Effects of anion substitution on hydration behavior and water uptake of the red-spotted toad, Bufo punctatus: is there an anion paradox in amphibian skin?

Amphibians absorb water osmotically across their skins and rely on chemosen sory information from the skin to assess the suitability of hydration sourc es. The time spent with skin in contact with a moist surface provides a qua ntitative measure of their ability to perceive the ionic and osmotic proper ties of aqueous solutions. Dehydrated toads given hyperosmotic (250 mM) sol utions of NaCl or Na-gluconate showed significantly longer periods of hydra tion behavior on the gluconate solution, but they lost water osmotically wh en immersed in either solution. Similarly, dehydrated toads given 250 mM so lutions of NaCl,. Na-acetate, Na-phosphate or Na-gluconate showed a progres sively greater length of hydration time on solutions with the larger mol. w t anions. These results are consistent with the chemosensory phenomenon pre viously described in mammalian tongue as 'anion paradox'. On dilute (50 mM) solutions of NaCl or Na-gluconate, the hydration time was not different be tween anions, despite toads gaining water more rapidly when immersed in dil ute NaCl than in Na-gluconate solutions. The differing behavioral results w ith hyperosmotic and hypoosmotic salt solutions suggest that chemosensory t ransduction through toad skin involves both transcellular and paracellular pathways.