K⁺ REABSORPTION BY THE LOWER MALPIGHIAN TUBULE OF RHODNIUS PROLIXUS: INHIBITION BY Ba²⁺ AND BLOCKERS OF H⁺/K⁺-ATPases

Active K⁺ reabsorption by the lower Malpighian tubule of the blood-feeding hemipteran Rhodnius prolixus does not involve the amiloride-sensitive K⁺/H⁺ exchangers or V-type H⁺-ATPases implicated in secretion of ions from haemolymph to lumen in the upper tubule. Amiloride, N-ethylmaleimide, 4-chloro-7-nitrobenzo-2-oxa-1,3-diazol and bafilomycin A₁ inhibit haemolymph-to-lumen secretion of Na⁺ and K⁺ by the upper Malpighian tubule, but have little or no effect on lumen-to-haemolymph reabsorption of K⁺ by the lower tubule. The effects of inhibitors of H⁺/K⁺-ATPases, including omeprazole and SCH 28080, suggest that a pump similar to the H⁺/K⁺-ATPase of the gastric mucosa is involved in KCl reabsorption. The presence of K⁺ channels in the basolateral membrane in the lower Malpighian tubule is suggested by inhibition of KCl reabsorption by basolateral but not apical application of the K⁺ channel blocker Ba²⁺, and by blockade of K⁺-dependent changes in membrane potential by Ba²⁺. It is proposed, therefore, that K⁺ is pumped from lumen to cell by an ATP-dependent pump resembling the H⁺/K⁺-ATPase of the gastric mucosa, and that K⁺ leaks from cell to bathing saline (haemolymph) via an electrodiffusive pathway (i.e. K⁺ channels).