Calcium-sensing receptor regulation of renal mineral ion transport

Cell Calcium 35 (2004) 229–237

Calcium-sensing receptor regulation of renal mineral ion transport Jianming Ba a , Peter A. Friedman a,b,∗ a

Department of Pharmacology, University of Pittsburgh School of Medicine, E-1347 Biomedical Science Tower, Pittsburgh, PA 15261, USA b Department of Medicine, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261, USA Received 20 October 2003; accepted 27 October 2003

Abstract Extracellular calcium has long been known to affect the rate and magnitude of renal calcium and phosphate recovery. In this review, we consider some of these findings in light of our present understanding of the tubular localization of the calcium-sensing receptor (CaSR). Experiments directly implicating the CaSR in regulating calcium and phosphate transport are described. These results point to an important role of the CaSR in regulating PTH-dependent calcium absorption by cortical thick ascending limbs and on PTH-sensitive proximal tubule phosphate transport. Possible avenues for further investigation are suggested. © 2003 Elsevier Ltd. All rights reserved. Keywords: Calcium-sensing receptor (CaSR); Parathyroid hormone (PTH); Ca transport; Pi transport

1. Introduction Ca2+ plays a key role in numerous cellular processes, such as maintaining membrane potential and controlling hormonal secretion, cellular proliferation and differentiation [1]. The mechanisms governing extracellular calcium homeostasis maintain its near constancy to ensure continual availability of calcium ions for their multiple intra- and extracellular functions. Renal mineral ion disposition of calcium and phosphate is regulated by parathyroid hormone (PTH), VitaminD [2,3], and calcitonin. Urinary calcium excretion increases with rising circulating Ca2+ concentrations within a certain range. However, in the absence of these calcitropic hormones, the relationship between plasma and urinary calcium is magnified [4,5], indicating some additional factor or process contributes to determining urinary Ca2+ excretion. Cloning of the extracellular, G protein-coupled Ca2+ sensing receptor (CaSR) from bovine parathyroid gland proved that extracellular calcium ions serve as extracellular first “messengers” in addition to calcium’s well-recognized function as an intracellular second messenger [6]. The CaSR is now appreciated to be the molecular mechanism through which parathyroid cells and several other cell types recognize and respond to small, physiologically relevant changes of extracellular Ca2+ . This receptor plays a central role in

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0143-4160/$ – see front matter © 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.ceca.2003.10.016

extracellular calcium homeostasis. Molecular cloning of the CaSR also enabled the identification of inherited diseases with inactivating or activating receptor mutations that cause hypercalcemic or hypocalcemic syndromes, respectively [7]. These studies showed that extracellular calcium ions themselves modulate the functions of target tissues through their cognate Ca2+ -sensing G protein-coupled receptor in a manner analogous to that through which the calcitropic peptide hormone, PTH, acts on its target cells. Emerging evidence points to a role for the CaSR in regulating renal calcium and inorganic phosphate (Pi) transport. This review provides an overview of renal calcium and phosphate transport and its regulation. The distribution and function of the CaSR in regulating Ca2+ and Pi transport in kidney are discussed.

2. Overview of calcium and Pi transport in kidney 2.1. Calcium transport The kidneys play a major role in the integrated regulation of calcium homeostasis. Calcium absorption takes place throughout the nephron. Proximal tubules, thick ascending limbs of Henle’s loop, and distal tubules are the major sites of calcium absorption. Sixty to seventy percent of the filtered calcium is reabsorbed by proximal tubules [8], an additional 20–25% by thick ascending limbs, and 8–10% by distal tubules [9–11]. The net result of unregulated and regulated

