Better known as a delicacy in the Far East, the Chinese softshell turtle has generated some interest in the Nephrology world because of its unusual means of urea elimination. A paper was recently published in the Journal of Experimental Biology in which the authors described the process of urea excretion in these interesting animals. The primary habitat of the turtle is seawater, and saltwater marshes and swamps. As a result, most of the water in that environment is hyperosmolar such that eliminating urea via the kidneys could lead to excessive water losses. It had been previously noted that the turtles have a tendency to submerge their heads in water for prolonged periods. The researchers attached plastic bags to the cloacae of the turtles but, to their surprise, found that the urea concentration in the water where they submerged their heads was higher that that of the collected urine.
The turtles have buccopharyngeal villiform processes in their mouths that contain active urea transporters. They take water into their mouths and spit it out again in order to excrete urea waste. A transporter was isolated from the buccal mucosa that had a 70% homology with mouse and human UT-A2. This transporter was not present in the kidneys of the turtles indicating a lesser role for the kidneys in urea transport. The average urea concentration of the saliva was 36mmol/L as opposed to 2.4mmol/L in the serum. After an IP injection of a urea load, the serum concentration increased to 45mmol/L while the saliva concentration increased to an impressive 614mmol/L.
This evolutionary adaptation may have allowed them to invade this hostile environment.