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 chemosensory information from the skin to assess the suitability of hydration sources. The time spent with skin in contact with a moist surface provides a quantitative measure of their ability to perceive the ionic and osmotic properties of aqueous solutions. Dehydrated toads given hyperosmotic (250 mM) solutions of NaCl or Na-gluconate showed significantly longer periods of hydration behavior on the gluconate solution, but they lost water osmotically when immersed in either solution. Similarly, dehydrated toads given 250 mM solutions of NaCl, Na-acetate, Na-phosphate or Na-gluconate showed a progressively greater length of hydration time on solutions with the larger mol. wt anions. These results are consistent with the chemosensory phenomenon previously described in mammalian tongue as 'anion paradox'. On dilute (50 mM) solutions of NaCl or Na-gluconate, the hydration time was not different between anions, despite toads gaining water more rapidly when immersed in dilute NaCl than in Na-gluconate solutions. The differing behavioral results with hyperosmotic and hypoosmotic salt solutions suggest that chemosensory transduction through toad skin involves both transcellular and paracellular pathways.

Salt sensitivity and hydration behavior of the toad, Bufo marinus.

Toads, Bufo marinus, were placed on laboratory tissue saturated with water or with hyperosmotic (250 or 500 mM NaCl or KCl) solutions, and their behavior was observed for 5 min. Toads placed on water initially allowed their ventral skin to touch the surface without abducting the hind limbs. During this "seat patch down" (SPD) behavior toads appeared to be evaluating the suitability of a hydration source prior to initiating "water absorption response" (WR) behavior with the hind limbs fully abducted and the ventral skin pressed to the moist surface. Toads dehydrated by more than 10% showed significantly shorter periods of SPD behavior and initiated WR behavior more frequently than did hydrated toads. Dehydrated toads placed on 250 mM NaCl initiated WR behavior in only 18% of the trials, but spent significantly more time showing SPD behavior than they did on water, indicating that this concentration is marginally acceptable to them. Recordings from spinal nerve #6 showed an increase in activity when 250 mM NaCl or KCl solutions were perfused over the outer surface of the ventral skin. The response to KCl was significantly greater than NaCl. The addition of 10 microM amiloride to 250 mM NaCl resulted in a higher frequency of WR behavior and reversibly inhibited the neural response to 250 mM NaCl. These results suggest that epithelial Na+ channels in the skin serve a sensory function in this species. Neither the hydrated nor dehydrated toads initiated WR behavior on 250 or 500 mM KCl solutions, indicating that toads have a lower tolerance of K+ than of Na+ salts.