Summary

Using a comparative approach, the mechanisms involved in maintenance of the transmembrane K+ activity gradients in the larval body-wall muscles of two insects, Phormia terraenovae (Diptera) and Spodoptera exigua (Lepidoptera), have been investigated. Double-barrelled K+-selective microelectrodes were used to obtain simultaneous measurements of intracellular K+ activity and membrane potential, whilst ordinary microelectrodes were used to monitor input resistance. By application of a variety of general metabolic blockers, the K+ gradients in both P. terraenovae and S. exigua muscle were found to be maintained, at least in part, by a metabolic component. Differences in sensitivity to dinitrophenol of the two insects suggested that the ATP-dependence of maintenance of the K+ gradient was significantly higher in P. terraenovae than in S. exigua. Vanadate sensitivity suggested that both insects possess P-type ATPases. The K+ activity gradient in P. terraenovae muscles was also found to be ouabain-sensitive, indicating the involvement of a Na+/K+-ATPase. In contrast, the K+ gradient in S. exigua muscles proved to be totally insensitive to ouabain but sensitive to amiloride. Application of the H+/K+-ATPase-specific inhibitor SCH 28080 suggested the presence of an H+/K+ pump similar to the mammalian gastric H+/K+-ATPase in the lepidopteran muscles. P. terraenovae muscles, however, were found to be totally insensitive to this inhibitor. Using the anion (Cl-)-dependent transport inhibitors bumetanide and SITS (4-acetamide-4-isothiocyanostilbene-2,2-disulphonic acid), P. terraenovae muscles were shown not to possess a Cl--dependent K+ transport mechanism. In contrast, a bumetanide-sensitive K+/Cl- cotransporter was likely to be involved in maintenance of the K+ gradient in S. exigua muscle. An additional SITS-sensitive Cl-/HCO3- exchanger could also have some indirect involvement in K+ maintenance through regulation of the inward Cl- gradient. The results are integrated in two ionic models, one for each insect, which could account for the bulk of K+ transport in the body-wall muscles of these insects.