Submitted on September 18, 2007
Accepted on October 11, 2007
SMIT2 mediates all myo-inositol uptake in apical membranes of
rat small intestine
Rym Aouameur1, Sandra Da Cal1, Pierre Bissonnette1, Michael J Coady2, and Jean-Yves Lapointe2*
1 Physiologie, Universite de Montreal, Montreal, Canada
2 Physique, Universite de Montreal, Montreal, Canada
* To whom correspondence should be addressed. E-mail: jean-yves.lapointe{at}umontreal.ca.
This study presents the characterization of MI uptake in rat intestine as evaluated using purified membrane preparations. Three secondary active myo-inositol (MI) cotransporters have been identified; two are Na+-coupled (SMIT1 and SMIT2) and one is H+-coupled (HMIT). Through inhibition studies using selective substrates such as D-chiro-inositol (DCI, specific for SMIT2) and L-fucose (specific for SMIT1), we show that SMIT2 is exclusively responsible for apical MI transport in rat intestine; rabbit intestine appears to lack apical transport of MI. Other sugar transport systems known to be present in apical membranes, such as SGLT1 or GLUT5, lacked any significant contribution to MI uptake. Functional analysis of rat SMIT2 activity, via 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. While these functional characteristics essentially confirmed those found in rat intestinal apical membranes, a unique discrepancy was seen between the two systems studied in that the affinity for glucose was about 40-fold higher in vesicles (Ki = 0.94 ± 0.35 mM) than in oocytes. Finally, the transport system responsible for the basolateral efflux of glucose in intestine, GLUT2, did not mediate any significant radiolabelled MI uptake in oocytes, indicating that this transport system does not participate in the basolateral exit of MI from small intestine.
