Co‐transport of glycine and sodium across the mucosal border of the midgut epithelium in the marine shrimp, Penaeus marginatus.

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Abstract
1. Influxes of glycine and Na across the mucosal border of the epithelial cells lining the midgut lumen of a marine shrimp have been examined. 2. Both mucosal JG and JNa displayed Michaelis‐Menten kinetics and the influx of each solute was markedly dependent upon the presence of the other solute in the bathing solution. A portion of JG occurred in the absence of Na, but JNa appeared to be completely dependent upon glycine. 3. Glycine influx Kt and Jmax were dependent upon [Na] while Na influx Kt and Jmax depended upon [glycine]. 4. K acted as a competitive inhibitor of Na binding to the co‐transport carrier by altering the Kt of glycine influx, but was without significant effect on the amino acid Jmax. 5. The coupling coefficient of mucosal co‐transport (JNa/JG) varied with the [Na] being 1‐1 at low [Na] (471 mM). 6. A model is proposed in which Na and glycine share ternary (NaXG) and quaternary (Na2XG) carrier complexes during their coupled influxes, but in which glycine may also traverse the mucosal membrane alone by way of an unshared binary (XG) carrier complex.

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