Abstract
This study presents the characterization of MI uptake in rat intestine using purified membrane preparations (BBMv). The two Na+‐coupled myo‐inositol (MI) cotransporters identified (SMIT1 and SMIT2) can be differentiated through inhibition studies using the selective substrates D‐chiro‐inositol (DCI, specific for SMIT2) and L‐fucose (specific for SMIT1). Results show that SMIT2 is exclusively responsible for apical MI transport in rat intestine. Other sugar transport systems present in apical membranes (SGLT1 and GLUT5) lacked any significant contribution to MI uptake. Functional analysis of rat SMIT2 activity determined using electrophysiological studies in Xenopus oocytes, demonstrated similarities to the activities of SMIT2 from species (rabbit and human), displaying high affinities for MI (0.150 ± 0.040 mM), DCI (0.31 ± 0.06 mM) and Pz (0.016 ± 0.007 mM), low affinity for glucose (36 ± 7 mM) and no affinity for L‐fucose. Electrophysiological studies essentially confirmed those found in rat intestinal BBMv with exception of glucose affinity which was about 40‐fold higher in vesicles (Ki = 0.94 ± 0.35 mM) when compared to oocytes. Finally, GLUT2 expressed in oocytes did not mediate any significant radiolabelled MI uptake, indicating that this transport system does not participate in the basolateral exit of MI from small intestine.
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