Intracellular Mg2+ and magnesium depletion in isolated renal thick ascending limb cells.

Abstract

IntroductionMagnesium reabsorption and regulation within the kidney occur principally within the cortical thick ascending limb (cTAL) cells of the loop of Henle. Fluorometry with the dye, mag-fura- Homeostasis of total body magnesium occurs principally within the kidney by epithelial cells of the cortical segment of the thick ascending limb (cTAL)' of Henle's loop (1-3). Shareghi and Agus (4), using in vitro perfused tubules, provided evidence for passive magnesium transport probably moving through the paracellular pathway. These early observations were supported by the findings ofde Rouffignac and colleagues (1, 5). However, the later investigators also showed that magnesium transport may be altered in the absence of voltage and resistance changes and NaCl absorption, suggesting that magnesium transport may be active in nature (5).Although we have considerable understanding of transepithelial magnesium movement and some ofthe factors that control transport, little is known about the intracellular regulation ofmagnesium (1-5). This, has been due in part to the lack ofan adequate radiotracer of magnesium or other suitable quantitative methods (2, 6). More important was the deficiency of an adequate means of assessing free Mg2" activity. The recent development of fluorescent dyes for Mg2" should allow a better understanding of cellular free Mg2" movements (7). It is the free Mg2" concentration ([Mg2J]i) that is thought to be important in determining plasma membrane transport and entry into biochemical processes rather than total magnesium, much of which is bound to intracellular ligands (6).

Molecular identification of ancient and modern mammalian magnesium transporters

“…The data suggest that epithelial cells can somehow sense the environmental Mg 2ϩ concentration and, through transcription/translation-and posttranslation-dependent processes, alter Mg 2ϩ transport to maintain Mg 2ϩ balance. Our evidence is that about half of the upregulation of Mg 2ϩ transport with Mg 2ϩ restriction is due to transcription/translation-mediated increase in protein synthesis and half is due to posttranslational modification and recycling of transport proteins onto the plasma membrane (21,111). These conclusions from studies using isolated epithelial cells are consistent with our views of Mg 2ϩ conservation in the intact kidney (109,111,136).…”

“…However, these hormones do not provide selective control, inasmuch as they also affect Ca 2ϩ and, in some cases, Na ϩ and K ϩ transport (109). Our findings indicate that selective and sensitive control of Mg 2ϩ transport is regulated by intrinsic cellular mechanisms in intact animals and isolated cells cultured in media (21,111). For example, in cells cultured in low-Mg 2ϩ medium, Mg 2ϩ uptake is upregulated.…”

“…For example, in cells cultured in low-Mg 2ϩ medium, Mg 2ϩ uptake is upregulated. This adaptive increase in Mg 2ϩ entry was shown to be dependent, in part, on de novo transcription, since prior treatment of the epithelial cells with actinomycin D prevented the adaptation to low extracellular Mg 2ϩ (21). The data suggest that epithelial cells can somehow sense the environmental Mg 2ϩ concentration and, through transcription/translation-and posttranslation-dependent processes, alter Mg 2ϩ transport to maintain Mg 2ϩ balance.…”

“…Deficiency of this essential metal has been implicated in many diseases, ranging from hypertension, migraines, and asthma to osteoporosis and other bone disorders (119,151,172). Intracellular free Mg 2ϩ concentration is on the order of 0.5 mM, which is 1-2% of the total cellular Mg 2ϩ (21,109). Accordingly, intracellular Mg 2ϩ is maintained below the concentration predicted from the transmembrane electrochemical potential.…”

Familial hypomagnesaemia with hypercalciuria and nephrocalcinosis maps to chromosome 3q27 and is associated with mutations in the PCLN-1 gene

“…24,25 In addition, it has been shown in rats that magnesium also enters the cortical TAL cells via selective pathways, most likely through magnesium specific channels. 26 Magnesium transporters have only been reported in bacteria and yeast. [27][28][29] As the primary defect in FHHNC has been related to a reabsorption defect of magnesium in the TAL, investigation of the molecular aspects of FHHNC seemed promising in order to gain more insight into renal magnesium transport systems.…”