K+-dependent 3H-d-glucose transport by hepatopancreatic brush border membrane vesicles of a marine shrimp
The effects of sodium, potassium, sugar inhibitors, and membrane potential on 3H-d-glucose uptake by hepatopancreatic epithelial brush border membrane vesicles (BBMV) of the Atlantic marine shrimp, Litopenaeus setiferus, were investigated. Brush border membrane vesicles were prepared using a MgCl2/EGTA precipitation method and uptake experiments were conducted using a high speed filtration technique. 3H-d-Glucose uptake was stimulated by both sodium and potassium and these transport rates were almost doubled in the presence of an inside-negative-induced membrane potential. Kinetics of 3H-d-glucose influx were hyperbolic functions of both external Na+ or K+, and an induced membrane potential increased influx Jmax and lowered Km in both salts. 3H-d-Glucose influx versus [glucose] in both Na+ or K+ media also displayed Michaelis–Menten properties that were only slightly affected by induced membrane potential. Phloridzin was a poor inhibitor of 0.5 mM 3H-d-glucose influx, requiring at least 5 mM in NaCl and 10 mM in KCl to significantly reduce hexose transport. Several sugars (d-galactose, α-methyl-d-gluco-pyranoside, unlabeled d-glucose, d-fructose, and d-mannose) were used at 75 mM as potential inhibitors of 0.1 mM 3H-d-glucose influx. Only unlabeled d-glucose, d-fructose, and d-mannose significantly (p < 0.05) reduced labeled glucose transport. An additional experiment using increasing concentrations of d-mannose (0, 10, 25, 75, and 100 mM) showed this hexose to be an effective inhibitor of 0.1 mM 3H-d-glucose uptake at concentrations of 75 mM and higher. As a whole these results suggest that 3H-d-glucose transport by hepatopancreatic BBMV occurs by a carrier system that is able to use both Na+ and K+ as drivers, is enhanced by membrane potential, is relatively refractory to phloridzin, and is only inhibited by itself, d-fructose, and d-mannose. These properties are similar to those exhibited by the mammalian SLC5A9/SGLT4 transporter, suggesting that an invertebrate analogue of this protein may occur in shrimp.