Na(+)-D-glucose cotransport by intestinal BBMVs of the Antarctic fish Trematomus bernacchii

Intestinal nutrient absorption in fish adapted to low temperature was investigated by isolating, with a Mg(2+)-precipitation procedure, brush-border membrane vesicles (BBMVs) from intestines of the Antarctic teleost Trematomus bernacchii. In particular, D-glucose transport was analyzed by measuring both 1) fluorescence changes of the electrical potential-sensitive dye 3,3'-diethylthiadicarbocyanine iodide [DiS-C2(5)] and 2) intravesicular uptake of D-[14C]glucose. Results demonstrated that transport of D-glucose across intestinal BBMs of the Antarctic fish is stimulated by the presence of a transmembrane Na+ gradient (out > in) and was specifically inhibited by phloridzin. Furthermore, Na(+)-dependent D-glucose uptake was strongly enhanced by the presence of an electrical potential (inside-negative) across the membrane. There was a marked difference in temperature dependence of Na(+)-sugar cotransport between the Antarctic fish and a temperate fish, such as the European yellow eel., Na(+)-dependent D-glucose uptake in T. bernacchii intestinal BBMV reached its maximal rate at -2-0 degree C (close to fish living temperature) and was exponentially inactivated by incubation at higher temperatures. Kinetic analysis of D-glucose influx indicated the presence of a single Na(+)-dependent carrier process (apparent maximal carrier-mediated influx = 0.233 +/- 0.009 nmol.mg protein-1.min-1; apparent half-saturation constant for carrier-mediated influx = 0.157 +/- 0.026 mmol/l) and a nonsaturable transfer component (apparent diffusional permeability of membrane to the sugar = 0.233 +/- 0.016 microliter.mg protein-1.min-1). The Na(+)-dependent carrier-mediated mechanism was specific for sugars, since it was partially inhibited by the presence in the extravesicular medium of other monosaccharides, but not by ascorbic acid or amino acids of different types. These data suggest that in the intestine of Antarctic fish luminal D-glucose transport takes place by a specific Na(+)-dependent electrogenic secondary active transport working well at subzero temperatures.