Intestinal glucose transport and salinity adaptation in a euryhaline teleost

Glucose transport by upper and lower intestinal brush-border membrane vesicles of the African tilapia (Oreochromis mossambicus) was characterized in fish acclimated to either freshwater of full-strength sea water. D-[3H]-glucose uptake by vesicles was stimulated by a transmembrane Na gradient, was electrogenic, and was enhanced by countertransport of either D-glucose or D-galactose. Glucose transport was greater in the upper intestine than in the lower intestine and in sea water animals rather than in fish acclimated to freshwater. Glucose influx (10-s uptake) involved both saturable and nonsaturable transport components. Sea water adaptation increased apparent glucose influx K/sub t/, J/sub max/, apparent diffusional permeability (P), and the apparent Na affinity of the cotransport system in both intestinal segments, but the stoichiometry of Na-glucose transfer (1:1) was unaffected by differential saline conditions or gut region. It is suggested that increased sugar transport in sea water animals is due to the combination of enhanced Na-binding properties and an increase in number or transfer rate of the transport proteins. Freshwater animals compensate for reduced Na affinity of the coupled process by markedly increasing the protein affinity for glucose