Ionic Permeability of Insect Epithelia

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Abstract
In general, ionic regulation will depend on active transport by epithelia and also on the permeability properties of these tissues. Passive permeability has recently been studied in the hindgut of the desert locust Schistocerca gregaria using electrophysiological and radiotracer techniques. Although locust rectum has low electrical resistance, cell membranes provide the major route for transepithelial ionic diffusion; i.e., the locust rectum is a tight epithelium. Potassium permeability (PK) is apparently regulated by luminal K and osmotic concentrations (local control), and also by the peptide hormone CTSH (chloride transport-stimulating hormone). Transepithelial resistance declines when isolated recta are exposed to CTSH or its “second-messenger” cAMP (adenosine 3′:5′-cyclic monophosphate). Cyclic-AMP also stimulates K diffusion across recta by 400%. Intracellular cable analysis indicatesthat cAMP lowers apical and basal membrane resistances (Ra and Rb, respectively) by ˜80%; however different ionic permeabilities are affected at thelumen- and hemolymph-facing membranes: The cAMP-induced decline in Ra, requires potassium whereas δRb is Cl-dependent. The actions of cAMP on active transport and passive permeability are complementary and would allow remarkably efficient control over KC1 absorption in vivo. One hypothesis is as follows: CTSH elevates intracellular cAMP concentration by stimulating adenyl cyclase. Cyclic-AMP enhances transepithelial Cl absorption by stimulating a Cl pump in the apical membrane and also by increasing the Cl permeability of the basal membrane. Passive K absorption would also increase during cAMP stimulation since Cl transport results in a more positive luminal potential, and because cAMP elevates transrectal PK. The mechanisms by which membrane permeability is regulated in insects have not yet been studied, but these might involve the modulation of ion channels by cAMP- or calmodulin-dependent phosphorylation, Ca or calmodulin binding, methylation, or insertion of new channels into the membrane.

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