Dynamics of haemolymph sorbitol-6-phosphate and its control by ecdysteroid in the larvae of the silkworm, Bombyx mori

Abstract

Developmental changes in the concentration of sorbitol-6-phosphate (S-6-P) and trehalose in the Bombyx haemolymph were determined from fourth larval stadium to adult eclosion by gas-liquid chromatography. In the fourth stadium, haemolymph trehalose concentration transiently decreased after ecdysis and then gradually increased from day 1 to day 3, followed by an abrupt increase immediately before the last larval ecdysis. The trehalose concentration decreased again after the last larval ecdysis, recovered to a level similar to that which was found on days 1–3 of the fourth stadium, and remained relatively constant during the feeding period of the fifth stadium. Concomitant with the onset of the wandering stage, the trehalose concentration decreased to a low level, but gradually increased again during the pupal period. Haemolymph S-6-P concentrations were always higher than trehalose concentrations during the feeding period of the fourth and fifth stadia and 2.5-fold higher immediately before spinneret pigmentation. S-6-P concentration began to decrease to a low level (less than 1 mM) concomitantly with the occurrence of spinneret pigmentation, one day earlier than the onset of the wandering stage. In contrast to the slow increase in the trehalose concentration after the wandering stage, the S-6-P concentration remained low throughout the pupal period. After the dilution of larval haemolymph by the replacement of larval haemolymph with insect Ringer's solution, or an artificial increase in haemolymph S-6-P and trehalose concentration by an injection of each of S-6-P and trehalose, S-6-P concentration did not recover to the level before manipulation, whereas trehalose concentration became similar to the initial level in a few hours, indicating that S-6-P concentration is not under homeostatic control, in contrast to trehalose. Injections of ecdysone or 20-hydroxyecdysone induced a decrease in the haemolymph S-6-P concentration in a dose-dependent manner. These findings suggest that S-6-P accumulates in the haemolymph during the feeding period, as a storage form of sugar and ecdysteroid is involved in the sharp decrease in haemolymph S-6-P concentration at the end of the feeding period in Bombyx larvae.