Concentration of Myo-inositol in Skeletal Muscle of the Rat Occurs without Active Transport

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Abstract

The cellular uptake of nonphosphorylated myo-inositol (MI) and its incorporation into phosphoinositide in the rat epitrochlearis muscle was measured. Cellular uptake of [2-3H]MI was determined by the difference between total uptake and [2-3H]MI present in the extracellular fluid determined with [1-14C]mannitol. Cellular uptake was parabolic and directly proportional to medium MI concentrations between 25 and 3,200 μM. Saturation of a MI carrier was not evident. Moreover, uptake was not inhibited by 2 mM ouabain, 0.3 mM 2,4-dinitrophenol, or 22 mM glucose. Insulin, 100 mU/ml, was without effect on either cellular uptake of [2-3H]MI or its incorporation into phosphoinositides. In muscles that were preloaded with [2-3H]MI and then incubated in media that contained a constant amount of MI but no [2-3H]MI, 44.3% of the [2-3H]MI was released after 10 min increasing to 62.5% by 120 min. Cellular MI concentrations were 0.18 μmol/g wet tissue (four times plasma levels) in rapidly isolated and frozen epitrochlearis muscle. When muscle was incubated without MI, 48% of endogenous MI was lost rapidly. Restoration of cellular MI in 50 μM MI media occurred in two phases, a rapid uptake phase lasting 10 min and a subsequent slow phase of MI uptake.

Authors

Bruce A. Molitoris, Irene E. Karl, William H. Daughaday