CHAMBERLIN, M.E.: Developmental Changes in Insect Epithelial Ion Transport and Metabolism

Like other caterpillars, the tobacco hornworm (Manduca sexta) is essentially an eating machine that consumes large amounts of food to fuel the growth and development of the subsequent two developmental stages. Commensurate with its huge appetite is a large midgut, which is involved in the digestion and absorption of food as well as the transport of inorganic ions. The larval midgut, however, is destined to be destroyed and replaced by a pupal epithelium. Within the first two to three days of the fifth and final larval instar, there is a small increase hemolymph ecdysteroids. This small increase in ecdysteroids does not induce molting, but does commit the insect to metamorphosis. This commitment is manifested on day 5 of the instar (wandering stage), as indicated by the appearance of a dorsal vessel and cessation of feeding. At the same time midgut short-circuit current and transepithelial potential decline 80% and 49%, respectively. Mitochondrial metabolism is also altered upon the onset of wandering. Mitochondria isolated from the midguts of day 5 larvae oxidize palmitoyl carnitine and succinate at rates that are 40% and 60% of the respective rates in mitochondria isolated from day 4 larvae. In addition, the membrane potential is lower in mitochondria isolated from the midguts of wandering larvae. Because these physiological and metabolic changes coincide with the onset of wandering, the midgut appears to modulated by ecdysteroids. Furthermore, the changes in midgut metabolism and ion transport occur abruptly, indicating that the larval midgut is destroyed by programmed cell death, rather than by a gradual senescence of the tissue.