GLUCOSE ENTRY INTO RAT MESANGIAL CELLS IS MEDIATED BY BOTH NA+-COUPLED AND FACILITATIVE TRANSPORTERS

Since previous studies from our laboratory have demonstrated that increased glucose consumption by cultured rat mesangial cells is accompanied by an accelerated production of type IV and type VI collagen, we have now examined the manner by which glucose is transported into these cells. A progressive stimulation of glucose uptake by the mesangial cells was observed with increasing concentrations of NaCl so that at 145 mmol/l about twice as much glucose entered the cells as in its absence (substituted by choline chloride). Moreover, since phlorizin inhibited the NaCl-promoted uptake of glucose and this salt was found to increase the accumulation of alpha-methylglucoside in a manner which could not be duplicated by KCl or mannitol, both Na+-coupled and facilitative glucose transporters appeared to be present in the cells. K-m values of 1.93 mmol/l and 1.36 mmol/l were determined for the co-transport and facilitated transport pathways, respectively, with their V-max being 29.5 and 18.0 nmol . mg protein(-1). h(-1). Both uptake activities were found to be down-regulated by exposure of the cells to high glucose and furthermore the Na+-dependent transport could no longer be detected after about 12 passages of the cells. Hybridization of mesangial cell mRNA with cDNA probes revealed transcripts for the Na+/glucose co-transporter as well as GLUT1 and to a lesser extent GLUT4. The identification of the co-transporter in these non-polarized cells is pertinent to an understanding of the intracellular signals which can lead to the development of the diabetic glomerular lesions; in the hyperglycaemic state this carrier provides an additional route for accelerated glucose entry and furthermore by the attendant increase in Na+ flux may bring about an alteration in the ionic composition of the cell.