Specificity of glucose transport inhibitors in the frog lens

The unidirectional fluxes (influx and efflux) of glucose across frog lens membranes were investigated using the radio-labelled sugar analogue 3-O-methyl-D-[U-14C]glucose. The effect of various inhibitors of sugar transport on the movement of 3-O-methylglucose was studied and the specificity of the inhibition was estimated by carrying out concomitant measurements of lens sodium content. The movement of 3-O-methylglucose into the lens was rapid, and 50% equilibration occurred within 5 hr. The influx was reduced in the presence of phloretin, phloridzin, ouabain, iodoacetate and cytochalasin B, but only in the case of the latter was there no concomitant change in lens sodium content. Only cytochalasin B can therefore be regarded as a specific inhibitor of glucose transport. The efflux of 3-O-methylglucose was followed after 16 hr incubation with [14C]3-O-methylglucose. The efflux kinetics had a double exponential form and the half-time of the slower phase was 165 mins. The efflux of the slow phase was found to be sensitive to the presence of inhibitors. Phloretin and cytochalasin B produced the most marked reduction in efflux rate, but again only in the latter case was the effect reversible. Bidirectional transport of glucose therefore occurs in the lens and movement in both directions is reduced by cytochalasin B, which appears to be the only inhibitor of transport so far studied that does not disturb lens ion levels.