In a paper published this week in The Journal of Physiology, Frank 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
Contact: Melanie Thomson
Blackwell Publishing Ltd.