Km for sodium and the maximal cotransport current were potential dependent, whereas the apparent Km for L-alanine was not affected by transmembrane potential. The increase in Km for alanine with decreasing inward-directed sodium gradients suggested a simultaneous transport mechanism. These results are consistent with a cotransport model with potential-dependent binding or unbinding of sodium (high-field access channel) and a potential-dependent translocation step.
Hoyer
J
J
Max-Planck-Institut für Biophysik, Frankfurt a.M., Germany.
Gögelein
H
H
D000409:Alanine / Q000378:metabolism*; D026905:Amino Acid Transport Systems; D026921:Amino Acid Transport Systems, Neutral; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D002352:Carrier Proteins / Q000378:metabolism*; D066298:In Vitro Techniques; D007536:Isomerism; D007687:Kidney Tubules, Proximal / Q000166:cytology / Q000378:metabolism*; D008564:Membrane Potentials / Q000502:physiology; D010649:Phenylalanine / Q000378:metabolism / Q000494:pharmacology; D011817:Rabbits; D012964:Sodium / Q000378:metabolism*; D015222:Sodium Channels / Q000187:drug effects / Q000378:metabolism; D027981:Symporters
D026905:Amino Acid Transport Systems; D026921:Amino Acid Transport Systems, Neutral; D002352:Carrier Proteins; D015222:Sodium Channels; D027981:Symporters; C052168:sodium-alanine cotransporter; D010649:Phenylalanine; D012964:Sodium; D000409:Alanine