Kidney cortical collecting tubule

Ling BN, Eaton DC. Cyclosporin A inhibits apical secretory K-i-channels in rabbit cortical collecting tubule principal cells. Kidney Int 1993 44 974-984. 

The most predominant primary effects of mineralocorticoids are adequately observed upon the cortical collecting tubule cells strategically positioned in the kidneys to enhance substantially sodium reabsorption vis-a-vis potassium secretion. This eventually leads to an elevated aldosterone titer values that actually governs, controls, and monitors effectively sodium retention and potassium depletion thereby giving rise to volume expansion and weight gain, metabolic alkalosis, and hypertension. 

A detailed model of the slow processes in principal cells of the cortical collecting tubule of the mammalian kidney was buQt by Tang and Othmer [1996] and includes sodium, potassium, and chloride channels a sodium-potassium pump a calcium pump a sodium-potassium-chloride cotransporter and a sodium-calcium exchanger. 

The effectiveness of thiazide and loop diuretics is dependent on the concentration of the drug in the tubular lumen. Diuretics are delivered to the tubular lumen of the kidney by active transport by the proximal mbular cells. Osmotic diuretics, on the other hand, are freely filtered into the mbular lumen in the proximal tubule, whereas spironolactone gains access to mineralocorticoid receptors in the cortical collecting duct through diffusion from the systemic circulation. 

In unprocessed foods, potassium occurs mainly in association with bicarbonate-generating precursors such as citrate and, to a lesser extent, with phosphate. In processed foods to which potassium is added and in supplements, the form of potassium is potassium chloride. In healthy people, approximately 85% of dietary potassium is absorbed. Most potassium (approximately 77-90%) is excreted in urine, whereas the remainder is excreted mainly in feces, with much smaller amounts excreted in sweat. Because most potassium that is filtered by the glomerulus of the kidney is reabsorbed (70-80%) in the proximal tubule, only a small amount of filtered potassium reaches the distal tubule. The majority of potassium in urine results from secretion of potassium into the cortical collecting duct, a secretion regulated by a number of factors including the hormone aldosterone. An elevated plasma concentration of potassium stimulates the adrenal cortex to release aldosterone, which in turn increases secretion of potassium in the cortical collecting duct. 

P450 27B1 is expressed in many parts of the human kidney, including the distal convoluted tubule, the cortical and medullary part of the collecting ducts, and the papillary epithelia. Lower expression was observed along the thick ascending limb of the loop of Henle and Bowman s capsule. Some weaker expression was observed in glomeruli or vascular structures. In normal humans, the distal nephron is the predominant site of expression . 

ELECTROLYTE AND WATER BALANCE Aldosterone is by far the most potent endogenous corticosteroid with respect to fluid and electrolyte balance. Thus, electrolyte balance is relatively normal in patients with adrenal insufficiency due to pituitary disease, despite the loss of glucocorticoid production by the inner cortical zones. Mineralocorticoids act on the distal tubules and collecting ducts of the kidney to enhance reabsorption of Na+ from the tubular fluid they also increase the urinary excretion of K and H . 

CO2 can be hydrated spontaneously, but in the tubular cell the conversion of CO2 into H2CO3 is catalyzed by an enzyme called carbonic anhydrase. Carbonic anhydrase activities are greater in the cortical portion of the kidney than in the medulla. This is in agreement with micropuncture and stop-flow studies, which suggest that the urine is acidified in the distal part of the nephron, possibly in the collecting ducts. Yet the proximal tubule may also participate in hydrogen secretion. 

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