The Permeability of the Cuticular Lining of the Insect Alimentary Canal

The layer of cuticle which lines the foregut and hindgut of insects has the obvious function of mechanical protection for the underlying epithelial cells. This is of particular importance in herbivorous insects. However it is also important that it should have permeability properties appropriate to the function of the different regions of the gut that it lines. Phillips & Dockrill (1968) showed that the cuticular intima of the rectum of Schistocerca gregaria selectively restricts permeation; only small molecules penetrate it with any ease. The lining was more or less impermeable to molecules with a radius of more than 0·5·0·6 nm. Such a restricted permeability is important because the rectum reabsorbs virtually all the water from the contents of the lumen. This leads to a high concentration of substances in the remaining fluid. Some of these substances are toxic and at high concentrations they would interfere with the action of the rectal cells, on whose reabsorptive abilities the well-being of the insect vitally depends. The rectal cuticle denies such toxins access to the rectal epithelium.

It has been supposed that the mode of operation of the insect excretory system is based on non-selective collecting of substances into a primary excretory fluid by the Malpighian tubules followed by reabsorption of useful molecules in the rectum. If this were the case, then difficulties would arise if the Malpighian tubules were more permeable to dissolved substances than is the rectum, as some useful substances committed to the primary excretory fluid might not then be recoverable. In the locust Schistocerca gregaria, it seems that such a mismatching does occur. Maddrell & Gardiner (1974) showed that the Malpighian tubules of this insect are moderately permeable to such compounds as the trisaccharide maltotriose (MW 504) and inulin (MW 5200), whereas Phillips & Dockrill found the rectal intima to be virtually impermeable to substances of molecular weight greater than 400. However, it must be remembered that the fluid derived from the Malpighian tubules passes through the anterior hindgut before entering the rectum (Fig. 1 ). Perhaps absorption of relatively large substances might occur there. In the present paper, we have investigated the cuticular lining of the walls of the hindgut anterior to the rectum to see if the perm eability is higher than that of the rectum. For comparative purposes we have also examined the permeability properties of the cuticle lining the foregut (crop) and the cuticular intima of the colon, a short region of the hindgut between the main length of the anterior hindgut, the ileum, and the rectum (see Fig. 1 ). In addition we report on the permeability of the gut cuticle of adults of two species of cockroach, Grompha-dorhina portentosa and Leucophaea maderae and of the larvae of a Lepidopteran, Manduca sexta.

Adult Schistocerca gregaria Forskal of average weight 2·2 g were taken 1·2 weeks after their last moult from a laboratory culture maintained on a diet of cabbage and rabbit pellets. Laboratory-reared adult Gromphadorhina portentosa Schaum and Leucophaea maderae Burmeister, and 5th instar larvae of Manduca sexta Johannson were also used.

The experimental preparations of cuticle were prepared by removing the entire gut and allowing it to soak overnight in distilled water, after which the cuticular lining could easily be separated from the cells of the gut by peeling the two apart. An alter native pretreatment was to freeze the insects at - 20 °C; particularly with the cock roaches, the subsequent separation of the cuticle from the underlying epithelium was simple. The two techniques were found to give identical results. It was not possible to effect separation in freshly killed material.

The cuticular lining, in the form of a hollow tube, was pulled over a piece of poly-thene tubing of the appropriate diameter and tied in place with a silk ligature. The free end was then tied off so that the polythene tube carried a closed sac of cuticular material at its end. The other end of the tube was then fitted to a stainless-steel T piece as is shown in Fig. 2. A second narrower polythene tube attached to a fluid reservoir was then passed down the tube so that its end lay not more than I mm above the bottom of the preparation. Fluid was passed down this tube at a rate which could be controlled by altering the height of the reservoir. In the majority of the experiments this rate was kept at about 180 µl min⁻¹. The preparations were normally kept im mersed in distilled water until required. In use, each preparation was first tested for leakage by passing through a solution of amaranth, a deeply coloured red dye (MW 535). Following this, a K-rich saline was perfused through the inner polythene tube. The rapid removal of dye showed that circulation of fluid in the cuticular sac had left no perceptible unstirred regions. A K-rich saline was used as an approximation to the composition of the fluid that the hindgut cuticle might experience in vivo in contact with its luminal face. It had the following composition (mm); K+ 138, Na+ 14, Ca²+ 2, Mg²+ 8·5, Cl⁻ 159, HCO₃⁻ 10, H₂PO⁻₄ 4, and glucose 34; the pH of the solution was 6·7. The perfused preparation was then lowered into a small vial with 1·2 cm³ of saline in it, containing one of a series of radioactive test substances. The bathing saline was iso-osmotic to the K-rich saline and contained (mm); Na+ 143, K+ 9, Ca²+ 2, Mg²+ 8·5, Cl⁻ 159, HCO₃⁻10,H₂ PO₄,⁻ 4, and glucose 34 (pH 6·7). Samples of the effluent perfusate were first taken at about 2 min intervals to find out how long it took for its radioactive content to reach a steady state. In no case did this take more than 6 min. A period of at least 10 min was allowed in all following experiments. After this initial period, samples of the perfusate were taken for determination of the permeability of the cuticular sac to the particular substance under test. Samples were either taken every few minutes, in which case the radioactive content of the entire sample was determined, or the perfused fluid was allowed to collect for, say, 20 min and a 200 µl aliquot was removed for counting. Even with the most permeant substance used the perfusate never contained radioactivity at more than 1 % of the concentration in the bathing fluid. Back diffusion of the isotope can thus be ignored.

