Active transport of myo-inositol in rat pancreatic islets.

myo-Inositol transport by isolated pancreatic islets was measured with a dual isotope technique. Uptake was saturable with a half-maximal response at approx. 75 microM. With 50 microM-inositol, uptake was linear for at least 2 h during which time the free intracellular concentration rose to double that of the incubation medium. Inositol transport is therefore active and probably energized by electrogenic co-transport of Na+ down its concentration gradient as uptake was inhibited by ouabain, Na+ removal or depolarizing K+ concentrations. Inositol transport was abolished by cytochalasin B which binds to hexose carriers, but not by carbamoylcholine or Li+ which respectively stimulate or inhibit phosphoinositide turnover. Uptake of inositol was not affected by 3-O-methylglucose or L-glucose (both 100 mM) nor by physiological concentrations of D-glucose. The results suggest that most intracellular inositol in pancreatic islets would be derived from the extracellular medium. Since the transport mechanism is distinct from that of glucose, inositol uptake would not be inhibited during periods of hyperglycaemia.