A vacuolar-type proton pump energizes K+/H+ antiport in an animal plasma membrane.

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 Cl2) or through a bumetanide-sensitive Na+/K+/2Cl2 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). Cl2 follows passively, perhaps through apical Cl2 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). To extend studies of ion transport beyond that revealed by the techniques of classical physiology, it is necessary to adopt a molecular genetic approach, in which the relevant genes are identified, characterised and mutated. However, the generalised molecular genetic dissection of the major genes responsible for a stimulus–secretion pathway in any vertebrate epithelium remains a daunting task, as the genetic tools available are relatively unsophisticated. Encouraged by previous physiological work on the larger, but less well genetically mapped, Drosophila hydei (Bertram et al. 1991; Wessing and Eichelberg, 1978; Wessing et al. 1987), we have investigated the feasibility of adapting the established tubule secretion assay to the study of tubules of larval and adult Drosophila melanogaster. The results (i) demonstrate that these Malpighian tubules, to

The malpighian tubules of Drosophila melanogaster: a novel phenotype for studies of fluid secretion and its control.

The excretory system of insects comprises the Malpighian tubules and hindgut (ileum and rectum), the functions of which are controlled by diuretic and antidiuretic hormones. Strategies that have been used for the isolation, purification and characterisation of factors (putative hormones) that influence Malpighian tubule secretion and hindgut reabsorption in vitro are reviewed, along with information about their receptors, second messengers, effects on ion transport and structure/activity relationships. This is followed by a detailed description of the immunocytochemical localisation of identified diuretic and antidiuretic factors in neurosecretory cells and in neurohaemal organs from where they might be released into the haemolymph. Consideration is then given to the evidence for these factors functioning as circulating neurohormones to regulate Malpighian tubule secretion and hindgut reabsorption in vivo , and to how their activities are integrated for the maintenance of haemolymph volume and composition. Finally, mention is made of the potential use of diuretic and antidiuretic hormone agonists and/or antagonists as novel insecticides that act by disrupting the endocrine control of the excretory system.

Insect diuretic and antidiuretic hormones

Insect Malpighian tubules secrete fluid into the lumen as part of their function as excretory organs. The underlying ion transport is, when stimulated, faster than in any other known tissue. It is driven by the activity of an H+-transporting V-ATPase situated on the luminal cell membranes. This ATPase, together with cation/H+ antiporter(s), constitutes a common cation pump which can transport sodium ions, potassium ions or both. Treatments that selectively slow cation transport across the epithelium cause the secreted fluid to become alkaline, whereas those that selectively reduce the rate of anion passage lead to secretion of acid fluid.

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INSECT MALPIGHIAN TUBULES: V-ATPase ACTION IN ION AND FLUID TRANSPORT.

Specification of pattern is fundamental to thedevelopmentofamulticellularorganism.TheMalpighian (renal) tubule of Drosophila melanogaster is a simple epithe- lium that proliferates under the direction of a single tip cell into three morphologically distinct domains. However, sys- tematic analysis of a panel of over 700 P{GAL4} enhancer trap lines reveals unexpected richness for such an apparently simple tissue. Using numerical analysis, it was possible for- mally to reconcile apparently similar or complementary ex- pression domains and thus to define at least five genetically defined domains and multiple cell types. Remarkably, the positions of domain boundaries and the numbers of both principal and secondary (''stellate'') cell types within each domain are reproducible to near single-cell precision between individual animals. Domains of physiological function were alsomappedusingtransportorexpressionassays.Invariably, they respect the boundaries defined by enhancer activity. These genetic domains can also be visualizedin vivo, both in transgenic and wild-type flies, providing an ''identified cell'' system for epithelial physiology. Building upon recent ad- vances in Drosophila Malpighian tubule physiology, the present study confirms this tissue as a singular model for integrative physiology.

Functional domains are specified to single-cell resolution in a Drosophila epithelium (Malpighian tubuleyP-elementyepithelial physiologyygene expressionyenhancer trap)

Bafilomycin A1 is a potent inhibitor of urine formation by Malpighian tubules of Drosophila hydei: Is a vacuolar-type ATPase involved in ion and fluid secretion?

P. Zimmermann

Papers

Bafilomycin A1 is a potent inhibitor of urine formation by Malpighian tubules of Drosophila hydei: Is a vacuolar-type ATPase involved in ion and fluid secretion?