Sodium-alanine cotransport in oocytes ofXenopus laevis: Correlation of alanine and sodium fluxes with potential and current changes |
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Authors: | D Jung W Schwarz H Passow |
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Institution: | (1) Max-Planck-Institut für Biophysik, 6000 Frankfurt am Main 71, Federal Republic of Germany |
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Abstract: | Summary The sodium-dependentl-alanine transport across the plasma membrane of oocytes ofXenopus laevis was studied by means of 14C]-l-alanine,22Na+ and electrophysiological measurements. At fixed sodium concentrations, the dependence of alanine transport on alanine concentration follows Michaelis-Menten kinetics; at fixed alanine concentrations, the transport varies with sodium concentration with a Hill coefficient of 2. In the presence of sodium the uptake of alanine is accompanied by a depolarization of the membrane. Under voltage-clamp conditions this depolarization can be compensated by an inward-directed current. Assuming that this current is carried by sodium we arrive at a 21 stoichiometry for the sodium-alanine cotransport. The assumption was confirmed by direct measurements of both sodium and alanine fluxes at saturating concentrations of the two substrates, which also yielded a stoichiometry close to 21. The sodium-l-alanine cotransport is neither inhibited by furosemide (0.5 mmol/liter) nor by N-methyl amino isobutyric acid (5 mmol/liter). A 20-fold excess ofd-alanine overl-alanine caused about 60% inhibition. |
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Keywords: | Xenopus oocyte cotransport alanine flux sodium flux sodium current |
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