Uptake of myo-Inositol by Early-Somite Rat Conceptus: Transport Kinetics and Effects of Hyperglycemia

We have shown that myo-inositol in the cultured rat embryo is diminished whenever malformations are induced by hyperglycemia and that the malformations and reductions of tissue myo-inositol content are not corrected by aldose reductase inhibitors. This study was designed to evaluate the kinetics of myo-[³H]inositol uptake in vitro during 1-, 3-, and 24-h intervals in the 10.5-day rat conceptus (10–12 somites). We found that the equilibration between tissue and medium is relatively slow and that the concentration of free myo-inositol in tissue is only approximately threefold greater than in the medium even after 24 h. The integrated uptake of free myo-inositol by the intact 10.5-day conceptus is a saturable process with a K_m(246 ±16 μM) consistent with a low-affinity system. The net rate of accumulation into the tissue pool of free myo-inositol exceeds the rate of incorporation of the accumulated myo-inositol into lipid components. Ambient glucose inhibits net myo-inositol uptake in a concentration-dependent fashion, and the inhibition is competitive in nature. The glucose-mediated inhibitions of myo-inositol transport also compromise the concurrent incorporation of myo-[³H]inositol into lipid components, although to a lesser extent. These inhibitory effects are relatively specific for D-glucose and not replicated by equimolar additions of D-mannose or D-galactose. myo-inositol accumulation by the 10.5-day rat conceptus is also impaired by relatively specific inhibitors of D-glucose transport such as phloridzin or ouabain. Thus, our findings indicate that the concentrative uptake of myo-inositol by the early postimplantation rat conceptus occurs via a comparatively slow transport that may be Na⁺ dependent and is competitively affected by extracellular glucose. We suggest that the impairment of free myo-inositol uptake and retention by high glucose and the attendant diminution in the incorporation of the myo-inositol into phosphoinositides may contribute to the embryopathic potential of hyperglycemia.