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tubular calcium absorption is that in healthy adults, i.e. in calcium balance, the amount of calcium in the voided urine is about 1% of that filtered. Although the majority of calcium is reabsorbed by proximal tubules, the fine adjustment of calcium recovery occurs by hormone- and drug-sensitive mechanisms at downstream sites in thick limbs and especially in distal tubules. Because the ratio of calcium in the glomerular ultrafiltrate is 1.0–1.2 mM, it is generally accepted that proximal calcium transport is essentially an isösmotic process that is energetically passive. This suggests that calcium absorption in proximal tubules proceeds through the paracellular (lateral intercellular) pathway between adjacent cells. However, the slight rise in the ratio of ultrafilterable to luminal calcium may indicate some active transport. It is generally thought that >80–85% of renal calcium reabsorption is passive and <15–20% is active [12,13]. Calcium is transported in medullary and cortical thick ascending limbs by a combination of parallel transcellular and paracellular routes. Basal calcium absorption proceeds through the paracellular pathway, where its rate of movement is governed by the prevailing electrochemical driving forces. These, in turn, are established by the extent of sodium absorption. Transcellular calcium absorption appears to be quiescent under resting conditions. The cellular component of calcium absorption in thick limbs is regulated by PTH and calcitonin [14,15], though the magnitude of such stimulation varies between species [8]. Calcium absorption in distal convoluted tubules proceeds entirely through a cellular pathway. Passive calcium movement through the lateral intercellular spaces is negligibly small because of the markedly low permeability of the tight junctional membranes to calcium [10]. Cellular calcium absorption across polarized epithelial cells is a two-step process. Calcium entry across apical (mucosal, luminal) membranes is followed by extrusion across basolateral (serosal, contraluminal) membranes into interstitial fluid and thence into the circulation. Calcium influx down its electrochemical gradient across apical membranes is generally accepted to be mediated by calcium channels. Basolateral efflux, in contrast, involves energy-dependent extrusion that is accomplished by the plasma membrane Ca2+ -ATPase (PMCA) and the Na+ /Ca2+ exchanger. The calcium-selective epithelial calcium channel, TRPV5 (ECaC1), was cloned and localized to the apical membrane of distal convoluted tubules [16–18]. TRPV5 is a homotetramer that is constitutively active, has a 77 pS single channel conductance [19], and is insensitive to dihydropyridine or phenylalkylamine calcium channel blockers [20,21]. However, as yet, no stimulatory effect of PTH or of chlorothiazide on calcium transport mediated by TRPV5 has been reported. This may reflect the need for assembly of accessory subunits or other adaptor proteins. However, it remains possible that other multimeric calcium channels mediate hormone or drug-sensitive calcium entry.

2.2. Regulation of renal calcium absorption Renal calcium absorption is regulated primarily by PTH, which stimulates the rate of calcium absorption by distal convoluted tubules and cortical thick ascending limbs of Henle’s loop. Diuretics also potently affect renal calcium disposition. Thiazide and thiazide-type diuretics exert a calcium-sparing action by promoting absorption by distal convoluted tubules. This effect can be directly demonstrated on perfused nephron segments [10,22] and in clonal distal convoluted tubule cells [23]. Thus, it is not due to changes of extracellular fluid volume and is independent of PTH status. Since thiazide diuretics inhibit sodium absorption while stimulating calcium transport, sodium and calcium movement are inversely related in distal convoluted tubules. In thick ascending limbs of Henle’s loop, furosemide and related “loop” diuretics inhibit passive calcium absorption, thereby augmenting its excretion. This is due to the secondary reduction of the transepithelial voltage that attends blockade of Na–K–2Cl cotransport. The driving force for passive calcium absorption is dictated by the prevailing voltage, which in turn, is set by the rate of Na–K–2Cl absorption and apical membrane K+ recycling that generates the lumen-positive voltage. Upon inhibition of sodium transport, the transepithelial voltage decreases pari passu and with it, calcium movement falls. Thus, in thick ascending limbs, sodium and calcium absorption proceed in parallel; decreases of sodium absorption are accompanied by decreases of calcium absorption. 2.3. Pi transport Renal regulation of inorganic phosphate (Pi) absorption is a major determinant of extracellular phosphorous homeostasis. Adequate supplies of Pi, derived from plasma, are a prerequisite for normal cell and body functions such as intracellular metabolism, bone growth and remodeling. Renal Pi absorption is tightly controlled by hormonal and nonhormonal factors. Renal proximal tubules are the major site of Pi transport in the kidney. Under normal physiologic and dietary conditions, approximately 70–80% of the freely filtered Pi is reabsorbed along the proximal tubules [24]. The rate of Pi transport in proximal convoluted tubules is approximately three fold higher than that in proximal straight tubules. Little Pi reabsorption occurs in the loop of Henle. It remains controversial whether Pi reabsorption proceeds in distal convoluted tubules or other terminal nephron sites. The mechanisms of proximal Pi transport have been extensively studied and are well characterized [25]. Pi is taken up from the apical cell surface by a sodium-dependent transport mechanism [26], which is rate limiting and the site of the physiological regulation and pathophysiological disorders of renal phosphate transport. Three NaPi-cotransporters have been identified [26]. The type IIa NaPi cotransporter accounts for about 70% of brush-border membrane (BBM)

