Active transport by the cecropia midgut. V. Loss of potassium transport during larval-pupal transformation.

Few serious attempts have been made to study either alterations of active ion transport during ontogenic development or the developmental consequences of altered ion environments within and around cells (see Kroeger, 1966 and Harvey & Haskell, 1966 for references). Nevertheless, to cite but a few examples of the importance of the ionic mileau for molecular activities associated with development; protein synthesis in cells and cell-free systems from bacteria has a potassium optimum (Lubin & Ennis, 1964); ribosomal structure and activity require magnesium (Tissieres & Watson, 1958); and the complexing of DNA with histone depends on ionic strength (Huang, Bonner & Murray, 1964). The isolated midgut of the silkworm Hyalophora cecropia is favourable material for studies of alterations of ion transport during development. Both the oxidative metabolism and the timing of nucleic acid and protein synthesis have been examined in relation to the well-known endocrine system of Cecropia (for references see Krishnakumaran, Berry, Oberlander & Schneiderman, 1967). Active potassium transport, which dominates active ion transport in the larval midgut, has been reasonably well characterized (Harvey & Nedergaard, 1964; Haskell, Clemons, & Harvey, 1965; Anderson & Harvey, 1966; Harvey, Haskell & Zerahn, 1967; Harvey & Haskell, 1968). The cells of the larva do not ordinarily divide but simply become larger as the insect grows. Two principal hormones, ecdysone and juvenile hormone, appear to alter the expression of genes contained within the stable cellular population, although a specific action of neither of these hormones on the midgut has ever been reported. Because synthetic ecdysone (Siddall, Cross & Fried, 1966) and juvenile hormone (see Williams, 1967, have recently become available, a rigorous assessment of their effects on in vitro ion transport has become feasible. Cecropia larvae feed on an unlimited supply of leaves from deciduous trees and at maturity metamorphose to pupae which subsequently enter a prolonged dormant state known as diapause. The lumen of the larval midgut is swamped with water, nutrient, and potassium contained within the leaves. Evidently the midgut tissue has available considerable sources of energy to prevent the entrance of potassium into the blood while nutrients are absorbed from the midgut lumen. By contrast diapausing pupae are not exposed to the high concentrations of potassium from leaves because