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Characterisation of human monocarboxylate transporter 4 substantiates its role in lactic acid efflux from skeletal muscle
Monocarboxylate transporter (MCT) 4 is the major monocarboxylate transporter isoform present in white skeletal muscle and is responsible for the efflux of lactic acid produced by glycolysis. Here we report the characterisation of MCT4 expressed in Xenopus oocytes. The protein was correctly targeted...
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Published in: | The Journal of physiology 2000-12, Vol.529 (2), p.285-293 |
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Main Authors: | , , |
Format: | Article |
Language: | English |
Subjects: | |
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Online Access: | Get full text |
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Summary: | Monocarboxylate transporter (MCT) 4 is the major monocarboxylate transporter isoform present in white skeletal muscle and
is responsible for the efflux of lactic acid produced by glycolysis. Here we report the characterisation of MCT4 expressed
in Xenopus oocytes.
The protein was correctly targeted to the plasma membrane and rates of substrate transport were determined from the rate of
intracellular acidification monitored with the pH-sensitive dye 2â²,7â²-bis-(carboxyethyl)-5(6)-carboxyfluorescein (BCECF).
In order to validate the technique, the kinetics of monocarboxylate transport were measured in oocytes expressing MCT1. K m values determined for L-lactate, D-lactate and pyruvate of 4.4, > 60 and 2.1 mM, respectively, were similar to those determined
previously in tumour cells.
Comparison of the time course of [ 14 C]lactate accumulation with the rate of intracellular acidification monitored with BCECF suggests that the latter reflects
pH changes close to the plasma membrane associated with transport, whilst the former may include diffusion-limited movement
of lactate into the bulk cytosol.
K m values of MCT4 for these substrates were found to be 28, 519 and 153 mM, respectively, and for a range of other monocarboxylates
values were at least an order of magnitude higher than for MCT1. V max values appeared to be similar for all substrates.
K 0.5 values of MCT4 (determined at 30 mM L-lactate) for inhibition by α-cyano-4-hydroxycinnamate (991 μM), phloretin (41 μM),
5-nitro-2-(3-phenylpropylamino)benzoate (240 μM), p- chloromercuribenzene sulphonate (21 μM) and 3-isobutyl-1-methylxanthine (970 μM, partial inhibition) were also substantially
higher than for MCT1. No inhibition of MCT4 by 2 mM 4,4â²-diisothiocyanostilbene-2,2â²-disulphonate was observed.
The properties of MCT4 are consistent with published data on giant sarcolemmal vesicles in which MCT4 is the dominant MCT
isoform, and are appropriate for the proposed role of MCT4 in mediating the efflux from the cell of glycolytically derived
lactic acid but not pyruvate. |
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ISSN: | 0022-3751 1469-7793 |
DOI: | 10.1111/j.1469-7793.2000.00285.x |