DEVELOPMENTAL-CHANGES IN MIDGUT ION-TRANSPORT AND METABOLISM IN THE TOBACCO HORNWORM (MANDUCA-SEXTA)

The midgut of the tobacco hornworm (Manduca sexta) undergoes structural reorganization during the last larval stadium. Changes in midgut active ion transport and metabolism during this transition were studied by monitoring short-circuit current, maximal enzyme activities, and mitochondrial respiration. The short-circuit current of midguts from feeding caterpillars was over seven times higher than that of wandering larvae. No short-circuit current was detected in midguts from prepupae, whereas the midguts of pupae had a small but significant level of active ion transport. Paralleling the decline in active ion transport during larval development was the decline in the activity of citrate synthase, a key enzyme in oxidative metabolism. Aerobic metabolism was explored further by monitoring the oxidation of substrates by mitochondria isolated from the midguts of feeding and wandering larvae. Oxidation of all substrates, except ascorbate, was lower in mitochondria isolated from wandering larvae. These results indicate that the decline in active ion transport during larval development may be caused by a decline in mitochondrial energy production. Furthermore, this study demonstrates that the highly aerobic larval midgut is reorganized into a pupal epithelium with low rates of ion transport and oxidative metabolism.