Effect of extracellular pH on the myo-inositol transporter SMIT expressed in Xenopus oocytes

Abstract

 The myo-inositol transporter SMIT is expressed particularly at high extracellular osmolarity and serves to accumulate the osmolyte myo-inositol. Transport of myo-inositol is coupled to the cotransport of Na⁺ and is electrogenic. In Xenopus oocytes injected with mRNA encoding SMIT but not in water-injected oocytes, myo-inositol creates an inward current that is dependent on the ambient Na⁺ concentration. The present study 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 found 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 µM 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 as 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⁺ concentra-tion from 150 mM to 50 mM significantly altered the maximal observed current and increased the apparent K _m for myo-inositol. Acidification to pH 6.5 significantly 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-mediated myo-inositol transport at least partially by decreasing the affinity of the carrier for Na⁺. The impaired Na⁺ binding subsequently decreases binding and transport of myo-inositol.