Cation-dependent nutrient transport in shrimp digestive tract
Purified epithelial brush border membrane vesicles (BBMV) were produced from the hepatopancreas of the Atlantic White shrimp, Litopeneaus setiferus, using standard methods originally developed for mammalian tissues and previously applied to other crustacean and echinoderm epithelia. These vesicles were used to study the cation dependency of sugar and amino acid transport across luminal membranes of hepatopancreatic epithelial cells. 3H-d-glucose uptake by BBMV against transient sugar concentration gradients occurred when either transmembrane sodium or potassium gradients were the only driving forces for sugar accumulation, suggesting the presence of a possible coupled transport system capable of using either cation. 3H-l-histidine transport was only stimulated by a transmembrane potassium gradient, while 3H-l-leucine uptake was enhanced by either a sodium or potassium gradient. These responses suggest the possible presence of a potassium-dependent transporter that accommodates either amino acid and a sodium-dependent system restricted only to l-leucine. Uptake of 3H-l-leucine was significantly stimulated (P < 0.05) by several metallic cations (e.g., Zn2+, Cu2+, Mn2+, Cd2+, or Co2+) at external pH values of 7.0 or 5.0 (internal pH 7.0), suggesting a potential synergistic role of the cations in the transmembrane transfer of amino acids. 3H-l-histidine influxes (15 suptakes) were hyperbolic functions of external [zinc] or [manganese], following Michaelis–Menten kinetics. The apparent affinity constant (e.g., Km) for manganese was an order of magnitude smaller (Km = 0.22 μM Mn) than that for zinc (Km = 1.80 μM Zn), while no significant difference (P > 0.05) occurred between their maximal transport velocities (e.g., Jmax). These results suggest that a number of cation-dependent nutrient transport systems occur on the shrimp brush border membrane and aid in the absorption of these important dietary elements.