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

(i) With use of a novel experimental method, an improved estimate has been obtained for the value of the osmotic permeability (P$_{\text{OS}}$) of Rhodnius Malpighian tubules, 4.3 $\times $ 10$^{-3}$ cm s$^{-1}$ osmol$^{-1}$ l. Corrections for unstirred layer effects permit a further 35% increase in the value of P$_{\text{OS}}$, to 5.8 $\times $ 10$^{-3}$ cm s$^{-1}$ osmol$^{-1}$ l. (ii) In the lower Malpighian tubule, P$_{\text{OS}}$ decreases from 3.4 $\times $ 10$^{-3}$ cm s$^{-1}$ osmol$^{-1}$ l at the junction with the upper Malpighian tubule to 0.4 $\times $ 10$^{-3}$ cm s$^{-1}$ osmol$^{-1}$ l at the junction with the gut. In lower tubules stimulated with 5-HT, P$_{\text{OS}}$ is reduced over most of their length by about 50%. It is suggested that reduced values of P$_{\text{OS}}$ 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 P$_{\text{OS}}$ in the upper segment of the upper tubule are discussed in relation to proposed mechanisms for coupling water movements to ion transport during isosmotic secretion by the tubule.