Basolateral inositol transport by intestines of carnivorous and herbivorous teleosts

Myoinositol transport by isolated basolateral membrane vesicles was characterized from intestines of the herbivorous tilapia (Oreochromis mossambicus) and the carnivorous eel (Anguilla anguilla). [3H]myoinositol transport occurred by nonelectrogenic, facilitated diffusion independent of cation gradients and was inhibited by phloretin (Ki = 0.6 and 0.9 mM for tilapia and eel, respectively) but not by phloridzin. Kinetic analysis of myoinositol influx disclosed no differences in concentration yielding half-maximal influx or maximum influx between these species. D-Glucose inhibition of myoinositol influx was shown to be noncompetitive. Additional inhibition studies with a range of sugars demonstrated that aldohexoses in the C-1 chair conformation were preferred substrates. Myoinositol had no inhibitory effect on D-glucose transport. Preloading vesicles with myoinositol transstimulated [3H]myoinositol uptake, while the use of internal D-glucose was without effect. These basolateral data support the contention that epithelial myoinositol carriers are separate from D-hexose transport systems present on the same membrane but are modulated by hexose binding to a regulator site on the myoinositol transporter. Furthermore, the comparison of two fish species suggests that genetic dietary transport adaptation occurs on the brush-border membrane, while no such adaptation is present at the epithelial basolateral pole.