Measurements of osmotic permeability in the Malpighian tubules of an insect, Rhodnius prolixus Stål

Abstract (i) With use of a novel experimental method, an improved estimate has been obtained for the value of the osmotic permeability (Pos) of RhodniusMalpighian tubules, 4.3 x 10-3 cm s-1 osmol-1 l. Corrections for unstirred layer effects permit a further 35% increase in the value of Pos, to 5.8 x 10-3 cm s-1 osmol-1 l. (ii) In the lower Malpighian tubule, Pos decreases from 3.4 x 10-3 cm s-1 osmol-1 l at the junction with the upper Malpighian tubule to 0.4 x 10-3 cm s-1 osmol-1 l at the junction with the gut. In lower tubules stimulated with 5-HT, Pos is reduced over most of their length by about 50% . It is suggested that reduced values of Pos in the lower portions of the lower tubule prevent water movements accompanying 5-HT-stimulated KCl resorption over the same area of the tubule. (iii) Values of Pos in the upper segment of the upper tubule are discussedin relation to proposed mechanisms for coupling water movements to ion transport during isosmotic secretion by the tubule.