METABOLIC RESPONSES TO DEHYDRATION BY LIVER OF THE WOOD FROG, RANA-SYLVATICA

The metabolic responses by the liver to the evaporative loss of up to 60% of total body water were quantified in spring-collected wood frogs , Rana sylvatica, a freeze-tolerant species. Dehydration stimulated ra pid hyperglycemia, liver glucose levels rising 3.8-fold to 90 nmol/mg protein (9.9 mu mol/g wet mass) by the time that 10% of total body wat er had been lost. Glucose accumulation occurred at the expense of live r glycogen reserves, which fell over the course of dehydration, and wa s supported by a 5.8-fold increase in the activity of glycogen phospho rylase a in the liver, made up of increases in both the total phosphor ylase activity expressed and the percentage of the enzyme in the activ e form. Analysis of changes in the levels of glycolytic intermediates in the liver over the course of dehydration showed sharp increases in glucose-6-phosphate and fructose-6-phosphate during the period of acti ve glucose synthesis but no change in the levels of fructose-1,6-bisph osphate or triose phosphates. This indicated that an inhibitory block on glycolysis at the phosphofructokinase reaction helped to promote th e diversion of glycogenolysis into glucose export. When water loss exc eeded 10%, cellular energetics were affected; ATP levels fell progress ively between 25 and 60% dehydration, but a concomitant drop in the to tal adenylate pool held the energy charge stable at 0.7-0.8 up to 35% dehydration. At extreme dehydration (50 and 60%), metabolic indicators of hypoxia stress appeared in the liver: lactate accumulated and the energy charge fell. The data show that a primary response to whole-bod y dehydration in wood frogs is the activation of liver glucose synthes is and this suggests that the production of glucose as a cryoprotectan t during freezing in this species is probably derived from a pre-exist ing amphibian volume-regulatory response to dehydration.