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

Abstract

(i) With use of a novel experimental method, an improved estimate has been obtained for the value of the osmotic permeability (Pos) ofRhodniusMalpighian tubules, 4.3 x 10-3cm s-1osmol-1l. Corrections for unstirred layer effects permit a further 35% increase in the value ofPos, to 5.8 x 10-3cm s-1osmol-1l. (ii) In the lower Malpighian tubule,Posdecreases from 3.4 x 10-3cm s-1osmol-1l at the junction with the upper Malpighian tubule to 0.4 x 10-3cm s-1osmol-1l at the junction with the gut. In lower tubules stimulated with 5-HT,Posis reduced over most of their length by about 50% . It is suggested that reduced values ofPosin 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 ofPosin 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.