Kidney collecting ducts

AQP6 is expressed in the intercalated cells of the kidney collecting duct. This channel is hardly permeable to water, but capable of transporting anions, including chloride, and is therefore thought to play a role in maintenance of body acid-base balance or in intracellular vesicle acidification. 

Other aquaporins with related sequences occur broadly. There are at least ten in mammals.359,3593 Plants, which must accommodate to heavy loss of water in hot dry weather, have aquaporins in both plasma membranes and tonoplasts.360 Bacteria also have aquaporins.356,361 A defect in aquaporin-2 of the kidney collecting duct leads to nephrogenic diabetes insipidus, in which the kidneys fail to concentrate urine in response to secretion of the hormone vasopressin.3553 362,363... 

It increases the water permeability of the kidney collecting duct cells by inducing translocation of aqua-porin proteins from intracellular storage vesicles into the apical plasma membrane.78 Vasopressin binds to... 

Nielsen S, Chou CL, Marpels D, Christensen El, Kishore BK, Knepper MA. 1995. Vassopressin increases water permeability of kidney collecting duct by inducing translocation of aquaporin-CD water channels to plasma membrane. Proc Natl Acad Sci USA 92 1013-1017. 

Brown, D., Weyer, P. and Orci, L. (1987). Nonclathrin-coated vesicles are involved in endocytosis in kidney collecting duct intercalated cells. Anat. Rec. 218, 237-242. 

Ishibashi, K., Sasaki, S., Fushimi, K., Yamamoto, T., Kuwahara, M., and Marumo, F. (1997). Immunolocalization and effect of dehydration on AQP3, a basolateral water charmel of kidney collecting ducts. Am.. Physiol. 272, F235-F241. 

ADH is secreted in response to an increase in plasma sodium chloride concentration or a decrease in circulating blood volume. ADH acts on kidney collecting duct cells to increase water reabsorption. In this way plasma sodium chloride concentration and/or blood volume are restored to normal. ADH also acts as a vasoconstrictor, which helps to maintain blood pressure if circulating blood volume has fallen. 

Cittanova ML, Lelongt B, Verpont MC, Geniteau-Legendre M, Wahbe F, Piie D, Coriat P, Ronco PM. 1996. Fluoride ion toxicity in human kidney collecting duct cells. Anesthesiology 84(2) 428-435. 

In normal human subjects, ANP infusion for one hour causes increased absolute and fractional sodium excretion, urine flow, GFR, and water clearance (53—55). As shown in many in vitro and in vivo animal studies, ANP achieves this by direct effect on the sodium reabsorption in the inner medullary collecting duct, ie, by reducing vasopressin-dependent free-water and sodium reabsorption leading to diuresis and by indirect effect through increased hemodynamic force upon the kidney. ANP inhibits the release of renin and aldosterone resulting in the decreased plasma renin activity and aldosterone concentration (56,57). 

Til tee successive tubule portions contribute to the ASDN the late portion of the distal convoluted tubule, the connecting tubule, and the collecting duct. The recent observation that collecting duct-specific inactivation of aENaC in the mouse kidney does not impair sodium and potassium balance, suggests that the more proximal nephron segments (late distal convoluted tubule, connecting tubule) are mainly important for-achieving sodium and potassium balance. 

Urodilatin is a peptide similar to atrial natriuretic peptide, which is produced in the distal tubule of the kidney and promotes sodium excretion and diuresis by acting on receptors localized on the luminal site of the collecting duct of the nephron. 

The posterior pituitary is innervated by direct nervous stimulation from the hypothalamus, resulting in the release of specific hormones. The hypothalamus synthesizes two hormones, oxytocin and vasopressin. These hormones are stored in and released from the posterior pituitary lobe. Oxytocin exerts two actions (1) it promotes uterine contractions during labor, and (2) it contracts the smooth muscles in the breast to stimulate the release of milk from the mammary gland during lactation. Vasopressin is an antidiuretic hormone (ADH) essential for proper fluid and electrolyte balance in the body. Specifically, vasopressin increases the permeability of the distal convoluted tubules and collecting ducts of the nephrons to water. This causes the kidney to excrete less water in the urine. Consequently, the urine becomes more concentrated as water is conserved. 

