IANOWSKI, J.P.*; O’DONNELL, M.J.: Evaluation of Na⁺:K⁺:2Cl^– cotransport across the basolateral membrane in Malpighian (renal) tubule cells of Rhodnius prolixus.

Malpighian tubules (MTs) of Rhodnius prolixus are among the most rapid fluid-transporting epithelia known. The current model of serotonin-stimulated fluid secretion in Rhodnius MT cells proposes that transport of Na⁺, K⁺, Cl^– and osmotically-obliged water involves an H⁺-ATPase, Na⁺ (or K⁺)/H⁺ exchange and Cl^– channels in the apical membrane, and bumetanide-sensitive Na⁺:K⁺:2Cl^– cotransport across the basolateral membrane. Using double-barreled ion-selective intracellular microelectrodes we measured basolateral membrane potential and intracellular activity of Na⁺, K⁺ and Cl^–. Corresponding electrochemical potentials were then calculated. The effect of Na⁺ or K⁺-free bathing saline on intracellular Cl^– activity was studied. Our results show a large haemolymph to cell directed Na⁺ electrochemical potential. In contrast, K⁺ and Cl^– electrochemical potentials indicate active transport from haemolymph to cell. Cl^– movement from haemolymph to cell is K⁺ and Na⁺ linked. Thus we conclude that Na⁺:K⁺:2Cl^– cotransporter but not a K⁺:Cl^– cotransporter would permit the movement of ions across the basolateral membrane. A ouabain sensitive Na⁺/K⁺ pump also plays a role in determining the K⁺/Na⁺ ratio in fluid secreted by serotonin-stimulated cells. In contrast, Ba²⁺-sensitive K⁺ channels do not appear to alter secreted fluid K⁺ concentrations