Metabolic responses to dehydration by liver of the wood frog, Rana sylvatica

Abstract

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 rapid hyperglycemia, liver glucose levels rising 3.8-fold to 90 nmol/mg protein (9.9 μmol/g wet mass) by the time that 10% of total body water had been lost. Glucose accumulation occurred at the expense of liver glycogen reserves, which fell over the course of dehydration, and was supported by a 5.8-fold increase in the activity of glycogen phosphorylase a in the liver, made up of increases in both the total phosphorylase activity expressed and the percentage of the enzyme in the active 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 active glucose synthesis but no change in the levels of fructose-1,6-bisphosphate or triose phosphates. This indicated that an inhibitory block on glycolysis at the phosphofructokinase reaction helped to promote the diversion of glycogenolysis into glucose export. When water loss exceeded 10%, cellular energetics were affected; ATP levels fell progressively between 25 and 60% dehydration, but a concomitant drop in the total 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-body dehydration in wood frogs is the activation of liver glucose synthesis and this suggests that the production of glucose as a cryoprotectant during freezing in this species is probably derived from a pre-existing amphibian volume-regulatory response to dehydration.