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1 Department of Kinesiology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada; and 2 Department of Life Sciences (Sports Sciences), University of Tokyo, Tokyo 153, Japan
Rates of lactate uptake into giant
sarcolemmal vesicles were determined in vesicles collected from rat
muscles at rest and immediately after 10 min of intense muscle
contraction. This contraction period reduced muscle glycogen
rapidly by 37-82% in all muscles examined (P < 0.05) except the soleus muscle (no change P > 0.05). At an external lactate concentration of 1 mM lactate, uptake into giant
sarcolemmal vesicles was not altered (P > 0.05),
whereas at an external lactate concentration of 20 mM, the rate of
lactate uptake was increased by 64% (P < 0.05).
Concomitantly, the plasma membrane content of monocarboxylate
transporter (MCT)1 was reduced slightly (
10%,
P < 0.05), and the plasma membrane content of MCT4 was
reduced further (
25%, P < 0.05). In additional
studies, the 10-min contraction period increased the plasma membrane
GLUT4 (P < 0.05) while again reducing MCT4 (
20%,
P < 0.05) but not MCT1 (P > 0.05).
These studies have shown that intense muscle contraction can increase
the initial rates of lactate uptake, but only when the external lactate
concentrations are high (20 mM). We speculate that muscle contraction
increases the intrinsic activity of the plasma membrane MCTs, because
the increase in lactate uptake occurred while plasma membrane MCT4 was
decreased and plasma membrane MCT1 was reduced only minimally, or not
at all.
lactate transport; GLUT4; giant vesicles
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