Intermediary metabolism during low temperature acclimation in the overwintering gall fly larva,Eurosta solidaginis
The levels of glycogen, lipid, protein, polyols (glycerol and sorbitol), sugars, amino acids, adenylates, and other intermediary metabolites were measured in the overwintering, third instar larvae of the gall fly,Eurosta solidaginis, sampled at specified temperatures during a controlled (1°C per day decrease) low temperature acclimation of the larvae from 15° to − 30°C.
Glycogen reserves were depleted as temperature was decreased, the decrease in glycogen fully accounting for the observed increases in glycerol, sorbitol, glucose, and trehalose in the larvae at low temperatures. Protein and total glyceride reserves of the larvae, however, were not altered during low temperature acclimation.
Temperature specific patterns of glycerol and sorbitol accumulation were found. Glycerol concentrations, which were 65% of maximum at 15°C, reached a plateau in concentration of 235 μmol/g wet wt. between 5 and 0°C. Sorbitol first appeared in larvae at 5° C and then increased in concentration rapidly as temperature decreased further to reach a plateau level of 145 μmol/g wet wt. by −10°C.
The free amino acid pool increased in size by 50% during acclimation from 15 to −5°C, this increase due largely to a 24 μmol/g wet wt. increase in proline concentration and a smaller 4.4 μmol/g wet wt. increase in alanine.
Arginine phosphate and ATP levels, as well as energy charge and the ratio [ATP]/[ADP]·[Pi], remained high and constant in larvae acclimated to temperatures as low as −5°C but in larvae acclimated to −30°C phosphagen and ATP levels had declined by 54 and 29% respectively and energy charge had decreased from 0.92 to 0.82.
The data suggest that aerobic metabolism with continued polyol synthesis is fully active in these larvae at temperatures as low as −10°C. However, below −10°C, the temperature at which hemolymph freezing takes place, mitochondria appear to be metabolically inactive. Evidence for this includes the cessation of polyol, sugar, and amino acid accumulation by this temperature and the drop in arginine phosphate, ATP, and energy charge and build-up of lactate at −30°C.
The regulation of metabolism inE. solidaginis larvae during low temperature acclimation is discussed with particular emphasis on the possible metabolic ‘switches’ regulating the flow of carbon to glycerol versus sorbitol synthesis.
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