In humans, the sickled red blood cells caused by sickle cell disease condemns sufferers to a whole suite of painful symptoms and a shortened lifespan, all due to one amino acid change in the haemoglobin sequence. This causes haemoglobin molecules to clump together to form long, thin filaments,deforming the cell shape. People don't seem to be the only vertebrates afflicted with sickle cells: researchers had observed sickle cells in other creatures, especially fish. But, many left it at that. This lack of explanation piqued the curiosity of physiologist Michael Berenbrink; `No one asked why or how this happened', explains his colleague Pia Koldkjær. Therefore the University of Liverpool based scientists decided to investigate how and why red blood cell sickling happens in fish(p. 3451).
Because sickling had been seen in many cod-fish, the team chose to examine one of the cod's relatives, the whiting (Merlangius merlangus), which is common in the Mersey estuary near the University. Using a fishing line to catch fish from the pier, and by collaborating with local anglers, the team quickly caught all the whiting they needed. They immediately took blood samples from the some of the fish, before transporting other live fish back to the lab.
Examining the whiting blood samples taken from just-caught fish under the microscope, the team found that around 96% of the red blood cells in samples were sickled. `You hardly ever see a normal red blood cell,' says Koldkjær. Comparing these samples with those taken from just-caught trout and carp, however, they found that a similar proportion of the red blood cells in these fish were normal shape. The sickle cell count in whiting that had rested for 24 hours in a tank in the lab was 63%, falling to 11% after a week's rest, showing that the fish recovered from sickling. The fish also behaved as normal, as if they were unaffected by sickling.
The team also knew that extreme stress reduces the pH of a fish's blood, so reasoned that the stress of being caught on a fishing line could be causing the sickling they saw. When they reduced the pH of the solutions containing the cells, this was exactly what they found: lowering the pH caused the red blood cells to sickle, probably caused by the increased number of protons inside the cells. Dissolving more oxygen in the solution lowered the pH needed to sickle the cells, suggesting that high oxygen levels partially protect against sickling. Since oxygen pressures are higher in some parts of a fishes'circulation than others, such as the retina, this could have a protective effect in areas where very low pH values have been previously measured.
Not only does stress lower pH, but also raises adrenaline and noradrenaline levels. These catecholamines stimulate a receptor on the surface of the red blood cell, which in turn activates a sodium/proton pump via an enzyme cascade. The action of this pump has the net effect of increasing pH inside the cell, which protects against sickling. Adding the adrenaline-like molecule isoprotenerol, which reliably stimulates the receptor, enabled the cells to partially reverse the sickling process, even at very low blood pH values only encountered during severe exercise. Although, a big mystery still remains: why sickling happens. `There could be an advantage, such as protection from parasites', Koldkjær says, `but we just don't know yet'.