先日、カンファレンスで誰かが「亀とアシドーシス」がどうのこうのというのを聴いて調べてみたら、すばらしい亀の知恵についての研究に出会うことができた。さっそく英語でコラムにしたら、もう他の人が発表していた（論文は2000年だし、無理もない）。残念だが、せっかくだからここに書いておく。
Can you imagine a patient with lactic acid level of 1800mg/dl and pH of 7? Don’t worry, I am not talking about a human being. It’s about a turtle.
The painted turtle (Chrysemys picta), a freshwater species in North America, spends the whole winter in an ice-covered pond. Lactic acid inevitably accumulates as a byproduct of anaerobic metabolism, even though its metabolic rate is at minimum in a cold temperature (its heart rate can go down to 1 beat per 5-10 minutes!).
The lactic acid is an acid with pKa of 4, stronger than the acetic acid. Yet the pH of a turtle still remains at 7. A turtle doesn’t breathe or urinate when hibernating. How could it be possible? This article (News Physiol Sci 2000 15 181) explains the 2 surprising mechanisms of this natural wonder.
First, it has a body fluid buffering. Its plasma bicarbonate concentration (normally) is 40mEq/l! It also has large amount of peritoneal fluid and pericardial fluid that are very high in bicarbonate (80mEq/l and 120mEq/l, respectively).
Second, it uses its shell as a buffer! It’s not just a protective armor. When the pH goes down, its shell releases carbonate with calcium and magnesium. Protons are buffered with carbonate and CO2 diffuses out into the water. The shell also sequesters lactic acid (both lactate and proton).
The endogenous buffering system is seen across species. In a case of Homo sapiens, the bone is our biggest buffer. However, it is obviously not large enough to compensate this extremely high lactic acid level!
A turtle is a symbol of longevity in many cultures. In Japan it is said to live 10,000 years. Maybe what doesn’t kill you makes you stronger.