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NaPi cotransporter activity and is the target of PTH action [27]. Apical Pi uptake is dissipatively coupled to Na+ , which enters the cell down its electrochemical gradient. The Na+ electrochemical gradient is sustained by maintaining intracellular Na+ at low levels. This is achieved by the continuous extrusion of Na+ from the cell by the basolateral membrane Na+ ,K+ -ATPase. The cellular mechanisms involved in Pi efflux across basolateral membranes have not been elucidated but may be mediated by anion exchange. Transepithelial Pi absorption by proximal tubules essentially equals the unidirectional absorptive flux because back flux from the peritubular fluid to the lumen is negligible. 2.4. Regulation of renal phosphate absorption The principal determinant of overall Pi homeostasis is the regulation of proximal tubular Pi absorption. Apical membrane Pi uptake is the rate-limiting step in Pi absorption and the primary site of its regulation. The major factors that regulate renal Pi absorption include PTH, dietary Pi intake, and VitaminD. PTH inhibits Pi absorption by internalizing the apical brush-border membrane type IIa NaPi cotransporter [28]. Low dietary Pi intake profoundly augments the reabsorption of filtered Pi, whereas high dietary Pi leads to decreased proximal Pi reabsorption [29]. Vitamin D has been suggested to increase Pi reabsorption [30]. Emerging evidence points increasingly to an important role for FGF23 and FRP4 as additional regulatory factors involved in regulating renal Pi transport [31–35]. The influence of extracellular calcium on proximal phosphate absorption is complicated and unsettled and is discussed later. Calcium affects a number of hemodynamic, physical chemical, and passive transport properties that may have contributed to the reported disparities. To avoid these complications, Rouse and Suki examined the direct effect of calcium on phosphate absorption by single microperfused rabbit proximal tubules [36]. They found that luminal addition of calcium enhanced basal phosphate transport in S2, but not in S3, proximal tubules. The mechanism for this action was not elucidated but might be attributable to an activating effect on the calcium-sensing receptor, CaSR.

3. CaSR in the kidney The calcium-sensing receptor (CaSR) is abundantly expressed in the kidneys. CaSR mRNA transcripts are present essentially throughout the nephron, viz., glomerulus, proximal convoluted and proximal straight tubule, medullary and cortical thick ascending limbs, distal convoluted tubule, cortical and inner medullary collecting ducts [37]. CaSR protein expression is prominently found in proximal tubules, thick ascending limbs, and cortical collecting tubules [38]. Notably, the membrane domain at which the CaSR is found differs between nephron segments. In proximal tubules the CaSR is expressed at the base of apical brush-border mem-

