Equilibration of Hexose Concentration in Erythrocytes from Normal and Diabetic Rats

The uptake and efflux of radioactivity was monitored in rat erythrocytes exposed to or prelabeled with either 3-O-methyl-D-[U-¹⁴C]glucose or D-[U-¹⁴C]glucose. In the case of 3-O-methyl-D-[U-¹⁴C]glucose uptake, the half-life for equilibration of hexose concentration between the extracellular medium and the intracellular 3H₂O space increased from 12.5 to 32.5 min as the concentration of the D-glucose analog was raised from 8.3 to 33.3 mM. In erythrocytes preincubated for 60 min with 3-O-methyl-D-[U-¹⁴C]glucose at the latter two concentrations, the half-life for the fall in cell radioactive content, after correction for the residual radioactivity recovered in the cells after prolonged incubation, amounted to respectively 6.5 and 9.5 min. Thus, whether in terms of uptake or efflux, the equilibration of hexose concentrations across the plasma membrane represented a delayed phenomenon. In this respect, there was no difference between erythrocytes from control or diabetic rats. Comparable conclusions were reached in the study of influent and effluent radioactivity in rat erythrocytes exposed to or prelabeled with D-[U-¹⁴C]glucose. In such a case, the measurement of radioactive acidic metabolites and L-lactic acid indicated that, even after 90 min incubation, the concentration of the hexose in the intracellular 3H₂O space remained lower than its extracellular concentration. It is proposed that the delayed equilibration of hexose concentrations accounts, in part at least, for an artifact of isotopic dilution in the study of radioactive D-glucose metabolism by erythrocytes from control or diabetic rats.