Apparent Transport of Water by Insect Excretory Systems
01 Aug 1970-Integrative and Comparative Biology (The Oxford University Press)-Vol. 10, Iss: 3, pp 413-436
TL;DR: Insects are capable of producing strongly hyperosmotic urine but most species do not possess the anatomical equivalent of the mammalian kidney's couiitercurrent system, so it is currently postulated that this apparent transport of water is driven by local transport and recycling of solute within the lateral intercellular spaces of the epithelium of the rectal pad.
Abstract: Insects are capable of producing strongly hyperosmotic urine but most species do not possess the anatomical equivalent of the mammalian kidney's couiitercurrent system. Concentration of the excreta occurs in the rectum where water is absorbed against increasing osmotic gradients without strict dependence on simultaneous absorption of solute. Properties of this process are reviewed. It is currently postulated that this apparent transport of water is driven by local transport and recycling of solute within the lateral intercellular spaces of the epithelium of the rectal pad.
The most concentrated excreta so far reported are those of the mealworm, Tenebrio molitor . This species possesses a cryptonephridial complex in which the posterior end of the malpighian tubules is closely applied to the rectum and both are enclosed within a complex membranous sheath. Active transport of potassium chloride by the malpighian tubules into the complex creates a local high osmotic pressure within the complex which is responsible, in part if not completely, for removal of water from the rectal lumen. This system bears some resemblance to the countercurrent system of the mammalian kidney.
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TL;DR: This chapter examines processes which are related to three separate aspects of excretion, which are the excretion of substances that are not toxic but merely useless, and which if allowed to accumulate, would become obstructive.
Abstract: Publisher Summary Excretion may be thought of as those processes which remove substances from the metabolic pool for the negative reason that they interfere with ordered metabolism This chapter examines processes which are related to three separate aspects of excretion The first is the excretion of molecules which are undesirable, perhaps even poisonous, at all except exceedingly low concentrations The second is the excretion of molecules which are ordinarily useful or, indeed, essential to metabolism but which are present to excess Under this second heading comes excretion of water and ions, and this process is of course the outwardly directed part of osmoregulation The third process is the excretion of substances that are not toxic but merely useless Their excretion is worthwhile because if allowed to accumulate, they would become obstructive The storage of water in the rectum by larvae of Dysdercus towards the end of the feeding cycle in an instar is surprisingly the only case known of storage excretion in insects Deposit excretion, in contrast, seems to be widespread It appears that some insects have difficulty in eliminating uric acid from the body through the Malpighian tubules and rectum complex Several organs play a subsidiary role in the excretion of ions and water The anal papillae of a salt-water mosquito may act to extrude ions when the animal is in a hyper-osmotic medium Malpighian tubules secrete fluid containing many organic molecules, some of which enter the lumen passively, while others appear actively to be secreted there
241 citations
TL;DR: This chapter presents the information that has recently been obtained on reabsorptive processes and their hormonal control in those insects possessing rectal pads or papillae (i.e., papillate recta), particularly Schistocerca gregarza.
Abstract: Publisher Summary Selective reabsorption is responsible for regulatory changes in hindguts of most terrestrial insects; salt-water species experience the different problem of eliminating excess ingested salt rather than retaining water. This chapter presents the information that has recently been obtained on reabsorptive processes and their hormonal control in those insects possessing rectal pads or papillae (i.e., papillate recta), particularly Schistocerca gregarza. Insect excretory systems consist of the Malpighian tubules and the hindgut. The tubules produce primary isosmotic urine that is usually rich in KCl but low in Na + . They actively secrete harmful organic molecules (e.g., plant alkaloids) and they remove excess ions and water. However, the very large regulatory changes in the composition of the final excreta are generally achieved by selective reabsorption in the insect hindgut, particularly the rectum. This often leads to the production of either very hyposmotic or hyperosmotic urine or powder dry excreta. Insect excretory epithelia exhibit transport rates and they create concentration differences, equal to or greater than those reported for vertebrate epithelia. Given their range of habitats and “life-styles”, insects not surprisingly possess quite diverse excretory systems, but only a few hindguts have been studied in detail.
139 citations
01 Jan 1985
124 citations
TL;DR: This review is limited to mechanisms in the teleostean gill and amphibian skin because so much has been published relating to these that numerous aspects of interest can be considered, and to avoid over-simplified generalizations based on results obtained from the two vertebrate groups.
