Effects of High Glucose Levels on the Protein Kinase C Signal Transduction Pathway in Primary Cultured Renal Proximal Tubule Cells

Effects of High Glucose Levels on the Protein Kinase C Signal Transduction Pathway in Primary Cultured Renal Proximal Tubule Cells

Diabetes mellitus is associated with a wide range of pathophysiologic changes in the kidney. This study was designed to examine the mechanisms by which glucose modulates the expression of polarized membrane transport functions in primary cultured rabbit renal proximal tubule cells. Results are as follows: The rate of 30 minute Rb⁺ uptake was significantly higher(137.76±5.40%) in primary renal tubular cell cultures treated with 20 mM glucose than that of 5 mM glucose. Not the level of mRNA for the α subunit of Na, K-ATPase but that of β subunit was elevated in primary cultures treated with high glucose. The initial rate of methyl-α-D-glucopyranoside(α-MG) uptake was significantly lower(71.91±3.02%) in monolayers treated with 20 mM glucose than that of 5 mM glucose. There was a tendency of an increase in phlorizin binding site in cells treated with 5 mM glucose. However, 3-O-methyl-D-glucose(3-O-MG) uptake was not affected by glucose concentration in culture media. TPA inhibited Rb⁺ uptake by 63.61±1.94 and 45.80±1.36% and α-MG uptake by 48.54±3.69 and 41.87±6.70% in the cells treated with 5 and 20 mM glucose, respectively. Also TPA inhibited mRNA expression of Na/glucose cotransporter in cells grown in 5mM glucose medium. cAMP significantly stimulated α-MG uptake by 114.65±5.70% in cells treated with 5mM glucose, while it did not affect α-MG uptake in cell treated with 20 mM glucose. However, cAMP inhibited Rb⁺ uptake by 76.69±4.16 and 66.87±2.41% in cells treated with 5 and 20 mM glucose, respectively. In conclusion, the activity of the renal proximal tubular Na,K-ATPase is elevated in high glucose concentration. In contrast, the activity of the Na/glucose cotransport system is inhibited. High glucose may in part affect the activity of the Na,K-ATPase and the Na/glucose cotransport system by controlling the protein kinase C and/or A signal transduction pathway in primary cultured renal proximal tubule cells.