If you've ever tried scuba diving, you'll know just how tricky regulating your buoyancy is, yet fish do it with ease. So how do they suddenly inject oxygen into their buoyancy-adjusting swim bladders when the pressure exerted on them by the water can exceed tens of atmospheres? Jodie Rummer from the University of British Columbia explains that fish have a specially adapted oxygen-carrying haemoglobin that can suddenly dump all its oxygen when the conditions are highly acidic; and the swim bladder has an acid-producing gland that provides the perfect conditions. However, there is one catch: the specially adapted haemoglobin evolved 270 million years before this type of swim bladder appeared, so why did this so-called ‘Root effect’ haemoglobin evolve?
This is the question that puzzled Rummer and principle investigator Colin Brauner. They suspected that this adaptation must have evolved to deliver oxygen to muscle. But there was another catch: haemoglobin's low affinity for oxygen under acidic conditions would make it impossible to pick up oxygen at the gill, making it almost useless as a regular oxygen deliverer. Rummer and Brauner realised that there must be some fast-acting acid switch that could trigger the sudden release of oxygen at the tissues but rapidly return the pH to neutral in the red blood cell before the blood passed back through the gills.
After months of brainstorming, Rummer and Brauner hit on an ingenious scheme that could allow the specialised haemoglobin to release oxygen in normal tissue (p. 2319). According to Rummer, stressed fish have a safety mechanism that stops their red blood cells becoming too acidic when the muscle works hard and is producing high levels of carbon dioxide that is converted into acidic protons and bicarbonate. The pair also knew that a hormone called noradrenaline switches on a pump that pumps protons out of the cell and into the plasma to maintain a neutral pH in the red blood cell so that the haemoglobin it carries is ready to pick up oxygen when it returns to the gill. The duo realised that fish could short-circuit this protection mechanism and send the red blood cell pH plummeting, but only if there was an enzyme called carbonic anhydrase in the plasma to recombine the acid protons and bicarbonate into carbon dioxide that could quickly diffuse back into the red blood cell. This would drop the red blood cell pH instantaneously to cause an acidosis and release the oxygen.
The duo had to test the idea. ‘We have these three elements that have to happen: we have to have blood that is acidified, is exposed to noradrenaline and the plasma must be exposed to carbonic anhydrase,’ explains Rummer. So she incubated trout blood with carbon dioxide to produce acidic conditions, switched on the protective acid-extracting pump with an analogue of noradrenalin and then added carbonic anhydrase to see if the haemoglobin would release its oxygen.
Amazingly, it did. The carbonic anhydrase short-circuited the protective pump, creating the acidic conditions that would release the oxygen from the red blood cell's haemoglobin. And Rummer calculates that the drop in red blood cell pH could release 25 times more oxygen than if the pH remained neutral. The team also found that there could be other acid-pumping systems on the red blood cell membrane that could enable oxygen delivery by the specialised haemoglobin even at moderate activity levels.
So, having found that the fish's specialised haemoglobin could – in theory – have evolved to deliver oxygen to exercising muscles, Rummer and Brauner are keen to find out if fish do this in practice.