myo-Inositol uptake and metabolism in isolated rat liver cells.

The uptake of myo-inositol into isolated rat liver parenchymal cells and its incorporation into phosphatidylinositol were studied by isotopic and chemical means. The uptake was concentrationand time-dependent, but a maximal rate could not be defined. The intracellular concentration of myo-inositol never exceeded the extracellular concentration, and the uptake was not affected by inhibitors of mitochondrial ATP synthesis. High concentrations of D-glucose (10-100 m ~ ) and of scyllo-inositol (0.4-4 mM) inhibited the uptake of 0.1 mM myo-inositol and its incorporation into phospholipid, as did cytochalasin B (10-100 p ~ ) and phloretin (0.1-4 m ~ ) , cytochalasin D was without effect. Phloretin and cytochalasin B were also shown to inhibit myo-inositol efflux. [2-3H]myo-Inositol was incorporated into phosphoinositides (95% into phosphatidylinositol and 4% into phosphatidylinositol-4-P plus phosphatidylinositol-4,5P2) in a concentrationand time-dependent manner. The medium [3H]myo-inositol concentration for halfmaximum synthesis of phosphatidylinositol was about 0.4 m ~ . All subcellular fractions showed incorporation of isotope into phosphatidylinositol, but this was highest in the smooth endoplasmic reticulum. Uncouplers of oxidative phosphorylation caused only a slight decrease in phospholipid labeling, whereas Mn2+ (0.1-10 mM) and ethylene glycol bis( 8-aminoethyl ether)N,N,iV',W-tetraacetic acid (5 m ~ ) increased it significantly. Epinephrine (1-10 p ~ ) , vasopressin (0.1 p ~ ) , angiotensin I1 (0.1 p ~ ) , and glucagon (0.1 p ~ ) significantly stimulated (by 14-32%) the incorporation of 0.1 mM [3H] myo-inositol into phospholipid and reduced the intracellular concentration of [3H]myo-inositol. The ionophores A23187 (1 p ~ ) and valinomycin (10 p ~ ) were without effect. The effects of epinephrine were not evident before 30 min and were blocked by the a1adrenergic antagonist prazosin. It is concluded that myo-inositol uptake by liver cells is nonactive and occurs by a carrier-mediated process which is different from that for glucose. [3H]myo-Inositol taken up by liver cells is incorporated into phosphatidylinositol by a process chiefly involving the phosphatidylinosito1:myo-inositol exchange enzyme of the endoplasmic reticulum. Hormones which act on the liver mainly by mobilizing intracellular Ca2+ cause a