High-energy turnover at low temperatures: recovery from exhaustive exercise in Antarctic and temperate eelpouts.

Earlier work on Notothenioids led to the hypothesis that a reduced glycolytic capacity is a general adaptation to low temperatures in Antarctic fish. In our study this hypothesis was reinvestigated by comparing changes in the metabolic status of the white musculature in two related zoarcid species, the stenothermal Antarctic eelpout Pachycara brachycephalum and the eurythermal Zoarces viviparus during exercise and subsequent recovery at 0 degreesC. In both species, strenuous exercise caused a similar increase in white muscle lactate, a drop in intracellular pH (pHi) by about 0.5 pH units, and a 90% depletion of phosphocreatine. This is the first study on Antarctic fish that shows an increase in white muscle lactate concentrations. Thus the hypothesis that a reduced importance of the glycolytic pathway is characteristic for cold-adapted polar fish cannot hold. The recovery process, especially the clearance of white muscle lactate, is significantly faster in the Antarctic than in temperate eelpout. Based on metabolite data, we calculated that during the first hour of recovery aerobic metabolism is increased 6.6-fold compared with resting rates in P. brachycephalum vs. an only 2.9-fold increase in Z. viviparus. This strong stimulation of aerobic metabolism despite low temperatures may be caused by a pronounced increase of free ADP levels, in the context of higher levels of pHi and ATP, which is observed in the Antarctic species. Although basal metabolic rates are identical in both species, the comparison of metabolic rates during situations of high-energy turnover reveals that the stenothermal P. brachycephalum shows a higher degree of metabolic cold compensation than the eurythermal Z. viviparus. Muscular fatigue after escape swimming may be caused by a drop of the free energy change of ATP hydrolysis, which is shown to fall below critical levels for cellular ATPases in exhausted animals of both species.