Hyperglycemia-Induced Changes in Na⁺/myo-Inositol Transport, Na⁺-K⁺-ATPase, and Protein Kinase C Activity in Proximal Tubule Cells

In many tissues, hyperglycemia alters the activities of the Na⁺-dependent myo-inositol (Na/MI) transporter, Na⁺-K⁺-ATPase, and protein kinase C (PKC). However, little is known concerning adaptive changes in renal proximal tubular function after acute or chronic hyperglycemia. We examined hyperglycemia-induced changes in Na/MI transport, Na⁺-K⁺-ATPase activity, and PKC activity using three proximal tubule–like cell lines (JTC12, LLC-PK₁, and OK/E cells) and primary cultures of human proximal tubular epithelium (HK cells) cultured for varying periods in low- or high-glucose media, myo-Inositol (MI) transport was mediated by a high-affinity (K_m ∼50 mumol/l) Na⁺-dependent saturable process in the four cell lines. Hyperglycemia produced a time-dependent and persistent increase in Na/MI transport in all cell lines. Chronic hyperglycemia increased the K_m for MI transport in LLC-PK₁ cells and increased the Vmax in both LLC-PK₁ and JTC12 cells. Glucose competitively inhibited Na/MI transport in all low-glucose cells and in high-glucose HK, JTC12, and OK/E cells but had no effect on transport in high-glucose LLC-PK₁ cells. Acute hyperglycemia also produced time-dependent increases in Na⁺-K⁺-ATPase activity in all cell lines, a change that persisted only in HK cells. A 24-h exposure to high glucose had no effect on PKC activity in any of the cell lines but increased Ca/phospholipid-dependent PKC activity in membrane fractions from chronically high-glucose LLC-PK₁ and OK/E cells. These data suggest that hyperglycemia causes acute changes in proximal tubule function and long-lived adaptive responses in Na/MI transport and the PKC signaling pathway.