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Abstract Magnesium Transport in the Renal Distal Convoluted Tubule. Physiol Rev 81: 51–84, 2001.—The distal tubule reabsorbs ;10% of the filtered Mg21, but this is 70–80% of that delivered from the loop of Henle. Because there is little Mg21 reabsorption beyond the distal tubule, this segment plays an important role in determining the final urinary excretion. The distal convoluted segment (DCT) is characterized by a negative luminal voltage and high intercellular resistance so that Mg21 reabsorption is transcellular and active. This review discusses recent evidence for selective and sensitive control of Mg21 transport in the DCT and emphasizes the importance of this control in normal and abnormal renal Mg21 conservation. Normally, Mg21 absorption is load dependent in the distal tubule, whether delivery is altered by increasing luminal Mg21 concentration or increasing the flow rate into the DCT. With the use of microfluorescent studies with an established mouse distal convoluted tubule (MDCT) cell line, it was shown that Mg21 uptake was concentration and voltage dependent. Peptide hormones such asparathyroid hormone, calcitonin, glucagon, and arginine vasopressin enhance Mg21 absorption in the distal tubule and stimulate Mg21 uptake into MDCT cells. Prostaglandin E2 and isoproterenol increase Mg21 entry into MDCT cells. The current evidence indicates that cAMP-dependent protein kinase A, phospholipase C, and protein kinase C signaling pathways are involved in these responses. Steroid hormones have significant effects on distal Mg21 transport. Aldosterone does not alter basal Mg21 uptake but potentiates hormone-stimulated Mg21 entry in MDCT cells by increasing hormone-mediated cAMP formation. 1,25-Dihydroxyvitamin D3, on the other hand, stimulates basal Mg21 uptake. Elevation of plasma Mg21 or Ca21 inhibits hormone-stimulated cAMP accumulation and Mg21 uptake in MDCT cells through activation of extracellular Ca21/Mg21-sensing mechanisms. Mg21 restriction selectively increases Mg21 uptake with no effect on Ca21 absorption. This intrinsic cellular adaptation provides the sensitive and selective control of distal Mg21 transport. The distally acting diuretics amiloride and chlorothiazide stimulate Mg21 uptake in MDCT cells acting through changes in membrane voltage. A number of familial and acquired disorders have been described that emphasize the diversity of cellular controls affecting renal Mg21 balance. Although it is clear that many influences affect Mg21 transport within the DCT, the transport processes have not been identified.

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Abstract An immortalized cell line (designated MDCT) has been extensively used to investigate the cellular mechanisms of electrolyte transport within the mouse distal convoluted tubule. Mouse distal convoluted tubule cells possess many of the functional characteristics of the in vivo distal convoluted tubule. In the present study, we show that MDCT cells also possess a polyvalent cation-sensing mechanism that is responsive to extracellular magnesium and calcium. Southern hybridization of reverse transcribed-polymerase chain reaction (RT-PCR) products, sequence determination and Western analysis indicated that the calcium-sensing receptor (Casr) is expressed in MDCT cells. Using microfluorescence of single MDCT cells to determine cytosolic Ca21 signaling, it was shown that the polyvalent cation-sensing mechanism is sensitive to extracellular magnesium concentration ([Mg21]o) and extracellular calcium concentration ([Ca21]o) in concentration ranges normally observed in the plasma. Moreover, both [Mg21]o and [Ca21]o were effective in generating intracellular Ca21 transients in the presence of large concentrations of [Ca21]o and [Mg21]o, respectively. These responses are unlike those observed for the Casr in the parathyroid gland. Finally, activation of the polycationsensitive mechanism with either [Mg21]o or [Ca21]o inhibited parathyroid hormone-, calcitonin-, glucagon- and arginine vasopressin-stimulated cAMP release in MDCT cells. These studies indicate that immortalized MDCT cells possess a polyvalent cation-sensing mechanism and emphasize the important role this mechanism plays in modulating intracellular signals in response to changes in [Mg21]o as well as in [Ca21]o.

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