Ammonia is an extremely unpleasant and toxic compound, which is very inconvenient for fish; they produce it as the end product of nitrogen metabolism. How fish handle ammonium excretion has long fascinated physiologists. Most fish were thought to dispose of ammonium simply by leaking it out of their gills but when Shigehisa Hirose from the Tokyo Institute of Technology discovered a new family of proteins (Rhesus proteins) in pufferfish with a similar amino acid sequence to other ammonium transport proteins, it became clear that ammonium disposal was more complex. Soon after, Chris Wood found that freshwater trout dispose of ammonia through Rhesus proteins in the membrane of a specialised gill cell, known as a pavement cell, so Hirose and Wood teamed up to find out how saltwater pufferfish handle ammonia excretion ().
Exposing pufferfish to high levels of ammonia and analysing the expression patterns of Rhesus proteins and other transporters involved in ammonium excretion, Hirose, Wood and their colleagues, Michele Nawata, Tsutomu Nakada and Akira Kato, found that, in addition to passively leaking ammonium through their pavement cells, pufferfish can actively pump ammonium out of their gills through another cell type, known as a mitochondrion rich cell, with Rhesus proteins on the external surface. The team also found that pufferfish can switch off Rhesus protein expression in their leaky gills if external ammonium levels are high – to prevent the toxin leaking back in – and pufferfish could also use Rhesus proteins to transport carbon dioxide out through their gills.
