Homeostasis of the extracellular fluid in insects is achieved by a two-part excretory system. Primary urine, isosmotic to the haemolymph and usually rich in potassium, is secreted by the Malpighian tubules and subsequently modified during passage through the hindgut, although modification of its volume and composition can also occur in downstream tubule segments. Phillips (1981) and Bradley (1985) provided excellent and comprehensive reviews of the functioning of the excretory system as a whole, and a more recent account appears in a wider review by O'Donnell (1997). Hormonal control of the processes involved is covered in detail by Coast et al. (2002).
Malpighian tubules: structure and function Insect Malpighian tubules are the most intensively studied invertebrate excretory organs (O'Donnell 1997). They are slender blind-ending tubes (varying in number from 2 to 200 depending on the species), consisting of a single cell layer with basal and apical membrane foldings which greatly increase the surface area exposed to haemolymph (Bradley 1985). This simple tubular epithelium makes an excellent in vitro preparation: Malpighian tubules will survive and secrete for long periods in appropriate Ringer solutions, they respond rapidly to stimulants, and their large cells are suited to impalement with microelectrodes. Ion transport mechanisms involved in fluid secretion are reviewed by Nicolson (1993), Pannabecker (1995) and Beyenbach (1995).
The driving force for Malpighian tubule secretion, once described as a 'common cation pump', is now recognized to be a proton pump or vacuolar-type H+ -ATPase (V-ATPase) located on the apical membrane (Fig. 4.6). Its role is evident from the effects of the specific blocker bafilomycin A1 and from intracellular and luminal pH measurements (Zhang et al. 1994; Harvey et al. 1998; Beyenbach et al. 2000). Electrogenic transport of H+ into the tubule lumen establishes a proton gradient which energizes secondary transport of cations via apical K +/H + and Na +/H + antiporters. With the exception of bloodsucking insects, K+ is normally the major cation in the primary urine. The 'common
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