Characterization of a Mg-Dependent, Na-Inositol Co-Transport Process in Cardiac Sarcolemmal Vesicles

[3H]-myo-Inositol transport into cardiac sarcolemmal vesicles is both a time- and Na+-dependent process. Transport was stimulated 3-fold by 1 μM valinomycin suggesting the process is electrogenic. myo-Inositol transport was dependent upon the presence of Mn2+ or Mg2+ but not Ca2+. Kinetic analysis revealed a high affinity transport process that was saturable (Vmax, 650 ± 71 pmoles myo-inositol/mg protein/min; Km, 37.7 ± 1.3 μM) and a low affinity process that was unsaturable up to 1.0 mM substrate. Transport was not inhibited by 1.0 mM of either L-glucose, D- or L-galactose. However, D-glucose and L-fucose at 1.0 mM were inhibitory. Higher concentrations (30 mM) of each of the sugars inhibited transport. myo -Inositol transport was also inhibited by the inositol isomers (1.0 mM), D-chiro-inositol, epi-inositol and scyllo-inositol with scyllo-inositol being most effective. Kinetic analysis established sycllo -inisitol as a competitive inhibitor of cardiac myo -inositol transport, increasing the Km for substrate three-fold (122 ± 21 μM). These data indicate that inositol transport across cardiac sarcolemma is a Mg2+-dependent Na+ co-transport process that is electrogenic and stereospecific.