Uric acid levels during last larval instar of Manduca sexta, an abrupt transition from excretion to storage in fat body

Abstract

Levels of uric acid in the whole body of the tobacco hornworm, Manduca sexta increased steadily for the 9 days of the fifth instar. However, concentrations in the haemolymph were lowest during the transition from the feeding stage to the wandering stage (days 3, 4), the time when there was a switch from uric acid excretion by the Malpighian tubule-hindgut system to storage in the fat body. Haemolymph volumes, determined for larvae between 2 and 6 days into the fifth instar by isotope dilution with [ 14C]-inulin, were used to calculate rates of incorporation of uric acid into Malpighian tubules and fat body of larvae injected with [ 14C]-uric acid. These labelling studies indicated that the Malpighian tubules ceased to remove uric acid from the haemolymph some time between the last 6 hr of day 3 of the fifth instar and the first 18 hr of day 4. At the same period, fat body removed significant quantities of uric acid from the haemolymph. The times of initial decreases and increases in levels of uric acid in haemolymph and fat body, respectively, indicated that storage in the fat body started before cessation of elimination via the Malpighian tubule-hindgut system.