Neurohypophysial Hormones and Homeostasis in the Crab-Eating Frog, <i>Rana cancrivor</i><i>a</i>

Abstract

Mechanisms of homeostasis in a euryhaline amphibian, Rana cancrivora, the crab-eating frog, are reviewed and discus-sed. While plasma osmolality of R. cancrivora is always greater thanthe bathing fluid (from 0 to 800 mOsmole), urine is always dilutewith reference to plasma. Homeostasis is maintained by the factthat the skin, though permeable to water, is insensitive to the actionof neurohypophysial hormones, the potential differences, short circuit current and net influx of Na+ across it being smaller than inother anuran amphibians. The hypertonicity of the plasma when the frog is in saline solutions is achieved by an increase of plasma NaCl and urea concentration. The mechanism by which plasma urea concentration increases from 40 mM when the frog is in fresh water to 400 mM when in brackish water can be explained by an initial diffusion of urea from the bladder, followed by an enhanced activity of the enzymes of the ornithine-urea cycle. Rapid diffusion of urea into muscles protects the latter against damage due to hypertonicity of the plasma and extracellular fluid. Neurohypophysial hormones appear to act mainly on the bladder where they appear to increase the permeability to urea. The gastrointestinal tract does not play a major role in the mechanism of adaptation to hypertonic solutions. The significance of the different mechanisms used by R. cancrivora to maintain homeostasis are discussed in relation to similar problems in other anuran amphibians.

Diet of Rana cancrivora in fresh water and brackish water environments

“…Gordon, Schmidt-Nielsen & Kelly (1961) showed this, using frogs collected from brackish water; and Colley, Rowe, Huggins, Elliott & Dicker (1972) showed it in frogs collected from fresh water. It has been pointed out by Dicker & Elliott (1973) that whether the frogs are collected from fresh water or from brackish water, the plasma urea level is always higher than in other anuran amphibians; and Chew & Elliott (1971), and Chew, Elliott & Wong (1972) have discussed the function of urea when the rain or tide changes the salinity of the environment of the frog. It is clear therefore that an adequate intake of nitrogenous food will be necessary for R. cancrivora, so that it may have the necessary urea for osmoregulation.…”