The permeability of a cuticular sac to a particular test substance was calculated from the equation P = C/ Nat, where C is the total number of radioactive counts collected in time t (s), N is the concentration of radioactive material in the bathing fluid (cpm cm⁻³), and a is the area of the cuticular sac (cm²). Initially, measurement of the area of cuticle in a preparation was carried out in two ways. First the length and diameter of the sac were measured with an ocular micrometer and the area of cuticle calculated assuming the sac to be cylindrical. Next the sac was cut off the polythene tube and the piece of cuticle distal to the bottom ligature was cut off. The sac was then cut longitudinally and laid out as a flat sheet on a measuring grid. Its dimensions were determined, again with an ocular micrometer, and its area determined. The two methods gave such closely similar results that in all later experiments only the first method was used.

The cuticular lining of the crop in Schistocerca is so extensive (its area is about 4 cm2) that measurements were made on only a short section of it taken about half way along its length.

One difficulty we had in our earlier experiments is worth recording. [³H]inulin appeared to cross the cuticular sacs at surprisingly high rates. This was found to be due to trace levels of radiolabelled impurity which did not appear in subsequent chromatographic checks for purity. We believe that tiny amounts of tritium label may have exchanged with hydrogen in the solvent water molecules. With such a slowly permeating substance as inulin, even a few extra counts from such a spurious source can greatly affect the results. In the experiments described we report only the results obtained with ¹⁴C-labelled inulin.

Table I summarizes our findings on the permeability of the cuticle lining the ali-mentary canal of adult Schistocerca. Included in the table are corresponding figures from the earlier work of Phillips & Dockrill (1968). On the important point of the relative permeabilities of the cuticle lining the anterior hindgut and rectum, our results show that the cuticle lining the anterior hindgut is scarcely more permeable to most of the test molecules than is the cuticle lining the rectum.

Compared with the values determined by Phillips & Dockrill, our results agree in showing only negligible permeability to substances as large as inulin, but we found the rectal cuticle to be somewhat more permeable for most of the other substances investigated. The differences are not large, but we made some attempts to find a factor which might explain them. One possibility was that the locusts we used were 1·2 weeks after their final moult, whereas Phillips & Dockrill used insects some 2 weeks older. To see if this factor could cause differences, we repeated some of our experiments on older locusts, The results (Table 2) showed that there was no significant difference in the permeability of rectal cuticle taken from young or old adult insects. Another possibility was that our experiments were done using a saline of near neutral pH (6·7) and those of Phillips & Dockrill at pH 5·5. We therefore investigated the permeability of rectal cuticle using alternately the same more acid solution used by Phillips & Dockrill and the K-rich near-neutral saline as before. The results are set out in Table 3 and show that acid conditions do somewhat increase the permeability to negatively charged molecules and depress the permeability to substances carrying a net positive charge. However, the permeability to more nearly neutral substances such as glycine, glucose, and inulin is not noticeably changed.

The permeability of the cuticle lining the foregut we found to be very low (Table 1). Interestingly, the permeability of the cuticular lining of the colon was also low, about 15 times lower than the cuticle lining either the ileum, which lies anterior to it, or the rectum, posterior to it,

As Phillips & Dockrill (1968) pointed out, it is advisable to test permeability at more than one concentration of a test substance to rule out the possibility that there might be a non-linear relationship between flux rate and concentration. In the cases of five test molecules and using two concentrations differing by about two orders of magnitude we checked this point on cuticle from colon, foregut and ileum. The results (Table 4) show that in no case was the permeability value for a test molecule significantly different at the different concentrations.

We were surprised to find (Table 1) that the permeability of the cuticle lining the ileum was apparently not noticeably higher than that of the rectal lining. In case a small difference had been lost in the variation due to slight differences in age and/or the physiological state of locust used, we compared the permeability of sacs of cuticle taken from the ileum and rectum of the same insect (Table 5). For most substances, the cuticle lining the ileum did show apparently slightly higher permeabilities, but in only two cases, those of urea and Cas+ ions, was the difference significant. Evidently, the cuticle lining the ileum and rectum is very similar in its permeability properties.

Table 6 sets out the results of determinations of the permeability of cuticle lining different regions of the foregut and hindgut in these other species. The number of determinations made are not large so that it would be unwise to draw other than general conclusions from the results. Nonetheless it is clear that, in the cockroach species, the lining of the foregut is as little permeable as it is in Schistocerca, and that even in Manduca sexta, it is considerably less permeable than the cuticle of the hind-gut. In Manduca, the permeability of the cuticle lining the ileum differs little from that lining the rectum. It is thus similar in this respect to the cuticle lining the hind-gut of Schistocerca. The cuticle lining the ileum in the two cockroach species appears to be rather less permeable than that of Manduca and Schistocerca. Finally, the hind gut cuticle in Manduca does not show the great difference in permeability to Ca²+ and SO₄² - ions that is apparent in Schistocerca.

