Salt and water balance in the toad Bufo viridis during recovery from two different osmotically stressful conditions

Toads, Bufo viridis, were subjected to two modes of osmotic stress: on soil of water potential approximately 5 atm and continuous partial immersion in 250 mmol/l NaCl solution. In both conditions, plasma osmolality was greatly elevated involving a large increase in urea concentration and was maintained hyperosmotic to the external environment. After acclimation to either condition, toads were allowed access to tap water, and the concentrations of body fluids and gross body weight were followed over a 7-day period. The toads bathed only until preacclimation gross weight was regained, although plasma osmolality remained elevated. Weight remained stable thereafter. Excess plasma Na+ and Cl- were eliminated within a few days, whereas the urea level diminished very slowly. K+ was closely controlled throughout, both during acclimation and recovery. The results suggest that B. viridis is equipped with a series of set points for osmotic pressure that enable it to maintain different steady states according to the prevailing conditions, the magnitude of the shift depending on the level of the imposed osmotic stress.