Betaine Efflux Research Articles

Osmoregulatory fluxes of myo-inositol and betaine in renal cells.

Renal medullary cells contain high concentrations of "compatible" organic osmolytes, such as myo-inositol, betaine, sorbitol, and glycero-phosphorylcholine. These organic osmolytes accumulate as an osmoregulatory response to the high and variable interstitial NaCl concentration that is part of the urinary concentrating mechanism. Madin-Darby canine kidney (MDCK) cells in culture were previously shown to accumulate myo-inositol and betaine in response to increased NaCl. These organic osmolytes are taken up by sodium-dependent active transport into the cells from the medium. The maximum concentration is not reached until 2-4 days after an increase in medium osmolality. The purpose of this study was to characterize the response to a decrease in medium osmolality of cells that had been grown at a high osmolality. The initial response to decreased osmolality was a rapid, transient efflux of both myo-inositol and betaine from the cells. Efflux was greatest during the first 15 min and resulted in a reduction of cell myo-inositol and betaine by almost 13 and 22%, respectively, after 3 h. Active myo-inositol and betaine influx fell more slowly, reaching a lower limit after approximately 1-2 days. The reduced influx was followed by progressive decrease in cell myo-inositol and betaine to approximately 30% of the initial value after 6 days. Thus, after a decrease in medium osmolality, MDCK cell myo-inositol and betaine fell because of a rapid, transient increase in efflux and a slow, sustained decrease in active influx.

Uncoupler-stimulated oxidation of choline by rat-liver mitochondria

The main product of uncoupler-stimulated oxidation of choline by rat-liver mitochondria is betaine, which is found almost exclusively extramitochondrially. The uncoupled oxidation of choline is stimulated by intramitochondrial phosphate. The effect of intramitochondrial phosphate is to induce adenine nucleotide efflux, which in its turn allows the efflux of betaine from the mitochondria.

The efflux of betaine from rat-liver mitochondria, a possible regulating step in choline oxidation

1. Rat-liver mitochondria accumulate betaine during coupled “State 3” oxidation of choline. 2. At low salt concentrations, betaine efflux from mitochondria occurs, when also adenine nucleotides leak out of the mitochondria. 3. At high salt concentrations betaine efflux is slow. 4. It is concluded that the overall oxidation of choline by rat-liver mitochondria may be limited by the efflux of betaine.