The results presented in this paper show, unexpectedly, that in Schistocerca the permeability of the cuticle lining the ileum is not significantly more permeable than that lining the rectum. This was surprising because, as emphasized earlier (p. 228), it would appear that useful substances appearing in the primary excretory fluid elaborated by the Malpihigan tubules could not readily be reabsorbed through cuticle of a low permeability. Three factors reduce this difficulty. First, in our experiments the permeability of the cuticle lining the ileum and rectum was significantly higher than Phillips & Dockrill (1968) found for the rectum. A second and more important point is that, as recently shown (Knowles, 1975), some insect Malpighian tubules actively reabsorb sugars such as glucose and trehalose from the lumen. It may well be, therefore, that the concentration of trehalose (the most important carbohydrate constituent of the haemolymph) in the fluid entering the midgut is already very low, so that difficulties of absorption through the cuticle may not be important. Thirdly, the other important organic constituents of the haemolymph are amino acids and lipids. The presence of amino acids in the primary excretory fluid does not cause the same difficulties as do larger molecules. This is because they can readily penetrate the hindgut cuticle and can therefore be reabsorbed from the lumen as, indeed, Balshin & Phillips (1971) have shown does occur. Lipids in circulation, which are of great importance to the locust as the main fuel for flight (Weis-Fogh, 1952), avoid being excreted by being circulated, not as free diglyceride, but in the form of lipoprotein, a conjugate of diglyceride with protein (Gilbert, 1967). Such large molecules would not cross the walls of the Malpighian tubules of the locust at significant rates (Maddrell & Gardiner, 1974).

The colon (Figs. 1, 3) is rather narrower in diameter in Schistocerca than is the ileum or rectum and is S-shaped in the dorsoventral direction. Our findings show that its cuticular lining is remarkable in that it is very much less permeable than is the Jining of the ileum or rectum. The functions of the colon include that of breaking up the continuous flow of material from the ileum. To do this, the colon first constricts the cylinder of materials in the lumen and then, by bending, breaks the peritrophic membrane between the separated pieces (Goodhue, 1963). This presumably yields suitable lengths of material which can then be individually processed by the rectum prior to elimination as faecal pellets. After these manoeuvres, the colon contains no solid matter and very little fluid. Presumably, then, reabsorption of useful substances does not occur in the colon and our finding that its cuticle has a greatly reduced permeability is entirely in accord with its known function.

Our findings on the permeability of the cuticle lining the foregut and hindgut of Schistocerca are summarized in Fig. 3.

In the locust Schistocerca, the cockroaches Gromphadorhina and Leucophaea, and in Manduca the cuticle lining the crop has a very low permeability. This presumably explains earlier qualitative findings that the crop wall in several insect species has a low permeability (Treherne, 1957, 1958, 1967). Such a low permeability allows this region of the gut to be used as a temporary store for ingested material very different in its osmotic concentration from the haemolymph. The rate at which the crop con tents are passed on to the much more permeable midgut is regulated in a manner dependent on the osmotic concentration of the contents (Treherne, 1957). It seems reasonable to propose that this would serve to minimize the effects of osmotic water movements between the haemolymph and the lumen of the midgut.

From his work on the cuticular lining of the rectum of Schistocerca, Phillips has concluded that it behaves as if it contained water-filled pores of radius 0·6·0·7 nm lined with negative charges having a pK of about 4 (Phillips, 1977). Our results are consistent with this proposal. For example, we find (Table 1) that the cuticle is almost impermeable to inulin which has an equivalent hydrated molecular radius of about 1·2 nm (Durbin, 1961). In addition, we have found that the cuticle lining the hindgut of Schistocerca is very much more permeable to cations than it is to anions (Tables 1, 3, and 5). This suggests that the cuticle carries a preponderance of fixed negative charges. Finally we find that although changing the pH from 6·7 to 5·5 (Table 3) decreases the permeability of the cuticle to cations and increases it to anions, the effects are small, as they would be if the pK of the fixed charges were about 4.

The significance of the differential permeability to anions and cations may lie in the facts that metabolically produced anions are important subjects for excretion in insects (Smith, 1962) and that insect Malpighian tubules possess active transport systems capable of accelerating their removal from the haemolymph (Maddrell et al,1974). In an insect such as Schistocerca that concentrates the contents of the hindgut very much (Phillips, 1964), the low permeability of the hindgut cuticle to anions is appropriate in that it reduces the tendency for excreted anions to be returned to the haemolymph.

Larvae of Manduca which feed on a diet relatively rich in water do not need to resorb water to the same extent. Substances in the hindgut lumen presumably do not therefore become so concentrated and so a low permeability to anions is not so obviously an advantage. It is perhaps not surprising then that the hindgut cuticle of Manduca does not in fact show nearly such a large difference in its permeability to Ca²+ and SO₄²-ions as does that of Schistocerca.