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Kinetics of Myo‐inositol transport in rat ocular lens

Myo‐inositol (MI) influx as a function of concentration in rat lens consisted of a saturable component, fit by a rectangular hyperbola, and a linear component which was more distinct at high myo‐inositol concentrations suggesting passive diffusion. The hyperbolic component was half‐maximally saturated (Kt) at 61.3 μM and had a maximal transport rate (Jmax) of 44.6 μMol/kg wet wt/h. The linear component had an apparent permeability coefficient of 1.44 × 10−6 s−1. Sorbitol, which distributed rapidly in the extracellular space (6.83 ml/100 g wet wt), also appeared to enter the intracellular space with a permeability coefficient of 1.37 × 10−6 s−1, similar to that of myo‐inositol. The influx of myo‐inositol was critically dependent on the concentration of extracellular sodium consistent with a sodium‐myo‐inositol contransport. The kinetics of influx activation by sodium suggested an apparent 2:1 coupling ratio for sodium and myo‐inositol. When potassium was used as sodium substitute, a significantly stronger influx inhibition was observed than with nondepolarizing sodium substitutes, indicating that myoinositol was driven by the electrochemicl gradient of sodium rather than the chemical gradient only. Reducing the extracellular Na concentration increased the MI concentration at which transport was half‐maximally activated, suggesting an ordered binding sequence of Na followed by MI. Myo‐inositol influx was competitively inhibited by phlorizin with an inhibitory coefficient (Ki) of 35 μM. Phloretin also was capable of inhibition but with a much lesser efficacy. Myoinositol desaturates from the lens at a rate of 0.00862 h−1. Approximately 19% of the efflux can be inhibited with phlorizin, suggesting that it represents carrier‐mediated flux. The phlorizin insensitive flux has a rate of 0.00695 h−1 or 1.93 × 10−6 s−1, similar to the Na‐independent passive influx. MI influx is due to a Na‐dependent, phlorizin‐sensitive active transport while the efflux consists largely of a phlorizin‐independent passive leakage. © 1995 Wiley‐Liss, Inc.