Mitochondrial placement and function in insect ion-transporting cells

Abstract

The ion-transporting epithelia of insects possess some unusual morphological adaptations which promote close juxtaposition of mitochondria and the ion-transporting plasma membranes. A particularly striking example of this adaptation is provided by the movement of branches of mitochondria into and out of the apical microvilli in the Malpighian tubules. In the hemipteran Rhodmus prohxus, the microvilli in the resorptive lower tubule are small and contain no mitochondria during the non-transporting period. When ion transport is stimulated,either in vivo or in vitro, there is a concomitant growth in microvillar volume and surface area. In addition, branches of mitochondria enter these microvilli. It has been shown that these mitochondrial movements are driven by an actinassociated process involving the microvillar core microfilaments. The stimulation for this movement in vivo is the insect diuretic hormone. In the lepidopteran Calpodes ethhus, the rates of fluid transport which the Malpighian tubules can sustain vary during the insect's life stages. Larvae and adults show rapid transport, while pupal Malpighian tubules show none. In the larvae and adults, microvilli in the Malpighian tubules are large and contain mitochondria. In the pupae, reduced transport is associated with mitochondrial retraction and microvillar shrinkage. These ultrastructural changes appear tobe regulated by the insect's developmental hormones. In the Malpighian tubules of adult female mosquitoes of the genus Aedes, intracellular infection by microfiliarial nematodes has hen shown to cause mitochondrial retraction and reduced rates of fluid transport. A model is presented which serves to summarize currently proposed mechanisms of membrane and mitochondrial function in the ion-transporting epithelia of insects. © 1984 by the American Society of Zoologists.