Stoichiometry of the renal sodium-L-lactate cotransporter.
We re-examined the electrical and stoichiometric properties of the Na+-L-lactate cotransporter using highly purified brush-border membrane vesicles prepared from the whole cortex of rabbit kidney. A valinomycin-induced K+ diffusion potential (interior-negative) stimulated Na+ gradient-dependent L-lactate uptake. However, this stimulation reflected catalytic rather than energetic activation as an inside-negative membrane potential did not induce net uphill lactate accumulation in the presence of Na+ but in the absence of a Na+ concentration gradient. Additional evidence for electroneutrality of the cotransporter was the finding that, under voltage-clamped conditions, L-lactate flux was a hyperbolic function of extravesicular Na+ concentration with a Hill coefficient (n) of 1.0. Moreover, the plot of V/[Na+]n versus V was linear for n = 1, indicating that one Na+ ion is co-transported with an anionic lactate1- molecule. Finally, addition of L-lactate to vesicles under Na+ equilibrium conditions failed to generate an inside-positive membrane potential as monitored by 3,3'-dipropylthiodicarbocyanine iodide fluorescence quenching, arguing against Na+-L-lactate cotransport by an electrogenic process. Taken together, these data indicate that the luminal Na+-L-lactate co-transporter is electroneutral with a stoichiometry of 1.
Barbarat
B
B
Department of Physiology, Faculté de Médecine Xavier Bichat, Paris, France.
Podevin
R A
RA
D000818:Animals; D052160:Benzothiazoles; D001692:Biological Transport / Q000187:drug effects; D002232:Carbocyanines; D002352:Carrier Proteins / Q000378:metabolism*; D004563:Electrochemistry; D005456:Fluorescent Dyes; D005947:Glucose / Q000494:pharmacology; D007672:Kidney Cortex / Q000648:ultrastructure*; D007773:Lactates / Q000378:metabolism; D019344:Lactic Acid; D008564:Membrane Potentials / Q000187:drug effects; D008871:Microvilli / Q000378:metabolism; D027501:Monocarboxylic Acid Transporters; D011188:Potassium / Q000378:metabolism; D011817:Rabbits; D012964:Sodium / Q000378:metabolism / Q000494:pharmacology; D013050:Spectrometry, Fluorescence; D027981:Symporters; D014634:Valinomycin / Q000494:pharmacology
D052160:Benzothiazoles; D002232:Carbocyanines; D002352:Carrier Proteins; D005456:Fluorescent Dyes; D007773:Lactates; D027501:Monocarboxylic Acid Transporters; D027981:Symporters; C056939:sodium lactate cotransporter; D014634:Valinomycin; D019344:Lactic Acid; C012944:3,3'-dipropyl-2,2'-thiadicarbocyanine; D012964:Sodium; D005947:Glucose; D011188:Potassium