CO-TRANSPORT OF GLYCINE AND SODIUM ACROSS MUCOSAL BORDER OF MIDGUT EPITHELIUM IN MARINE SHRIMP, PENAEUS-MARGINATUS

Influxes of glycine and Na across the mucosal border of the epithelial cells lining the midgut lumen of a marine shrimp were examined. Both mucosal JG [glycine influx rate] and JNa [Na influx rate] 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. Glycine influx Kt [concentration resulting in one-half maximal entry] and Jmax [maximum influx] were dependent upon [Na] while Na influx Kt and Jmax depended upon [glycine]. 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. The coupling coefficient of mucosal co-transport (JNa/JG) varied with the [Na] being 1.1 at low [Na] (65 mM) and 2.0 at high [Na] (471 mM). 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.