Na+-dependent uptake of 1, 5-anhydro-D-glucitol via the transport systems for D-glucose and D-mannose in the kidney epithelial cell line, LLC-PK1

1, 5-Anhydro-D-glucitol, a 1-deoxy form of D-glucose, is one of the major polyols in human and rat blood plasma, and is regarded as a sensitive marker of glycemic control in diabetic patients. Although renal tubular reabsorption of 1, 5-anhydro-D-glucitol is thought to maintain the physiological plasma level of this polyol, the mechanism of its cellular uptake has not yet been established. In the present study, the transport characteristics of 1, 5-anhydro-D-glucitol in a kidney epithelial cell line, LLC-PK1, were investigated. The uptake of 1, 5-anhydro-D-glucitol by the LLC-PK1 cell monolayers was found to be a highly Na+-dependent process. The initial uptake rate of 1, 5-anhydro-D-glucitol was inhibited by the presence of D-glucose, D-mannose and methyl-α-D-glucoside, a nonmetabolizable D-glucose analogue. D-Mannose was taken up partially by LLC-PK1 cells in a Na+-dependent manner. 1, 5-Anhydro-D-glucitol had an inhibitory effect on the uptake of both methyl-α-D-glucoside and D-mannose. Phlorizin inhibited the uptake of methyl-α-D-glucoside and 1, 5-anhydro-D-glucitol, but not of D-mannose. In contrast, phloretin inhibited the uptake of both 1, 5-anhydro-D-glucitol and D-mannose, but not the uptake of methyl-a-D-glucoside. The apparent Michaelis-Menten constant and maximum velocity values for 1, 5-anhydro-D-glucitol uptake were 29 mM and 240 pmol/mg protein/min, respectively. These findings suggest that the uptake of 1, 5-anhydro-D-glucitol across the apical mem-branes of LLC-PK1 cells is mediated by the Na+/D-glucose cotransport system and probably by the Na+/D-mannose cotransport system.