Glycerol metabolism in a freeze-tolerant arctic insect: an in vivo 13C NMR study.

Freeze-tolerance in larvae of Gynaephora groenlandica is enhanced by the accumulation of glycerol in the winter. Since summer larvae remain freeze-tolerant despite the lack of glycerol, we investigated glycerol metabolism as a function of acclimation and body temperature using noninvasive 13C NMR spectroscopy. Major constituents of hemolymph isolated from cold- and warm-acclimated larvae were identified with the aid of standard NMR spectra and confirmed by TLC and GLC. Spectra obtained on live, warm-acclimated larvae showed the presence of lipids, glycogen, glucose, trehalose and amino acids. Similar spectra of cold-acclimated or previously frozen larvae showed the additional presence of glycerol. In vitro time-lapse 13C spectra of D-[1-13C]glucose added separately to hemolymph or extracted fat body tissue showed that glycerol is synthesized from glucose in the fat body tissue and distributed to the peripheral tissue via hemolymph. In vivo time-lapse 13C spectra of cold- and warm-acclimated larvae were obtained after injection with D-[1-13C]glucose to monitor the production of labeled metabolic intermediates and end-products. [13C]Glycerol was produced between -30 degrees C and 30 degrees C but accumulated only below 5 degrees C. Above 5 degrees C glycerol was degraded and the 13C label incorporated mainly into glycogen. The mechanism underlying temperature control of glycerol biosynthesis and degradation may provide a clue to the role of glycerol in enhancing freeze-tolerance in these insects.