SMIT2 mediates all myo-inositol uptake in apical membranes of rat small intestine

Abstract

Aouameur R, Da Cal S, Bissonnette P, Coady MJ, Lapointe J-Y. SMIT2 mediates all myo-inositol uptake in apical membranes of rat small intestine. Am J Physiol Gastrointest Liver Physiol 293: G1300-G1307, 2007. First published October 11, 2007; doi:10.1152/ajpgi.00422.2007.-This study presents the characterization of myo-inositol (MI) uptake in rat intestine as evaluated by use of purified membrane preparations. Three secondary active MI cotransporters have been identified; two are Na ϩ coupled (SMIT1 and SMIT2) and one is H ϩ 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 other species (rabbit and human) displaying high affinities for MI (0.150 Ϯ 0.040 mM), DCI (0.31 Ϯ 0.06 mM), and phlorizin (Pz; 0.016 Ϯ 0.007 mM); low affinity for glucose (36 Ϯ 7 mM); and no affinity for L-fucose. Although 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 constant for glucose was ϳ40-fold lower in vesicles (Ki ϭ 0.94 Ϯ 0.35 mM) than in oocytes. Finally, the transport system responsible for the basolateral efflux transporter of glucose in intestine, GLUT2, did not mediate any significant radiolabeled MI uptake in oocytes, indicating that this transport system does not participate in the basolateral exit of MI from small intestine. brush border; glucose; transport; oocytes; phlorizin THE PHYSIOLOGICAL IMPORTANCE of myo-inositol (MI) is generally considered to reflect its role in signal transduction as a precursor to phosphoinositides and inositol phosphates. In addition, MI acts as a "compatible osmolyte" in specific tissues, such as brain and kidney medulla, where variations in milieu osmolarity may threaten normal cell function. In response to an increase in osmolarity, intracellular MI concentration may rise up to 500-fold above its plasma concentration of ϳ30 M (9, 11, 12), where it prevents the accumulation effects of high ionic concentrations, which leads to DNA degradation (11a). This has been well documented in brain where conditions such as trauma (30), edema and hypernatremia (26,27,38) have been shown to increase MI levels. To reach such high intracellular concentrations, secondary active transport systems are required for MI.

Functional Characterization of Pneumocystis carinii Inositol Transporter 1

“…Standard treatments of this neurological disease include lithium salts, (e.g., lithium chloride), carbamazepine, and valproate (26). Studies have hypothesized both that lithium can inhibit myo- inositol biosynthesis and that it can decrease the activity of high-affinity myo- inositol transport, along with 2 other bipolar therapies, valproate and carbamazepine, based on studies in cultured astrocytes (26). We evaluated the effects of these drugs on the uptake of radiolabeled myo- inositol (Fig.…”

Effects of hyperosmolarity on the Na+-myo-inositol cotransporter SMIT2 stably transfected in the Madin-Darby canine kidney cell line

“…Preliminary assays have shown that linearity of uptake was maintained for ϳ60 min under all conditions used throughout this study (tracer or saturating conditions, isotonic and hypertonic media), which validated the chosen uptake time of 30 min. Discrimination between SMIT1 and SMIT2 activities was performed, as presented elsewhere (1,4,31), using specific uptake conditions. This strategy, which compares activities in the absence of inhibitor (total uptake) to those in the presence of either 100 mM L-fucose (blocking condition for SMIT1) or saturating MI (10 mM, blocking both SMIT1 and SMIT2), enables a functional isolation of both SMIT1 (subtracting L-fucose condition from total) and SMIT2 (subtracting saturating condition from Lfucose).…”

Expression of mRNA for glucose transport proteins in jejunum, liver, kidney and skeletal muscle of pigs

“…The solute carrier family 5 (SLC5A) comprises of six Na + -glucose cotransporters (sodium glucose-linked transporters, SGLT; ref. 54), of which SGLT5 has not been functionally characterised so far and SGLT6 is, in fact, an intestinal and renal Na + -myo-inositol transporter (therefore renamed SMIT2) with very low affinity to glucose (1,29). The second family of glucose transporters is SLC2A, comprising of 13 members (GLUT1- GLUT 13); the last member (GLUT13) again being a myo-inositol transporter (renamed HMIT; ref.…”