Antidiuretic hormone promotes the reabsorption of water from the tubules of the kidney, or antidiuresis. Specifically, it acts on the collecting ducts and increases the number of water channels, which increases the diffusion coefficient for water. This results in the body s conservation of water and the production of a low volume of concentrated urine. The reabsorbed water affects plasma osmolarity and blood volume. This effect of ADH on the kidney occurs at relatively low concentrations. At higher concentrations, ADH causes constriction of arterioles, which serves to increase blood pressure. Antidiuretic hormone secretion is regulated by several factors ... 

The vasa recta are modified peritubular capillaries. As with the peritubular capillaries, the vasa recta arise from efferent arterioles. However, these vessels are associated only with the juxtamedullary nephrons and are found only in the medullary region of the kidney. The vasa recta pass straight through to the inner region of the medulla, form a hairpin loop, and return straight toward the cortex. This structure allows these vessels to lie parallel to the Loop of Henle and collecting ducts. 

Takenaka M et al. Gene expression profiling of the collecting duct in the mouse renal inner medulla. Kidney Int 2000 57 19-24. 

The answer is c. (Hardman, pp 649—650.) Acute hyperuricemia, which often occurs in patients who are treated with cytotoxic drugs for neoplasic disorders, can lead to the deposition of urate crystals in the kidneys and their collecting ducts. This can produce partial or complete obstruction of the collecting ducts, renal pelvis, or ureter. Allopurinol and its primary metabolite, alloxanthine, are inhibitors of xanthine oxidase, an enzyme that catalyzes the oxidation of hypo xanthine and xanthine to uric acid. The use of allopurinol in patients at risk can markedly reduce the likelihood that they will develop acute uric acid nephropathy. 

The dominant mercury pool in the body is the kidney [23, 31]. The kidneys contained over 85% of the body burden of mercury 15 days or more after a single injection of mercuric chloride into rats [32]. Maximum levels in the rat kidney were attained in less than a day after doses of mercuric chloride [33], The renal cortex contained the highest levels [34-36], the maximum concentrations being found in the proximal tubular system, while mercury was close to background levels in the glomeruli and collecting ducts. 

After peroral intake of nickel sulphate in mice, kidney damage appeared at the corticomedullary junction [296]. There was loss of renal tubular epithelial cells and protein loss, giving numerous hyaline casts in the renal tubules and collecting ducts, especially prominent in the renal papillae. 

Dworzack, D.L. and Grantham, J.J. (1975). Preparation of renal papillary collecting duct cells for study in vitro. Kidney Int. 8 191-194. 

ADH, a nonapeptide, released from the posterior pituitary gland promotes re-absorption of water in the kidney. This response is mediated by vasopressin receptors of the V2 subtype. ADH enhances the permeability of collecting duct epithelium for water (but not for electrolytes). As a result, water is drawn from urine into the hyperosmolar inter-stitium of the medulla. Nicotine augments (p. 110) and ethanol decreases ADH release. At concentrations above those required for antidiuresis, ADH stimulates smooth musculature, including that of blood vessels ( vasopressin ). The latter response is mediated by receptors of the Vi subtype. Blood pressure rises coronary vasoconstriction can precipitate angina pectoris. Lypres-sin (8-L-lysine vasopressin) acts like ADH. Other derivatives may display only one of the two actions. 

Aquaporins help water to pass through biological membranes. They form hydrophilic pores that allow H2O molecules, but not hydrated ions or larger molecules, to pass through. Aquaporins are particularly important in the kidney, where they promote the reuptake of water (see p. 328). Aquaporin-2 in the renal collecting ducts is regulated by antidiuretic hormone (ADH, vasopressin), which via cAMP leads to shifting of the channels from the ER into the plasma membrane. 

The functional unit of the kidney is the nephron. It is made up of the Malpighian bodies or renal corpuscles (consisting of Bowman s capsules and the glomerulus), the proximal tubule, Henle s loop, and the distal tubule, which passes into a collecting duct. The human kidney contains around one million nephrons. The nephrons form urine in the following three phases. 

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