Electrophysiology of K+ Transport by Midgut Epithelium of Lepidopteran Insect Larvae: The Transbasal Electrochemical Gradient

Abstract

Basal membrane potential (Vb) and intracellular K+ activity [(K+)i] were recorded, using microelectrodes, in isolated, superfused, short-circuited midgut of fifth instar larvae of Manduca sexta. The electrochemical gradient across the basal membrane was favourable for K+ entry as long as [K+]b was 32mequivl−1 or greater. In 20 mequiv 1−1 K+ Vb rose so that in some cells the basal electrochemical gradient was unfavourable for K+ entry. In 10 mequivl−1 K+, the basal electrochemical gradient of all cells was unfavourable for K+ entry. This result suggests that an active K+ pump may augment passive basal K+ entry. Addition of 2mmoll−1 Ba2+ to midgut resulted in substantial hyperpolarization of Vb accompanied by relatively small changes in (K+)i; the net effect was to move (K+); farther away from electrochemical equilibrium with external K+. Identification of recorded cells by ionophoretic injection of Lucifer Yellow showed that both major cell types of the epithelium (goblet and columnar cells) had similar control values of Vb and (K+)i; and responded similarly to Ba2+, suggesting the presence of effective chemical or electrical coupling between the transporting goblet cells and the non-transporting columnar cells. Hypoxia reduced transepithelial K+ transport, both in the absence and in the presence of Ba2+. In the absence of Ba2+, (K+)i; was within a few millivolts of equilibrium and the effect of hypoxia was small. In the presence of Ba2+, when (K+)i; was far from equilibrium with extracellular K+, hypoxia markedly depolarized the basal membrane. The results are compatible with the suggestion that Ba2+ partially blocks basal K+ entry, but does not directly affect the apical pump. Hypoxia inhibits the apical pump. Since the active transepithelial transport of K+ was reduced after Ba2+ treatment even though (K+)i; was unchanged, it appears that the activity of the apical pump is primarily controlled by the voltage step across the apical membrane.