Ion specificity and stoichiometry of the cardiac inositol transporter

Cardiac myocytes have a high affinity. Mg(2+)-dependent, electrogenic, Na-inositol co-transporter on the plasma or sarcolemma (SL) membrane (Rubin and Hale, 1993). The ion specificity and stoichiometry of this transport process is unknown. Using bovine cardiac SL vesicles, we have determined that transmembrane movement of myo-[3H]inositol requires Na+ and is not supported by K+ or Li+. Furthermore, replacing Cl- with a non-permeant anion has no effect on inositol transport. Carrier-mediated inositol efflux indicated that efflux was Na(+)-dependent and electrogenic but independent of extravesicular Mg2+ in the efflux media. The transport stoichiometry of 1 Na+ for one inositol was derived by determining the null point of myo-[3H]inositol flux under conditions where the inside and outside concentration (ratio) of myoinositol and Na+ were controlled. The results suggest that Na+, inositol, and Mg2+ bind to the same side of the membrane for transport to occur and that the stoichiometry of Na+ and inositol transport is 1:1.