Carrier-dependent and carrier-independent uptake of myo-inositol in cultured retinal pigment epithelial cells: activation by heat and concentration.
Transport of myo-inositol (MI) was studied in primary cultures of bovine retinal pigment epithelial (RPE) cells. At low external concentrations (0.01-1 mM), uptake appeared to follow saturation kinetics, although the reciprocal forms of the rate equations did not fit either Lineweaver-Burk or Eadie-Hofstee plots. Increasing external concentrations dramatically changed the pattern of MI entry. At two to three orders of magnitude higher than physiological concentrations, a second saturation occurred (pseudo saturation). Cells incubated with 20 microM [3H]MI for 60 min had a ratio of intracellular to extracellular radioactivity greater than or equal to 8, indicating active transport. MI transport reduction by Na+ replacement or inhibitors (phlorizin, ouabain, amiloride, KSCN, iodoacetamide, MI analogues) was greater when RPE cells were incubated with low (20-400 microM) than with high (10-20 mM) MI concentrations. Cells incubated with 20 microM MI at 53 or 65 degrees C showed increased transport (up to 560%) compared with cells at 22 degrees C. The effect on MI uptake (20 microM) of Na+ replacement also was reduced at 53 degrees C. The uptake of MI involved at least two transport systems. The major mechanism at low external MI concentrations (physiological levels) was a carrier-mediated active process. At high external MI levels, uptake occurred by a diffusion process. A lipotropic effect of MI may be responsible for this increased rate of diffusion.
Khatami
M
M
Department of Ophthalmology, Scheie Eye Institute, School of Medicine, University of Pennsylvania, Philadelphia 19104.
D000818:Animals; D002352:Carrier Proteins / Q000502:physiology*; D002413:Cations, Divalent; D002417:Cattle; D002452:Cell Count; D002478:Cells, Cultured; D004247:DNA / Q000032:analysis; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D007294:Inositol / Q000378:metabolism*; D007424:Intracellular Fluid / Q000032:analysis; D007700:Kinetics; D008055:Lipids / Q000302:isolation & purification; D010857:Pigment Epithelium of Eye / Q000166:cytology / Q000378:metabolism*; D012964:Sodium / Q000494:pharmacology
D002352:Carrier Proteins; D002413:Cations, Divalent; D008055:Lipids; D007294:Inositol; D004247:DNA; D012964:Sodium