Proximal tubule reabsorption

In children with lead encephalopathy, proximal tubule reabsorptive defects characterized by the Fanconi syndrome have been observed [13]. The Fanconi syndrome appears when blood lead levels approach 150 //g/ dL. It is rapidly reversed by chelation therapy designed to treat the far more dangerous lead encephalopathy. The proximal tubule reabsorptive defect can regularly be induced experimentally in rats fed dietary lead [14]. In both children and experimental animals, acute lead nephropathy is consistently associated with acid-fast intranuclear inclusions in proximal tubule... 

Increased proximal tubule reabsorption of solutes and water -... 

Ociasionally. a patient may experience hypercalcemia or liypcr-uiicemia after long-term use of a thia/idc or thia/idc-like diuretic. Tliis results from diuretic-induced reduction of the pti-irem s plasma volume and a concomitant compeasulory increase in the proximal tubule reabsorption of luminal fluid and solutes. In such a situation, mote Ca and uric acid than usual will be reabsorbed proximally. The seriousness of these two adverse cffccLs depends in part on the duration and extent of the plasma volume r uction. 

Sodium is reabsorbed in the proximal and the distal tubules. A major fraction of filtered sodium and water is reabsorbed in the proximal tubule. Reabsorption is isotonic, and consequently the sodium concentration in the proximal tubule is not markedly changed. However, sodium reabsorption in the proximal tubule is not passive. A number of experiments have suggested... 

Birn, H., Willnow, T. E., Nielsen, R., Norden, A. G., Bonsch, C., Moestrup, S. K., Nexo, E., Christensen, E. I. 2002. Megalin is essential for renal proximal tubule reabsorption and accumulation of transcobalamin-B(12). Am J Physiol Renal Physiol, 282, F408-16. Burger, R. L., Schneider, R. J., Mehlman, C. S., Allen, R. H. 1975. Human plasma R-type vitamin B 12-binding proteins. II. The role of transcobalamin I, transcobalamin III, and the normal granulocyte vitamin B 12-binding protein in the plasma transport of vitamin B12.J Biol Chem, 250, 7707-13. 

Hydroxy vitamin D pools ia the blood and is transported on DBF to the kidney, where further hydroxylation takes place at C-1 or C-24 ia response to calcium levels. l-Hydroxylation occurs primarily ia the kidney mitochondria and is cataly2ed by a mixed-function monooxygenase with a specific cytochrome P-450 (52,179,180). 1 a- and 24-Hydroxylation of 25-hydroxycholecalciferol has also been shown to take place ia the placenta of pregnant mammals and ia bone cells, as well as ia the epidermis. Low phosphate levels also stimulate 1,25-dihydtoxycholecalciferol production, which ia turn stimulates intestinal calcium as well as phosphoms absorption. It also mobilizes these minerals from bone and decreases their kidney excretion. Together with PTH, calcitriol also stimulates renal reabsorption of the calcium and phosphoms by the proximal tubules (51,141,181—183). 

The kidney contains the major site of renin synthesis, the juxtaglomerular cells in the wall of the afferent arteriole. From these cells, renin is secreted not only into the circulation but also into the renal interstitium. Moreover, the enzyme is produced albeit in low amounts by proximal tubular cells. These cells also synthesize angiotensinogen and ACE. The RAS proteins interact in the renal interstitium and in the proximal tubular lumen to synthesize angiotensin II. In the proximal tubule, angiotensin II activates the sodium/hydrogen exchanger (NHE) that increases sodium reabsorption. Aldosterone elicits the same effect in the distal tubule by activating epithelial sodium channels (ENaC) and the sodium-potassium-ATPase. Thereby, it also induces water reabsotption and potassium secretion. 

Excretion via the kidney can be a straightforward question of glomerular filtration, followed by passage down the kidney tubules into the bladder. However, there can also be excretion and reabsorption across the tubular wall. This may happen if an ionized form within the tubule is converted into its nonpolar nonionized form because of a change in pH. The nonionized form can then diffuse across the tubular wall into plasma. Additionally, there are active transport systems for the excretion of lipophilic acids and bases across the wall of the proximal tubule. The antibiotic penicillin can be excreted in this way. 

Sodium SGLTl -dependent unidirectionai transporter Small intestine and kidney Active uptake of glucose from lumen of intestine and reabsorption of glucose in proximal tubule of kidney against a concentration gradient... 

Increasing calcium reabsorption from the proximal tubules in the kidney... 

Calcium is freely filtered along with other components of the plasma through the nephrons of the kidney. Most of this calcium is reabsorbed into the blood from the proximal tubule of the nephron. However, because the kidneys produce about 1801 of filtrate per day, the amount of calcium filtered is substantial. Therefore, the physiological regulation of even a small percentage of calcium reabsorption may have a significant effect on the amount of calcium in the blood. Parathyroid hormone acts on the Loop of Henle to increase the reabsorption of calcium from this segment of the tubule and... 

Phosphate, which is also freely filtered with plasma through the nephrons of the kidney, is reabsorbed into the blood from the proximal tubule. Parathyroid hormone acts on this segment to decrease phosphate reabsorption and increase the amount excreted in the urine. 

Coupled with CP reabsorption by way of transcellular (through the epithelial cell) and paracellular (in between the epithelial cells) pathways second half of the proximal tubule... 

Water reabsorption. Water is reabsorbed passively by way of osmosis from many regions of the tubule. As with sodium and chloride, 65% of the filtered water is reabsorbed from the proximal tubule. An additional 15% of the filtered water is reabsorbed from the descending limb of the Loop of Henle. This reabsorption occurs regardless of the water content of the body. The water enters the tubular epithelial cells through water channels, also referred to as aquaporins. These channels are always open in the early regions of the tubule. 

The answer is e. (Katzung, p 254.) Furosemide affects the re absorption of uric acid in the proximal tubule. It increases uric acid reabsorption... 

After initial filtration many proteins are actively reabsorbed (endocytosed) by the proximal tubules and subjected to lysosomal degradation, with subsequent amino acid reabsorption. Thus, very little intact protein actually enters the urine. 

Proximal tubule Cells of the PCT are responsible for bulk transport of solutes, with approximately 70-80% of the filtered load of sodium chloride (active processes) and water (passive, down the osmotic gradient established by sodium reabsorption) and essentially all of the amino acids, bicarbonate, glucose and potassium being reabsorbed in this region. 

Kidney medulla From the metabolic point of view the kidney is virtually two organs, the cortex and the medulla. The cortex contains the glomeruli, through which the blood is filtered, the proximal tubules and part of the distal tubules, from which ions and molecules are reabsorbed. The cortex is well supplied with blood so that ATP is generated by the oxidation of fuels. The medulla is metabolically quite different. Here the ATP is required for the reabsorption of ions from the loop of Henle. Some ATP is generated by anaerobic glycolysis, since the supply of blood, and therefore of oxygen, to the medulla is much poorer than to the cortex. This reflects control of the uptake of water and Na+ ions into the blood by the counter current mechanism. This depends on a slow flow of the blood in the capillaries. 

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