SUGAR RELEASE AND PENETRATION IN INSECT FAT BODY: RELATIONS TO REGULATION OF HAEMOLYMPH TREHALOSE IN DEVELOPING STAGES OFHYALOPHORA CECROPIA

Abstract

1. The levels of sugar (chiefly trehalose) have been determined in haemolymph and in unrinsed fat body (corrected for adhering haemolymph) of developing stages of the silkmoth Hyalophora cecropia, as well as a few individuals of other insect species. In cecropia, the fat body trehalose level was relatively stable at about 45 millimolal in mature larvae and about 20 millimolal in pupae, while that in the haemolymph ranged from somewhat above the tissue level in larvae to less than half the tissue level in diapause pupae.2. During brief incubation in culture medium, fat body released 40-60% of its trehalose content. In larval tissue only, this release was sensitive to temperature, being blocked at 1° C. In fat body from diapausing pupae only, it was blocked by the presence of 1% or more of haemolymph protein, bovine serum albumin or polyvinylpyrrolidone in a medium containing amino acids.3. Penetration of 14C-solutes into fat body cells was measured. Trehalose and sucrose penetrated readily into larval fat body but not into pupal fat body; penetrability was restored in the early pharate adult. Glucose and glycerol distributed in 40-5% of cell water at all stages of development. Uptake was unaffected by dinitrophenol or by dilution with unlabelled solute. The data suggest that the mechanism of transport is simple diffusion restricted by molecular size.4. The partial release of internal sugar during incubation of fat body and the partial occupation of intracellular water by exogenous solutes indicate the existence of some form of compartmentation in the fat body.5. The developmental changes in retentiveness and penetrability to trehalose in cecropia fat body may explain changes in the steady state level of haemolymph sugar. In the larva, feedback inhibition of intracellular trehalose synthesis does not take place until trehalose has built up throughout the haemolymph, whereas in the diapausing pupa, the fat body retains trehalose, and feedback can occur within the cells while haemolymph trehalose remains low.