Info

Ion transport

Haemocoel Hypo-osmotic absorbate

Rectal lumen

Osmotically driven

H2O H2O

Cuticle

Apical microvilli^

Mitochondria Septate junction

Septate junction Tight junction

Rectal Lumen Osmotically Driven

Figure 4.8 Fluid reabsorption by the rectal pad cells of Schistocerca gregaria. Anatomy on the left and physiology on the right: ion movements are shown by closed arrows and water movement by open arrows. Note that mitochondria are concentrated at the apical and lateral membranes.

Source: Bradley (1985).

Haemocoel Hypo-osmotic absorbate

Figure 4.8 Fluid reabsorption by the rectal pad cells of Schistocerca gregaria. Anatomy on the left and physiology on the right: ion movements are shown by closed arrows and water movement by open arrows. Note that mitochondria are concentrated at the apical and lateral membranes.

Source: Bradley (1985).

follows. With the resulting build-up of hydrostatic pressure, fluid is forced in the direction of the haemolymph, but reabsorption/recycling of ions converts it into a hypo-osmotic absorbate (Phillips et al. 1986). In contrast, the ileum lacks the elaborate lateral membranes which are necessary for ion recycling (Irvine et al. 1988), so that in this tissue the absorbed fluid cannot be hypo-osmotic to the luminal contents. In both rectum and ileum, Cl_ ions are actively transported from lumen to cell across the apical membrane. It seems unlikely that a V-ATPase drives CP transport in the locust hindgut, for reasons discussed by Phillips (1996a). Proline in the primary urine is also reabsorbed by the rectum and, in fact, is the main respiratory substrate for this tissue.

Antidiuretic factors may be broadly defined as decreasing Malpighian tubule secretion, increasing hindgut reabsorption, or inhibiting whole animal water loss (Gade et al. 1997). Characterization of neuropeptides acting on hindgut reabsorption is far less advanced than of those controlling Malpighian tubules, except in the case of locusts. The rate of faecal water loss decreases 10-fold after locusts are transferred from succulent plant material to dry diets (Loveridge 1974). One candidate for locust antidiuretic hormone is chloride transport stimulating hormone (CTSH), which acts via cyclic AMP to stimulate active CP transport and thus fluid reabsorption in the rectum. However CTSH has been only partially purified (Phillips et al. 1986). The major stimulant of ileal reabsorption in locusts is ion transport peptide (ITP), isolated from the corpora cardiaca: the complete primary structure of 72 amino acid residues was deduced from its cDNA nucleotide sequence (Meredith et al. 1996). ITP has high sequence homology with a large family of crustacean hormones, and acts via cyclic AMP to increase CP transport and apical cation conductances (Phillips et al. 1998). Recent molecular studies show that ITP homologues are widespread in insects (Coast et al. 2002). The coordinated functioning of the two parts of the locust excretory system was investigated by Coast et al. (1999). Diuretic peptides of the CRF-related and kinin families were without effect on ileum or rectum, but their synergistic action on Malpighian tubules was more rapid than ITP stimulation of hindgut tissues. This separate control and different timing of secretory and reabsorptive processes may permit initial elimination of excess water before fluid recycling predominates.

The hindgut also plays an important role in acid-base regulation (reviewed recently by Harrison 2001). Locusts recover from acute acid challenge (HCl injected into the haemocoel) by ileal and rectal transport of acid-base equivalents, but the contribution of the Malpighian tubules is minor because of proton recycling in fluid secretion (Phillips et al. 1994). Feeding state is important in the excretory capacity to remove acid loads, disturbances being less easily dealt with in starved animals (Harrison and Kennedy 1994).

Rectal complex

Another type of structural complexity occurs in the rectal complex (cryptonephric system) of many Coleoptera (both larvae and adults) and larval Lepidoptera (which as adults possess rectal pupillae). It has been best studied in the mealworm Tenebrio molitor: both structure and function were described in classic detail by Ramsay (1964). The distal (blind) ends of the Malpighian tubules are not free in the haemolymph, but are held against the rectal wall by a perinephric membrane. The use of ion-sensitive microelectrodes provided direct evidence for K+ transport into the Malpighian tubules as the driving force for reabsorption. Ion concentrations in the tubule lumen can reach values of 3.35 M K+ and 3.10 M CP in the rectal complex of Onymacris plana, even more extreme than corresponding values for T. molitor (Machin and O'Donnell 1991; O'Donnell and Machin 1991). This creates a powerful osmotic gradient which draws water from the rectal lumen first into the surrounding perirectal space and then into the tubule lumen (Fig. 4.9). Reabsorbed water progressively dilutes the highly concentrated fluid secreted by the posterior perirectal tubules, and final osmotic equilibration probably occurs as the tubules leave the rectal complex.

In beetles the rectal complex is concerned with highly efficient dehydration of faeces, and the same mechanism is responsible for water vapour absorption (see Section 4.2.4). Caterpillars normally have a high dietary water intake, but faecal water

Transported fluid

—>- KCI transport C> Water vapour absorption

Transported fluid

Perinephric membrane

Malpighian tubule

Perirectal space Rectal epidermis

Cuticle Open anus

Figure 4.9 Longitudinal section of the rectal complex of Tenebrio molitor, showing the various compartments.

Perinephric membrane

Malpighian tubule

Perirectal space Rectal epidermis

Cuticle Open anus

Figure 4.9 Longitudinal section of the rectal complex of Tenebrio molitor, showing the various compartments.

Note: Values are osmolalities (Osmol kg-1), and arrows show direction of water movement. Osmolalities decrease from posterior to anterior in all compartments. The rectal lumen is shown partially air-filled during water vapour absorption (see Section 4.2.4).

Source: Machin (1983).

content can be modulated according to water need in Manduca sexta caterpillars fed on artificial diet (Reynolds and Bellward 1989). Field observations on the same species showed that faecal water content decreases during the day to offset increased evaporative losses (Woods and Bernays 2000). Faecal water loss in caterpillars is apparently under neuroendocrine control, but the complexity of the rectal complex makes it difficult to distinguish between diuretic and antidiuretic events occurring in different compartments (Gade et al. 1997). Neurohaemal processes of the rectal nerve within the rectal complex showed immunoreactivity to both the CRF-like diuretic hormone of Manduca (Mas-DP1) and to leucokinin IV (Chen et al. 1994), and Audsley et al. (1993) showed that Mas-DP1 stimulates fluid absorption by the rectal complex (i.e. is antidiuretic), probably via a diuretic effect on the cryptonephric tubules. A single peptide can, thus, stimulate both the free segments of the tubules and the rectal complex to accomplish fluid recycling through the excretory system.

Water recycling in the midgut

In unfed locusts some of the primary urine entering the gut moves anteriorly rather than posteriorly and is reabsorbed in the midgut and caecae (Dow 1981): This may reduce the load on the hindgut, which has a relatively small surface area compared to that of the Malpighian tubules (Phillips 1981). Countercurrent flow of fluid in the midgut is important in water recycling in caterpillars (Reynolds et al. 1985) and tenebrionid beetles (Nicolson 1991, 1992), and in the latter may be a means of hydrating dry food (Terra et al. 1985). The midgut epithelium contains diffuse endocrine cells of largely unknown function (see, for example, Veenstra et al. 1995), some of which could be involved in water recycling.

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