EFFECT OF EXTRACELLULAR PH ON THE MYOINOSITOL TRANSPORTER SMIT EXPRESSED IN XENOPUS OOCYTES

The myo-inositol transporter SMIT is expressed particularly at high ex tracellular osmolarity and serves to accumulate the osmolyte myo-inosi tol. Transport of myo-inositol is coupled to the cotransport of Na+ an d is electrogenic. In Xenopus oocytes injected with mRNA encoding SMIT but not in water-injected oocytes, myo-inositol creates an inward cur rent that is dependent on the ambient Na+ concentration. The present s tudy has been performed to elucidate the pH dependence of myo-inositol -induced currents. Therefore, Xenopus oocytes were injected with mRNA encoding SMIT and two-electrode voltage-clamp studies were performed. The myo-inositol-induced currents in oocytes expressing SMIT were foun d to have a sigmoidal dependence on the ambient pH between pH 5.5 and 8.5 with an apparent K-i of 0.21+/-001 mu M H+ and a Hill coefficient of 1.80+/-0.16. Kinetic analysis of the myo-inositol-induced currents at pH 8.0 and -90 mV holding potential revealed a Hill coefficient of 0.93+/0.07 and an apparent K-m, for myo-inositol of 0.031+/-0.003 mM a s well as a Hill coefficient of 1.64+/-0.24 and an apparent K-m of 38. 8+/-4.1 mM for Na+. A decrease of the Na+ concentration from 150 mM to 50 mM significantly altered the maximal observed current and increase d the apparent K-m for myo-inositol. Acidification to pH 6.5 significa ntly increased the apparent K-m for myo-inositol and for Na+ to 0.057/-0.005 mM and 73.9+/-4.8 mM, respectively. The Hill coefficients for myo-inositol and Na+ were not affected and remained close to 1 for myo -inositol and 2 for Na+. In summary, acidification impedes SMIT-mediat ed myo-inositol transport at least partially by decreasing the affinit y of the carrier for Na+. The impaired Na+ binding subsequently decrea ses binding and transport of myo-inositol.