Studies on the low temperature induced biogenesis of glycerol by adult Protophormia terranovae

Physiological and biochemical changes accompanying cold stress in the diapausing adult arctic blowfly, Protophormia terranovae, have been observed. In the laboratory, this insect survives prolonged periods at temperatures in the range of ?1°C to +4°C. Concentrations of free glycerol in excess of 10% of fresh body weight have been measured and the rate of its synthesis is greater at +1°C to +4°C than at ?1°C to 0°C. Under these conditions Protophormia also undergoes significant weight loss (up to 58% over 39 days) presumably in part due to dehydration. Its respiration rate decreases as expected when first shifted from 20°C to 0°C but the rate declines an additional 70% after exposure to 0°C for 24 hr. This lowest rate, which is then maintained, when considered with the initial faster one suggests positive thermal modulation is coupled to inverse thermal compensation during cold stress. This was not observed with nondiapausing Protophormia.Increments in free glycerol are accompanied by decreases in the insect's total glycogen reserves but upon rewarming, they return to pre-cold stress levels. While pre-stress glycogen stores are insufficient to provide for most of the free glycerol which accumulates, ingested carbohydrate present in the crop provides sufficient quantities. Studies with ¹⁴C] glucose indicate it is also metabolically active at low temperature.Neutral glyceride glycerol cannot contribute to net synthesis of free glycerol in significant amounts since the steady state concentrations present in pre-cold stressed insects decrease only slightly during cold stress. Furthermore, the specific radioactivity of acyl glyceride glycerol labelled in vivo with 2-³H] glycerol before cold stress, remains unchanged during hibernation indicating that acyl glycerides are not turning over glycerol units produced by catabolism of hexose. The results of these studies argue that carbohydrate and not lipid glycerol is the source of the free glycerol which accumulates in Protophormia at low temperatures. The relationship of the above results to possible mechanisms which should permit glycerol accumulation under aerobic conditions are discussed.