METABOLIC EFFECTS OF DEHYDRATION ON AN AQUATIC FROG, RANA-PIPIENS

Cellular responses to dehydration were analyzed in six organs of leopard frogs Rana pipiens. Frogs at 5 degrees C endured the loss of up to 50 % of their total body water content but water contents of individual organs were strongly defended, Skeletal muscle water content was strongly affected by dehydration, dropping from 80.7 % of wet mass in controls to 67.2 % in frogs that had lost 50 % of their total body water, However, water contents of internal organs dropped by only 3-8 % of their wet masses, Water contents of ah organs except skeletal muscle were fully restored by 24h of rehydration in water at 5 degrees C, Dehydration had no consistent effect on the protein content of five organs but in a sixth, the kidney, protein levels were elevated (by 60-72 %) at the higher levels of dehydration and during rehydration, Dehydration led to a rapid increase in glucose concentration in the liver; compared with control values of 13+/-2 nmol mg(-1) protein, levels were doubled by 12.2 % dehydration and continued to increase to a maximum of 307+/-44 nmol mg-l protein (20 mu mol g-l wet mass) in 50 % dehydrated frogs, Glucose accumulation was supported by a decrease in liver glycogen content and a parallel rise in glucose 6-phosphate levels, but not in the levels of other glycolytic intermediates, confirming that glycogenolytic flux was being directed into glucose synthesis, Blood glucose levels also increased as a function of increasing dehydration, reaching values 13.8 times higher than controls, but only the kidney and brain showed a significant accumulation of glucose over the course of dehydration, All organs (except skeletal muscle) had increased lactate levels during dehydration, particularly at 36.6 and 50 % dehydration, Severe dehydration also compromised cellular energetics, with ATP levels dropping by 44 % in liver of 50 % dehydrated frogs, However, frogs readily recovered from dehydration stress and, after rehydration for 24 h, organ lactate levels were sharply reduced relative to 50 % dehydrated frogs and liver energy status was largely restored, The hyperglycaemic response elicited by dehydration in this semi-aquatic frog mirrors the extreme hyperglycaemia seen in freeze-tolerant frogs during comparable dehydration exposure at 5 degrees C or during freezing, This suggests that the cryoprotectant response of freeze-tolerant species may have grown out of a pre-existing hyperglycaemic response to dehydration, a common stress experienced by all anurans,