Analysis of epithelial transport by measurement of K+, Cl- and pH gradients in extracellular unstirred layers: Ion secretion and reabsorption by Malpighian tubules of Rhodnius prolixus

The pH and concentrations of K⁺ and Cl^- in the unstirred layer (USL) associated with the basolateral surfaces of the upper and lower Malpighian tubules of Rhodnius prolixus were measured using extracellular ion-selective microelectrodes. When stimulated with 5-hydroxytryptamine (5-HT) in vitro, the upper Malpighian tubule secretes Na⁺, K⁺, Cl^- and water at high rates; the lower Malpighian tubule reabsorbs K⁺ and Cl^- but not water. …