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Introduction to Solid Supported Membrane Based Electrophysiology
Published on: May 11, 2013
Anionic leak currents through the Na+/monocarboxylate cotransporter SMCT1.
Michael J Coady1, Bernadette Wallendorff, Francis Bourgeois
1Groupe d'étude des protéines membranaires (GEPROM Université de Montréal, C.P. 6128, succ. centre-ville, Montréal, Québec H3C 3J7, Canada. michael.coady@umontreal.ca
American Journal of Physiology. Cell Physiology
|October 30, 2009
Summary
The sodium-coupled monocarboxylate transporter 1 (SMCT1) exhibits an anionic leak current, distinct from its known function in transporting short-chain fatty acids. This leak current is significant when external sodium is reduced, suggesting a potential secondary physiological role.
Area of Science:
- Molecular biology
- Membrane transport
- Physiology
Background:
- SMCT1 (Sodium-coupled monocarboxylate transporter 1) is expressed in various epithelia, including the colon, renal cortex, and thyroid.
- Previous studies indicated reduced SMCT1 cotransport with extracellular chloride replacement and suggested an anionic leak current.
- The interaction between monovalent anions and SMCT1 cotransport and leak currents required further investigation.
Purpose of the Study:
- To re-evaluate the interaction between small monovalent anions and SMCT1 cotransport and leak currents.
- To clarify the nature of the observed anionic leak current associated with SMCT1.
- To determine the physiological relevance of SMCT1's anionic leak current.
Main Methods:
- Electrophysiological recordings to measure SMCT1 currents.
- Substitution of extracellular anions (e.g., cyclamate, MES, nitrate, iodide, bromide, chloride) to assess their effect on SMCT1.
- Varying extracellular sodium concentrations and membrane potential to analyze leak current properties.
Main Results:
- The apparent chloride dependence of SMCT1 cotransport was due to inhibition by cyclamate, not chloride itself; other anions like MES function as substrates.
- Observed outward leak currents represent anion influx, favoring larger anions (NO3- > I- > Br- > Cl-), and are significant at reduced external Na+.
- The leak current exhibits outward rectification, dependent on external Na+, which blocks the current, especially at positive membrane potentials.
Conclusions:
- SMCT1 possesses a distinct anionic leak current, separate from its monocarboxylate transport function.
- This leak current is modulated by external sodium concentration and membrane potential.
- The identified anionic leak current may represent a novel physiological role for SMCT1 in relevant tissues.
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