Midgut Secretory Mechanisms

Insects are continuous (e.g., Lepidoptera and Diptera larvae) or discontinuous (e.g., predators and hematophagous insects) feeders. Synthesis and secretion of digestive enzymes in continuous feeders seem to be constitutive; that is, these functions occur continuously, whereas in discontinuous feeders they are regulated. It is widely believed (without clear evidence) that putative endocrine cells (Fig. 3I) occurring in the midgut could, like similar cells in vertebrates, play a role in regulating midgut events. The presence of food in the midgut is necessary to stimulate synthesis and secretion of digestive enzyme. This was clearly shown in mosquitoes.

Mosquitoes express constitutively small amounts of a trypsin called early trypsin. After a blood meal, early trypsins generate free amino acids and small peptides from blood proteins. These compounds are the initial signals that induce the synthesis and secretion of large amounts of late trypsins, which complete protein digestion.

Like all animal proteins, digestive enzymes are synthesized in the rough endoplasmic reticulum, processed in the Golgi complex, and packed into secretory vesicles (Fig. 4). There are several mechanisms by which the contents of the secretory vesicles are freed in the midgut lumen. During exocytic secretion, secretory vesicles fuse with the midgut cell apical membrane, emptying their contents without any loss of cytoplasm (Fig. 4A). In contrast, apocrine secretion involves the loss of at least 10% of the apical cytoplasm following the release of secretory vesicles (Fig. 4B). These have previously undergone fusions originating larger vesicles that after release eventually free their contents by solubilization (Fig. 4B). When the loss of cytoplasm is very small, the secretory mechanism is called microapocrine. Microaprocrine secretion consists of releasing budding double-membrane vesicles (Fig. 4C) or, at least in insect midguts, pinched-off vesicles that may contain a single or several secretory vesicles (Fig. 4D). In both apocrine and microapocrine secretion, the secretory vesicle contents are released by membrane fusion and/or by membrane solubilization due to high pH contents or to the presence of detergents.

Secretion by hemipteran midgut cells displays special features because the cells have perimicrovillar membranes, in addition to microvillar ones (Fig. 3H): double-membrane vesicles bud from modified (double-membrane) Golgi

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structures (Fig. 4E). The double-membrane vesicles move to the cell apex, their outer membranes fuse with the microvillar membrane, and their inner membranes fuse with the perimi-crovillar membranes, emptying their contents (Fig. 4E). Because apocrine and microapocrine mechanisms waste membrane and cytoplasm material, these mechanisms are preferred only when they present advantages over the exocytic mechanism. This occurs when a burst of digestive enzymes is needed, as in hematophagous flies after a blood meal, and when secretion occurs in a midgut region responsible for water absorption, a common situation in the anterior midgut of most insects. An exocytic mechanism in a water-absorptive region is not efficient, because the movement of fluid toward the cells would prevent uniform diffusion of the material secreted. Fluid movement has little effect on apocrine and microapocrine secretion because the enzymes are released from budded or pinched-off secretory vesicles far from cells. Since posterior midgut cells usually secrete fluid, no problem arises in the dispersion of material released by exocytosis by these cells. Microapocrine mechanisms seem to be an improvement relative to apocrine mechanisms, because they waste less material. This is consistent with the observation that apocrine mechanisms were found in less evolved grasshoppers and beetles, whereas microaprocine mechanisms were described in the more evolved moths.

See Also the Following Articles

Blood Sucking • Excretion • Feeding Behavior • Symbionts Further Reading

Chapman, R. F. (1998). "The Insects: Structure and Function." 4th ed.

Cambridge University Press, Cambridge, U.K. (See especially Chaps. 2—4). Cristofoletti, P. T., Ribeiro, A. F., and Terra, W. R. (2001). Apocrine secretion of amylase and exocytosis of trypsin along the midgut of Tenebrio molitor larvae. J. Insect Physiol. 47, 143—155. Daly, H. V., Doyen, J. T., and Purcell III, A. H. (1998). "Introduction to Insect Biology and Diversity." 2nd ed. Oxford University Press, Oxford, U.K. (See especially Chap. 15.) Dow, J. A. T. (1986). Insect midgut function. Adv. Insect Physiol. 19, 187-328.

Kerkut, G. A., and Gilbert, L. I. (eds.). (1985). "Comprehensive Insect Physiology, Biochemistry and Pharmacology," 13 vols. Pergamon Press, Oxford, U.K. (See especially Vol. 4, Chaps. 4-6.) Lehane, M. J., and Billingsley, P. F. (1996). "Biology of the Insect Midgut."

Chapman & Hall, London. Silva, C. P., Ribeiro, A. F., Gulbenkian, S., Terra, W. R. (1995). Organization, origin and function of the outer microvillar (perimicrovillar) membranes of Dysdercus peruvianus (Hemiptera) midgut cells. J. Insect Physiol. 41, 1093-1103. Terra, W. R. (1990). Evolution of digestive systems of insects. Annu. Rev.

Entomol 35, 181-200. Terra, W. R., and Ferreira, C. (1994). Insect digestive enzymes: Properties, compartmentalization and function. Comp. Biochem. Physiol. 109B, 1-62.

Terra, W. R. (2001). The origin and functions of the insect peritrophic membrane and peritrophic gel. Arch. Insect Biochem. Physiol. 47, 47-61. Vonk, H. J., and Western, J. R. H. (1984). "Comparative Biochemistry and Physiology of Enzymatic Digestion." Academic Press, London.

Wolfersberger, M. G. (2000). Amino acid transport in insects. Annu. Rev. Entomol. 45, 111-120.

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