Cellular mechanisms of glucose-induced myo-inositol transport upregulation in rat mesangial cells

Abstract

Uptake of myo-inositol (MI) is necessary to maintain normal cellular phosphoinositide signaling and function. MI transport is up-regulated in the cells of diabetic rat glomeruli compared with normal rat glomeruli [C. I. Whiteside, J. C. Thompson, and J. Ohayon. Am. J. Physiol. 260 (Renal Fluid Electrolyte Physiol. 29): F138-F144, 1991]. To identify mechanisms associated with upregulation of MI transport, rat mesangial cells were cultured in high (25.6 mM) vs. normal (5.6 mM) glucose. Specific Na(+)-dependent [3H]MI uptake (> 97%), using L-[14C]glucose as the nonspecific marker, was linear for 120 min in high and normal glucose. In high glucose, compared with normal glucose, there was no change in Michaelis-Menten constant values [29.1 +/- 0.6 vs. 30.3 +/- 0.7 microM (SE)], whereas maximum velocity (Vmax) was increased (2.024 +/- 52 vs. 1.132 +/- 115 fmol.mg protein-1.min-1, P < 0.001). Mannitol (20.0 mM), used as an osmotic control, had no effect on the upregulation of MI transport. Maximum upregulation of MI transport measured by Vmax (control taken as 100%) was observed after 8 h of exposure to high glucose (222 +/- 6% above control, P < 0.0001) or galactose (20.0 mM) (194 +/- 6%, P < 0.0001) and was sustained for up to 48 h. The protein synthesis inhibitors cycloheximide (20 micrograms/ml) or actinomycin D (5 micrograms/ml), the F-actin depolymerizing agent cytochalasin D (2 micrograms/ml), and the aldose reductase inhibitor Tolrestat (0.3 mM) independently prevented glucose- or galactose-induced upregulation of MI transport.(ABSTRACT TRUNCATED AT 250 WORDS)