Renal response of euryhaline toad (Bufo viridis) to acute immersion in tapwater, NaCl, or urea solutions

Green toads (Bufo viridis) were acclimated to either tap water, 230 mOsmol NaCl kg(-1) H2O (saline), 500 mOsmol NaCl kg(-1) H2O thigh saline), or 500 mmol L-1 urea. Renal functions for each acclimation group were studied on c onscious animals that had one ureter chronically catheterized. Reciprocal i mmersion of tap-water- and saline-acclimated toads in the opposite solution did not stress the animals osmotically, and plasma osmolality increased or decreased by no more than 15%. However, urine osmolality and ionic composi tion changed immediately and profoundly on exposure to the other solution. Exposure of tap-water-acclimated toads to saline decreased urine flow by 30 %, whereas the reciprocal immersion led to an increase of 30%. Immersion of tap-water-acclimated toads in high saline led to immediate cessation of ur ine dow whereas immersion of 500 NaCl- or urea-acclimated toads in tap wate r led to a large increase in urine now, with an overshoot that lasted 10 h (as a result of either salt or urea diuresis). Urine flow then stabilized a t a level 5-6 times higher than the value attained at high-salt environment . On immersion of 500 urea-acclimated toads in 500 NaCl, urine flow doubled , accompanied by a change in ion composition, without change in the osmolal ity. In all experimental conditions, plasma potassium concentration was mai ntained within a narrow range. The results show that the toad's kidneys con tributed efficiently both to osmo- and ionoregulation in a wide range of am bient solutions.