In case you missed Shark Week this year, I thought I would mention a cool fact about dogfish sharks (Squalus acanthias) that you probably did not learn about.
I read an interesting article from a lecture given by Dr. John N Forrest Jr, M.D. that was published in 2016 in Transactions of the American Clinical and Climatological Association. Sharks have a complicated system for regulating water and salt balance in the body that includes their gills, kidneys and rectal glands. The rectal gland in sharks was actually predicted to exist as early as 1930 by Homer Smith but it wasn’t until the 1959 that Wendell Burger at Mt. Desert Island Biological Laboratory in Maine actually discovered it. The shark rectal gland has been studied ever since as a model of salt regulation. In fact, it is a perfect illustration of the August Krogh principle that “For a large number of problems there will be some animal of choice, or a few such animals, on which it can be most conveniently studied.”
The purpose of the rectal salt gland is to secrete concentrated sodium chloride into the cloaca for removal from the body. This function is vital to the health of these saltwater animals as their kidneys are not able to effectively concentrate urine. Because the rectal gland is a lot easier to study than the concentrating mechanisms of the mammalian kidney, it has provided much insight into how kidneys are able to produce concentrated urine allowing animals to conserve water while getting rid of excess salts.
When I say this is a well-studied gland, I mean it. In searching for more information about it I came across many studies published by the American Physiological Society in their various journals not to mention those published by other societies. One of the most recent articles that I came across was a study published again by Dr. Forrest in the American Journal of Physiology – Cell Physiology that examined metabolic factors that could regulate sodium chloride secretion by targeting a specific chloride channel in the rectal gland called Cystic Fibrosis Transmembrane Conductance Regulator, or CFTR for short. Mutations in CFTR are responsible for the disease cystic fibrosis. His team discovered that adenosine monophosphate-activated protein kinase (AMPK), which is known to increase with metabolism in mammals, was able to reduce the secretion of chloride by the shark gland.
How many articles can you find? https://www.physiology.org/journal/ajpregu
JN Forrest Jr. The Shark Rectal Gland Model: A Champion of Receptor Mediated Chloride Secretion through CFTR. Transactions of the American Clinical and Climatological Association. 127: 162-175, 2016.
RI Neuman, JAM van Kalmthout, DJ Pfau, DM Menendez, LH Young, JN Forrest Jr. AMP-activated protein kinase and adenosine are both metabolic modulators that regulate chloride secretion in the shark rectal gland (Squalus acanthias). American Journal of Physiology – Cell Physiology. 314(4): C473-C482, 2018.