Abstract: Writing a review on transepithelial transport mechanisms within the limits imposed here is possible only if a choice of certain precise topics is made. We have decided to restrict ourselves to mechanisms in the teleostean gill and amphibian skin because so much has been published relating to these that numerous aspects of interest can be considered. Even among these, a choice must be made, a necessarily arbitrary one based on the personal experiences of the authors, their views of the problems involved, and their desire to express them clearly. For these reasons we have resisted the temp tation to include many references, citing only those works that are pertinent to the discussion of the problems analyzed rather than those of perhaps greatest intrinsic importance. We have limited ourselves to a study of sodium and chloride since they are the principal ions transported across the epithelia in the groups considered. Studies on the amphibian skin have greatly advanced our knowledge of transport mechanisms. The frog skin has acquired such a key position for two reasons: firstly, it can be studied with relative ease in vitro, and sec ondly, it was with this material that Ussing and his colleagues laid the rational foundations of the subject. Although skin and gill have many functional resemblances the gill has proved to be a much more difficult organ to study in vitro; research into the transport mechanisms in fish has therefore been conducted mainly in vivo. In view of this fundamental difference in approach, we have considered it wise to avoid over-simplified generalizations based on results obtained from the two vertebrate groups. We sincerely hope that readers will regard our proposed interpretations with constructive scepticism, for it is in this spirit that we have allowed our selves the liberty to express such hypotheses. Excellent reviews on the role of hormones in the regulation of hydro mineral metabolism in the lower vertebrates have recently been published. and the interested reader can refer to them for comprehensive information (7, 106, 123). In other publications concerning transport mechanisms he will find either a different approach or data complementary to ours (25, 82, 155, 178, 188).
89 citations
References
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TL;DR: The results suggest that water-to-solute coupling in epithelia is due to the ultrastructural geometry of the transport route, and a standing-gradient system can yield hypertonic fluids whose osmolarities are virtually independent of transport rate over a wide range.
Abstract: At the ultrastructural level, epithelia performing solute-linked water transport possess long, narrow channels open at one end and closed at the other, which may constitute the fluid transport route (e.g., lateral intercellular spaces, basal infoldings, intracellular canaliculi, and brush-border microvilli). Active solute transport into such folded structures would establish standing osmotic gradients, causing a progressive approach to osmotic equilibrium along the channel's length. The behavior of a simple standing-gradient flow system has therefore been analyzed mathematically because of its potential physiological significance. The osmolarity of the fluid emerging from the channel's open end depends upon five parameters: channel length, radius, and water permeability, and solute transport rate and diffusion coefficient. For ranges of values of these parameters encountered experimentally in epithelia, the emergent osmolarity is found by calculation to range from isotonic to a few times isotonic; i.e., the range encountered in epithelial absorbates and secretions. The transported fluid becomes more isotonic as channel radius or solute diffusion coefficient is decreased, or as channel length or water permeability is increased. Given appropriate parameters, a standing-gradient system can yield hypertonic fluids whose osmolarities are virtually independent of transport rate over a wide range, as in distal tubule and avian salt gland. The results suggest that water-to-solute coupling in epithelia is due to the ultrastructural geometry of the transport route.
772 citations
TL;DR: Equations are derived, utilizing the methods of irreversible thermodynamics, for the steady state flow of solvent and a single, neutral solute across a simple, non-homogeneous membrane system.
377 citations
TL;DR: Interrelationships between metabolism, NaCl transport, and water transport have been studied in an in vitro preparation of rat ileum and can be explained as a secondary effect due to its dependence on active salt transport.
Abstract: Interrelationships between metabolism, NaCl transport, and water transport have been studied in an in vitro preparation of rat ileum. When glucose is present in the mucosal solution, Na and Cl both appear to be actively transported from mucosa to serosa while water absorption is passive and dependent on net solute transport. Removal of glucose from the mucosal solution or treatment with dinitrophenol, monoiodoacetate, or anoxia inhibits active salt transport and as a result, water absorption is also inhibited. The dependence of water absorption on metabolism can be explained as a secondary effect due to its dependence on active salt transport. The relationship between salt and water transport has been discussed in terms of a model system.
315 citations
TL;DR: Application of the standing gradient hypothesis to the Malpighian tubules of Calliphora provides a model for urine formation in which the local gradients for osmotic water flow occur within the long narrow channels of the basal infolds and microvilli of the primary cells.
Abstract: The Malpighian tubules of Calliphora are described, emphasizing the possible role of surface specializations in solute-linked water transport. The tubules are composed of two cell types, primary and stellate, intermingling along the tubule length. The primary cells have long narrow basal infoldings and a microvillate luminal border, both intimately associated with mitochondria. The stellate cells have shorter and wider basal infoldings and their apical microvilli do not contain mitochondria. Application of the standing gradient hypothesis to this sytem provides a model for urine formation in which the local gradients for osmotic water flow occur within the long narrow channels of the basal infolds and microvilli of the primary cells. Stellate cells may modify the initial secretion by reabsorbing sodium.
226 citations