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Absorption of amino acids In vitro by the rectum of the desert locust (Schistocerca gregaria) Balshin, Michael


Rectal sacs of Schistocerca gregaria Forskal adults are capable of active absorption of those neutral amino acids naturally found in locust hemolymph. This absorption occurs against large electrochemical gradients, is inhibited by KCN, and is enhanced only slightly by solvent flow. Metabolic conversion of the amino acids during the translocation across the epithelium is not significant. The site of active absorption appears to be located on the apical border of the epithelium. Exit of the amino acids from the epithelium to the hemolymph may occur by passive diffusion. The absorptive process exhibits stereo-specificity, saturation kinetics, and competitive inhibition. Relative rates of absorption of different amino acids correlate approximately with their relative abundance in the hemolymph. A reciprocal correlation is found between the absorption rate and the length of the hydrocarbon side chain of the transported amino acid. The glycine transport system across the rectal wall involves a large Na⁺ dependent and a small Na⁺ independent component. Partial inhibition of glycine uptake is observed when CI⁻ or K⁺ are omitted from the incubation medium. The Na⁺ dependence of net ¹⁴C-glycine uptake into the rectal epithelium from the lumen is a consequence of both an increase in influx and decrease in efflux. The tissue concentration of Na⁺ is not affected by the presence or absence of external glycine. Net transfer of glycine across KCN poisoned rectal sacs is not reversed upon reversal of the Na⁺ concentration gradient. The present work suggests the involvement of a specific carrier in the mechanism of active transport of amino acids by the locust's rectum. It would be premature to assess, however, whether the translocation of amino acids is a primary transport process, or a secondary one coupled to net Na⁺ fluxes across the rectal wall. Nonetheless, the presence of such a mechanism would suggest that the rectum plays an important role in the regulation of amino acid levels in the hemolymph of insects.

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