Chloride channels and action potentials
This page on a different site links to a really good YouTube action potential video that sorts of brings home the work that the mitochondria have to do to keep neurons doing their job. Wikipedia authors have deposited a lot of information on the Na+ Pump. The Na+ pumps out three sodiums for every two potassium it pumps into the cell. This results in a net positive charge on the outside of the cell and a net negative charge on the inside. This is called an electrochemical gradient, or even potential. Energy stored in a potential gradient can be used to do work.
One of the things that the potential energy gradient can be used for is tranaporting glucose into the cell against a concentration gradient. This is done via the Na+ /glucose cotransporter family members. Glucose transporter family members simply allow glucose to diffuse into the cell with the concentration gradient.
SLGLT1 is the name of the sodium glucose transporter that allows our brains to have first dibs on the glucose in our blood. Other cells in the body have only glut1 and glut4 that allow for facilitated diffusion only.
Schneider S. Inositol transport proteins.
FEBS Lett. 2015 Apr 28;589(10):1049-58. free article
SMIT1 is a Na+/myo-inositol cotransporter that transports myo-inositol that is used in phosphatidyl inositol phospholipid synthesis. The gene name, SLC5A3 was used in a search of ProteinAtlas.org. l-Frucose is also transported by SMIT and was shown to inhibit in high concentrations myo-inositol transport in competitor experiments cited by the Schneider review. The Schneider review cites literature linking SMIT1 to Alzheimer’s Disease and Down Syndrome. The gene SLC5A3, is located on chromosome 21, the chromosome that has three copies instead of two in Down Syndrome. Note: this point may be worth discussing with a physician or studying more given the bad rap that high fructose corn syrup drinks have on brain function.
Chiba Y, and others. Glucose, Fructose, and Urate Transporters in the Choroid Plexus Epithelium.
Int J Mol Sci. 2020 Sep 30;21(19):7230. PMC free article
This review confirms what I was taught in medical school physiology class: there are Na+ glucose cotransporters in the choroid plexus of the brain pumping glucose out of the blood and into the CSF making sure our brain gets first dibs on glucose. However, their presence is not as cut and dry as I was taught back then. Protein Atlas was searched for SGLT2 as is also shown in the Chiba review cartoon. Looking back, what was taught in my physiology class might have been ahead of its time and biased because one of the professor’s research was devoted to glucose transport in skeletal muscle. The Chiba review discusses many more glucose facilitative glucose transporters in the brain .
The Chiba review acknowledges that SGLT2 is primarily a transporter of the kidney used to recover glucose from the filtrate less we pee out our precious glucose. They really are not that sure if it is in the choroid plexus of our brains pumping glucose from our blood and into our CSF. They do mention other Na+/glucose cotransporters that might be doing the job. Oddly, the Chiba review makes no mention of a Na+/inositol cotransporter. We can probably mentally replace SGLT2? with the Na+/inositol cotransporter in our minds when viewing this cartoon. The really big question is whether fructose and inositol (Schneider review) compete enough to make a difference in getting myo-inositol into the brain for the purpose of making inositol phosphate lipids. The structures of myo-inositol and D-fructose are not all that close so some skepticism is warranted at this point.