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branes. Expression decreases from S1 to S3 proximal tubule segments. By contrast, in thick ascending limbs the CaSR is found on basolateral cell membranes [38,39]. In cortical and inner medullary collecting ducts the CaSR is localized to apical plasma membranes [38–40]. The CaSR is expressed only in some of the type A intercalated cells of the cortical collecting duct [38]. The trafficking motifs responsible for directed membrane targeting of the CaSR have not been identified. 3.1. CaSR effects on renal calcium and Pi transport By regulating the amount of tubular Ca2+ and Pi reabsorption from the glomerular filtrate, the kidneys play a vital role in mineral ion homeostasis. A substantial body of literature, summarized here but reviewed in detail elsewhere [30], underscores the many but often conflicting effects of alterations of extracellular calcium on renal transport of calcium and phosphate. The molecular cloning of the CaSR, its localization within the kidney, and the recognition that inactivating and activating mutations are associated with important alterations of mineral ion homeostasis opened the opportunity of determining more directly whether CaSR activation accounts for the previously described effects of calcium on basal or hormone-dependent tubular calcium and phosphate absorption. CaSRs are expressed in both cortical and medullary thick ascending limbs [37,41]. The CaSR is abundantly expressed on basolateral membranes of thick ascending limbs (Fig. 1). Thus, it was anticipated that in cortical ascending limbs the CaSR monitors the Ca2+ composition of peritubular fluid. However, it was not known if CaSR activation regulated Na+ , K+ , Ca2+ , or Mg2+ absorption. Studies from Wang’s lab established that CaSR activation reduced the activity of the 70-pS apical membrane K+ channel in thick limbs [42]. This would reduce K+ recycling and thereby limit the rate of Na–K–2Cl cotransport. K+ recycling is also required for transcellular current flow and generation of the lumen-positive transepithelial voltage. Since passive Ca+ absorption in thick limbs proceeds through the lateral intercellular spaces, CaSR-mediated reductions of K+ current would be expected to curtail this component of Ca transport. PTH receptor expression along the nephron was initially determined by measuring PTH-activated adenylyl cyclase in individual microdissected nephron segments [43–45]. After the molecular cloning of the type 1 PTH receptor (PTH1R), the segmental pattern of PTH1R RNA transcript expression was determined [37,41]. Both functional and molecular localizations report abundant PTH receptor expression in thick ascending limbs. Some appreciable species differences attend the extent of PTH1R abundance in cortical thick limbs (present in all mammalian species examined) and medullary thick limbs, where the PTH receptor is prominent in some species but limited or absent in others [8]. The CaSR is expressed on both medullary and cortical thick ascending limbs. In contrast, the PTH1R is found only

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Fig. 1. Immunocytochemical localization of CaSR in mouse thick ascending limbs. Immunofluorescence showed strong localization in the basolateral region of thick ascending limbs. The CaSR was localized using a well-characterized CaSR-specific polyclonal antibody (#2; [90]) kindly provided by Dr. Dolores Shoback (UCSF). Sections were washed 2 × 5 min in high salt PBS (PBS containing 2.7% NaCl) and 1 × 5 min in PBS to reduce non-specific staining, and the secondary donkey anti-rabbit antibody conjugated to CY3 (Jackson Immunologicals) was applied for 1 h at room temperature at a dilution of 1:800. The sections were again washed in high salt PBS and in PBS before mounting in Vectashield anti-fade solution diluted 1:1 in Tris buffer, pH 9.0. Sections were examined using a Nikon Eclipse 800 epifluorescence microscope and images were captured with a Hamamatsu Orca CCD camera and IP Lab acquisition software (Scanalytics).

in cortical ascending limbs of the mouse, while the V2 vasopressin receptor is localized exclusively to medullary thick ascending limbs [44,46]. This pattern of CaSR, PTH1R, and vasopressin receptor expression is consistent with studies showing that hypercalcemia selectively inhibited PTH-stimulated cAMP formation by cortical thick limbs, whereas hypercalcemia suppressed vasopressin-induced cAMP accumulation by medullary thick ascending limbs [47]. Earlier studies established that elevated basolateral but not luminal calcium inhibited calcium (and magnesium) absorption by thick limbs [48]. Similar results have been reported by others [49]. As noted above, elevating extracellular calcium selectively depresses PTH-stimulated cAMP formation by cortical thick ascending limbs [47]. Based on these findings, we theorized that CaSR activation regulates PTH-sensitive calcium absorption by cortical ascending limbs. This idea was tested by examining the effects of CaSR activation on basal and PTH-dependent calcium absorption by cortical ascending limbs of Henle’s loop. CaSR activation with trivalent Gd3+ inhibited PTHstimulated active Ca2+ absorption (Fig. 2) [50]. CaSR activation with the Type II CaSR agonist NPS R-467 likewise suppressed PTH-dependent Ca2+ absorption by cortical

