KCl reabsorption by the lower malpighian tubule of rhodnius prolixus: inhibition by Cl − channel blockers and acetazolamide

Iono- and osmoregulation by the blood-feeding hemipteran Rhodnius prolixus involves co-ordinated actions of the upper and lower Malpighian tubules. The upper tubule secretes ions (Na +, K +, Cl −) and water, whereas the lower tubule reabsorbs K + and Cl − but not water. The extent of KCl reabsorption by the lower tubule in vitro was monitored by ion-selective microelectrode measurement of Cl − and/or K + concentration in droplets of fluid secreted by Malpighian tubules isolated under oil. An earlier study proposed that K + reabsorption involves an omeprazole-sensitive apical K +/H + ATPase and Ba 2+-sensitive basolateral K + channels. This paper examines the effects acetazolamide and of compounds that inhibit chloride channels, Cl −/HCO 3 − exchangers and Na +/K +/2Cl − or K +/Cl − co-transporters. The results suggest that Cl − reabsorption is inhibited by acetazolamide and by Cl − channel blockers, including diphenylamine-2-carboxylate(DPC) and 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), but not by compounds that block Na +/K +/Cl − and K +/Cl − co-transporters. Measurements of transepithelial potential and basolateral membrane potential during changes in bathing saline chloride concentration indicate the presence of DPC- and NPPB-sensitive chloride channels in the basolateral membrane. A working hypothesis of ion movements during KCl reabsorption proposes that Cl − moves from lumen to cell through a stilbene-insensitive Cl −/HCO 3 − exchanger and then exits the cell through basolateral Cl − channels.