Lactate and pyruvate transport is dominated by a pH gradient-sensitive carrier in rat skeletal muscle sarcolemmal vesicles

Muscle lactate transport studied in sarcolemmal giant vesicles

Lactate transport was studied in giant (median diameter 6.3 microns) sarcolemmal vesicles obtained by collagenase treatment of rat skeletal muscle. The lactate transport displayed stereospecificity, had a high temperature coefficient, and could be inhibited up to 90% with known transport inhibitors (PCMBS and cinnamate). In equilibrium exchange experiments, the L-lactate flux demonstrated saturation kinetics with Km = 23.7 mM and Vmax = 108 pmol cm-2 s-1. With lactate present on only one side of the membrane, (zero trans conditions), Vmax was reduced to 48 pmol cm-2 s-1. The flux rate displayed transacceleration. The lactate flux was coupled to a parallel H+ flux. Under equilibrium exchange conditions, the carrier-mediated lactate flux was not pH-dependent. In the zero trans experiments, H+ on the trans side acted as an inhibitor. The loaded form of the carrier reorients faster than the unloaded form, and the protonated form with no lactate bound reorients slowly or is immobile. When compared to intact muscles, the giant sarcolemmal vesicles retain their transport characteristics both qualitatively and quantitatively.

Lactate transport is mediated by a membrane-bound carrier in rat skeletal muscle sarcolemmal vesicles

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.