ABSTRACT
The insect renal (Malpighian) tubule has long been a model system for the study of fluid secretion and its neurohormonal control (Maddrell, 1981; Maddrell and O’Donnell, 1992). Classical physiology suggests a model for tubular secretion of iso-osmotic fluid in most insects, in which ions are thought to enter basally either through a series of ion channels (Na+, K+ and Cl−) or through a bumetanide-sensitive Na+/K+/2Cl− cotransport. Apical fluxes are energised by a plasma-membrane H+-pumping V-ATPase, driving secretion of Na+ or K+ through one or more exchangers, at least one of which is amiloride-sensitive and appears to be closely similar to the Na+/H+ exchanger of vertebrates (Maddrell and O’Donnell, 1992). Cl− follows passively, perhaps through apical Cl− channels. Water follows the major ions, and haemolymph solutes diffuse across the tubule wall passively via a paracellular route. There are also transcellular active transport processes for certain metabolites or toxins, such as acylamides (Maddrell et al. 1974) and plant alkaloids (Maddrell, 1976; O’Donnell et al. 1983).
