AJP - Renal Physiology, Vol 258, Issue 5 1282-F1287, Copyright © 1990 by American Physiological Society
Myo-inositol does not modulate PI turnover in MDCK cells under hyperosmolar conditions
J. A. Shayman and D. Wu
Department of Internal Medicine, University of Michigan, Ann Arbor.
The relationship between free cellular myo-inositol concentration and
phosphatidylinositol turnover was evaluated in Madin-Darby canine kidney
(MDCK) cells under isosmolar and hyperosmolar conditions. MDCK cells
exposed to high extracellular sodium chloride were documented to increase
their free myo-inositol content as measured by gas-liquid chromatography in
both a time- and concentration-dependent manner. Measurement of
phosphatidylinositol, phosphatidylinositol 4-monophosphate, and
phosphatidylinositol 4,5-bisphosphate mass failed to reveal changes under
conditions where the myo-inositol concentration was more than threefold
higher compared with control conditions. CDP diacylglycerol:myo-inositol
3-phosphatidyltransferase activity, measured in plasma membranes from MDCK
cells grown under control and hyperosmolar conditions, was kinetically
similar with comparable observed Michaelis constant (Km) and maximal rate
of enzyme reaction. Moreover, the apparent Km was significantly below the
estimated intracellular myo-inositol concentration consistent with exposure
of the transferase to saturating concentrations of myo-inositol under both
conditions. Finally, bradykinin-stimulated myo-inositol trisphosphate mass
was measured by use of a competitive binding assay. Both basal and
hormone-stimulated myo-inositol 1,4,5-trisphosphate levels were not
significantly different under control and hyperosmolar conditions. These
data indicate that bulk free myo-inositol content is unlikely to regulate
phosphatidylinositol turnover and myo-inositol trisphosphate formation
under hyperosmolar conditions.
