Just because cells don't have a heart doesn't mean that they can't have a 'heartbeat'. Biologists Howard Petty and colleagues from Wayne State University in Detroit, USA, now report that single cells undergo pulsations akin to those of a beating heart.
The researchers have made videos of the changes in concentration of certain chemical compounds, 'NADPH' and 'NADH' (collectively NAD(P)H), within individual blood cells called neutrophils, as they explain in
They saw a 'wave' of NAD(P)H travelling from one end of each sausage-shaped cell to the other and back again. The oscillations happened at a rate of about three per second. Petty's group also found evidence that the cells' acidity -- the concentration of hydrogen ions -- oscillated too. They saw a wave of high acidity pass back and forth along a cell when they added an acid-sensitive fluorescent dye.
Where do these waves come from? Changes in the concentrations of NAD(P)H and hydrogen ions in cells occur as a result of their metabolism, when they obtain energy by breaking down sugar in a process called 'glycolysis'. It has long been known that glycolysis can produce oscillations in the levels of the chemical reagents involved, but no one has previously seen it give rise to waves in single cells.
This work rounds off a curious story in the history of chemistry. In the 1950s, a Russian biochemist, Boris Pavlovitch Belousov, investigated a mixture of chemical compounds which he believed would mimic the process of glycolysis, with the advantage that he could brew them up in large quantities in a test tube. To his surprise, he found that, instead of simply reacting to produce a collection of end products, his mixture seemed to undergo a kind of see-saw process. First the reaction would go one way, then it seemed to go in reverse.
When Belousov tried to publish these findings in 1951, he was ridiculed. Everyone knew, other chemists said, that a chemical reaction could only go in one direction, not back and forth. To say otherwise was apparently to contradict one of the fundamental laws of thermodynamics, the science of change. It was not until the late 1960s that the idea of an oscillating chemical reaction began to be taken seriously, mainly thanks to the efforts of another Russian biochemist, Anatoly Zhabotinsky. He devised a different mixture which also showed oscillations but which was accompanied by a striking colour change that was hard to ignore.
Chemists now realise that oscillating reactions do not contradict thermodynamics. If such a reaction is conducted in a closed system, like a mixture in a test tube, the oscillations will gradually get weaker until in the end the reaction does reach a stable end state. But if the mixture is constantly fed with fresh reagents -- like the situation in living cells -- it can oscillate indefinitely.
Zhabotinsky and others found in 1969 that, if the oscillating mixture is not mixed well, it can generate so-called chemical waves. A difference in concentrations of the reagents can spread out from a single point in a series of expanding circular waves, like ripples in a pond. The regular waves that Petty's group have now seen are of just this type -- except that they are produced by glycolysis itself, not by an analogue of it, and are confined to single cells. In fact, the researchers point out that the cells are probably the smallest possible 'containers' in which such waves can arise. If they were any smaller there would be no room for the waves to form.
The comparison with heartbeats is more than fanciful. The regular pulse of a heart is the result of 'travelling waves' of electrical activity communicated from cell to cell in the heart muscle. The way that these pulsations arise and behave is so closely analogous to the formation of chemical waves that the mixtures devised by Belousov and Zhabotinsky have long been used as models for studying the dynamics of the heart.