Active Transport by Insect Malpighian Tubules of Acidic Dyes and of Acylamides

Abstract

Insect Malpighian tubules carry out active transport of two types of organic anion: acylamides (such as p-aminohippuric acid) and sulphonates (such as indigo carmine and amaranth). There are separate mechanisms for the transport of these two classes of compounds. The degree to which these compounds are concentrated depends critically on the passive permeability of the tubule wall. In the permeable Malpighian tubules of Calliphora, small transported molecules readily escape from the tubule lumen. At low rates of fluid secretion the net rate of dye transport is thereby very much reduced. As a result the rate of dye transport in this insect depends on the rate of fluid secretion, although the processes are not rigidly linked. In the less permeable tubules of Rhodnius and Carausius, dye secretion is not affected by the rate of fluid secretion. The active transport of these two types of compounds is a means of clearing from the haemolymph the conjugated compounds which are the products of detoxication of potentially toxic products of metabolism.