Bicarbonate ion transport: a mechanism for the acidification of urine in the turtle.

The uricotelic turtle Pseudemys scripta acidifies the urine to a pH as low as 4 in the urinary bladder. Data in this report show that the mechanism of acidification in this bladder is the transport of bicarbonate ion from lumen to serosa, and that the temperature to which the turtles are adapted prior to the in vitro experiment largely determines the direction of the transmural carbon dioxide gradient observed. This temperature effect also serves to reconcile apparently disparate data that were previously reported. A new technique for the direct determination of the partial pressure of carbon dioxide was employed.