Renal medulla, hypertonicity

Concentration of urine is made possible by generation of osmotic gradients in the renal medulla. This involves a counter-current process which pumps ions without water from the loops of Henle into the medullary interstitial fluid. When the filtrate passes down the collecting ducts lying parallel to the loops of Henle, water passes out of the ducts along an osmotic gradient, provided ADH is present. A hypertonic urine is therefore produced. 

Once choline has entered the cell, its normal fate is rapid phosphorylation by choline kinase (Fig. 3). In neurons choline is also converted to the neurotransmitter, acetylcholine. Choline can also be oxidized to betaine [-00C-CH2-N (CH3)3] in the liver and kidney. In liver, betaine is an important donor of methyl groups for methionine biosynthesis. Betaine is produced in mitochondria into which choline is transported by a specific transporter on the inner membranes. Next, choline is oxidized to betaine aldehyde by choline dehydrogenase on the inner leaflet of the mitochondrial inner membranes and the subsequent conversion to betaine is catalyzed by betaine-aldehyde dehydrogenase in the mitochondrial matrix. Betaine can be transported into kidney medulla by a betaine transporter. In renal medulla and many plants and organisms, betaine accumulates as an osmolyte (a small organic solute that accumulates in response to hypertonicity without adverse effects to the cell or organism) (J.S. Handler, 1992). Hypertonicity of the renal medulla is important for the kidney s ability to concentrate urine. 

The more distal portions of the collecting duct pass through the renal medulla, where the interstitial fluid is markedly hypertonic. In the absence of vasopressin, the collecting duct system is impermeable to water, and a dilute urine is excreted. In the presence of vasopressin, the collecting duct system is permeable to water, which is reabsorbed down a steep concentration gradient that exists between the tubular fluid and the medullary interstitium. 

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