The Strange Tale Of Muscle Lactate

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The Strange Tale Of Muscle Lactate: When The Villain Becomes Your

Friend

http://www.sciencedaily.com/releases/2007/05/070531191121.htm

In an article published in The Journal of Physiology, de Paoli

and colleagues, working at the University of Aarhus in Denmark, add

to the growing literature leading to a more complete understanding

of the physiological role of lactic acid production in muscle.

In the late 19th century, fermentation chemists realized that juice

left to ferment without adequate oxygen resulted in acid products.

Then, in the early 20th century, when physiologists stimulated

isolated frog muscles to contract until exhaustion, they found that

the tissues had accumulated high amounts of lactic acid. Since then,

the idea that lactic acid accumulation causes muscle fatigue has

persisted.

But did early scientists fail to address the various issues

adequately and interpret the results appropriately? Did they fail to

ask the essential question? " Why does nature make lactic acid? " , and

did they in effect put one and one together and make them a minus?

De Paoli and colleagues looked at the effects of lactic acid and

adrenaline on the processes that signal contractions in skeletal

muscles. Using rat muscles, the study examined the combined effect

of potassium ions, lactic acid and adrenaline on the electrical

signalling system that serves to forward the activating signals from

the brain to the muscle fibres where contraction takes place.

They showed that in combination, lactic acid and adrenalin serve to

help working muscles ward off the effects of potassium ions which

leak from the inside to the outside of working muscle cells and

negatively effect the signaling process by which muscles contract.

In this, the latest of a series of reports from the Aarhus group, in

combination with reports from other scientists in Scandinavia, the

UK, US and Canada, long-standing ideas about the role of lactic acid

in muscle are being overturned.

So, why do muscles contract? Usually, muscles contract because the

central and peripheral nervous system signals them to do so.

Why do the muscles make lactic acid? Lactic acid is the result of

the glycolytic energy production system. It is an energy source to

be used in muscle cells of origin, or adjacent fibres (cells), or

fibres in the heart and cells in the brain. Lactic acid is also the

material that the liver prefers to make glucose (sugar) for the

blood when exercise is prolonged. Lactic acid production in muscle

is stimulated in part by circulating adrenalin. Now, from de Paoli

and colleagues we learn that adrenalin and lactic acid also help

protect against the electrolyte imbalance across muscle membranes

brought on by the loss of potassium.

Why does potassium have such a negative effect? In the study, when

potassium ions outside the muscle fibres were increased to levels

seen during intense exercise, the ability of the signalling system

to forward electrical signals was profoundly reduced and the muscle

became paralysed. If, however, lactic acid and adrenaline were added

in combination, the function of the signalling system was largely

recovered and the contractile response of the muscles restored. It

was further shown that the positive effect of lactic acid was

specifically related to an acidification of the interior of the

muscle cells, which is one of the hallmarks of intense exercise.

The muscle lactic acid story, however, is still incomplete. It may

even be found that lactate production is adaptive because its

presence signals the activation of genes responsible for controlling

muscle function. So, it seems that there is wisdom in the way that

the body functions, a retrospective realisation that seems obvious,

and which for lactic acid is supported by a century of strides even

after a few false steps.