title: Lactate transport is mediated by a membrane-bound carrier in rat skeletal muscle sarcolemmal vesicles.
abstract: To study the kinetics of lactate transport in an isolated, nonmetabolizing system, skeletal muscle sarcolemmal membrane vesicles were purified from 22 female Sprague-Dawley rats. L(+)-[U-14C] Lactate at 10 concentrations demonstrated saturation kinetics with a Vmax of 139.4 nmol/mg/min, and an apparent Km of 40.1 mM. Threefold higher initial rates of L(+)-lactate uptake were seen at 37 degrees C than at 25 degrees C, indicating temperature sensitivity. Transport was stereospecific for the L(+) isomer: isotopic D(-) uptake rates remained linear at concentrations from 1 to 200 mM, and 1 mM D(-) remained 6-fold lower in net uptake after 60 min than the L(+) isomer. Furthermore, unlabeled 10 mM D(-)-lactate in the external medium could only inhibit 1 mM isotopic (L(+) uptake by 12%, whereas unlabeled 10 mM L(+)-lactate and pyruvate inhibited 82 and 71%, respectively. Additionally, 10 mM beta-hydroxybutyrate and acetoacetate could moderately inhibit (27 and 32%, respectively) 1 mM L(+)-lactate transport, but the unsubstituted aliphatic monocarboxylates (formate, acetate, propionate), tricarboxylic acid cycle intermediates (malate, succinate, oxaloacetate, alpha-ketoglutyrate, citrate), amino acids (alanine, aspartate, glutamate), and palmitate or adenosine in 10-fold excess could not effectively inhibit 1 mM L(+)lactate uptake under cis-transport conditions. 4,4'-Diisothiocyanostilbene-2,2'-disulfonic acid could inhibit L(+)-lactate transport by only 13%, so that lactate transport does not appear to be affected directly by Cl- or HCO3- fluxes. It was demonstrated that KCl could not evoke a membrane potential-induced overshoot of lactate uptake in the presence or absence of valinomycin. Moreover, gluconate could substitute for Cl-, indicating that Cl- flux does not contribute to a membrane potential-dependent component of the transport mechanism, suggesting an electroneutral translocation process. Protein-modifying reagents significantly inhibited 1 mM L(+)-lactate transport during pH-stimulated conditions (p-chloromercuriphenyl-sulfonic acid, 83%; N-ethylmaleimide, 86%; HgCl2, 56%; mersalyl, 63% inhibition). We conclude that the skeletal muscle lactate transporter is a membrane-bound protein, specifically associated with the sarcolemma, that demonstrates saturation kinetics, competition, stereospecificity, and sensitivity to temperature as well as various ionic cis-inhibitors. The lactate transporter is a potentially important regulator of lactate flux across skeletal muscle, and may help to regulate intracellular pH and intermediary metabolism during lactic acidosis.
authors.affiliation: Department of Physical Education, University of California, Berkeley 94720.
authors.forename: D A
authors.initials: DA
authors.lastname: Roth
authors.forename: G A
authors.initials: GA
authors.lastname: Brooks
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002264:Carboxylic Acids / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism*; D002712:Chlorides / Q000502:physiology; D005260:Female; D066298:In Vitro Techniques; D007700:Kinetics; D007773:Lactates / Q000378:metabolism*; D008564:Membrane Potentials; D011188:Potassium / Q000502:physiology; D051381:Rats; D011919:Rats, Inbred Strains; D012508:Sarcolemma / Q000378:metabolism*; D013237:Stereoisomerism; D013696:Temperature
chemical_list: D002264:Carboxylic Acids; D002352:Carrier Proteins; D002712:Chlorides; D007773:Lactates; D011188:Potassium