ascending limbs. CaSR activation had no effect on resting Ca2+ absorption. However, CaSR activation reduced passive paracellular Ca2+ absorption, thereby confirming the findings obtained by Desfleurs et al. [49]. The effects of CaSR activation were specific for Ca2+ absorption because there was no effect on Na+ transport [50]. The mechanism whereby CaSR activation inhibits PTH-dependent Ca2+ absorption has not been examined. Several possible pathways may be involved. Activation of the CaSR results in G-protein-dependent stimulation of phospholipase C with attendant inositol trisphosphate formation and rapid but transient release of Ca2+ from intracellular stores. Other CaSR signaling pathways, including activation of Gi, phospholipase A2, phospholipase D, mitogen-activated protein kinase, and phosphatidylinositol 4-kinase, have been described but are less well characterized [6,51–53]. Because PTH stimulation of Ca2+ transport in cortical ascending limb and distal convoluted tubule cells requires activation of protein kinase A [54], it is attractive to speculate that the negative regulatory effect of Gi blocks the stimulatory influence on Gs, thereby abrogating the action of PTH. Such an interpretation would be consistent with the inhibitory action of elevated calcium on PTH-stimulated cAMP formation as described above.

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Fig. 2. Inhibition of parathyroid hormone (PTH)-stimulated calcium absorption by Gd3+ . Mouse cortical thick ascending limbs were perfused and bathed with identical Na+ -containing solutions. After the control period, the peritubular bath was changed to one containing 10 nM bovine PTH(1–34), which increased net calcium absorption by 100%. Addition of 30 ␮M GdCl3 to the bathing solution inhibited PTH action. ∗ P < 0.01 Modified from [50].

As outlined earlier, cellular calcium absorption is a two-step process, where apical calcium entry is followed by exit across basolateral plasma membranes. This latter step is likely to be mediated by the Na+ /Ca2+ exchanger and the plasma membrane Ca2+ ATPase (PMCA) [55,56]. The role of the CaSR in regulating basolateral calcium efflux was examined in monolayers of MDCK cells, a cell model for the distal nephron [57]. MDCK cells constitutively express CaSRs and the PMCA [57,58]. Increasing basolateral calcium, or addition of the CaSR agonists Gd3+ or neomycin caused concentration-dependent inhibition of unidirectional

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Ca2+ absorption (Fig. 3) [57]. This effect was blocked with an inhibitor of phospholipase C. PMCA activity in membranes purified from cells exposed to high (5 mM) calcium was virtually abolished. Thus, the inhibitory action of CaSR activation was due to reduction of PMCA activity. Together, these results suggest that CaSR activation may exert coördinated regulatory effects on transcellular calcium transport that assure that calcium efflux remains stoichiometrically linked to calcium influx, thereby protecting cytosolic calcium levels. The strongest evidence supporting the role of the renal CaSR in regulating Ca2+ handling in the kidney comes from inherited human diseases with heterozygous loss-of-function CaSR mutations in familial hypocalciuric hypercalcemia, also called familial benign hypercalcemia. Such mutations lower the sensitivity of CaSR to extracellular Ca2+ so that a greater elevation of plasma Ca2+ is necessary to suppress PTH secretion from the parathyroids [59–61]. In contrast to loss-of-function CaSR mutations, gain-of-function CaSR mutations, as found in autosomal dominant hypocalcaemia (ADH), lead to hypercalciuria [62]. 3.2. CaSR regulation of renal phosphate transport Extracellular calcium influences renal phosphate absorption. Most studies report that acutely elevating plasma calcium increases Pi absorption independent of PTH [63–66]. Conversely, lowering extracellular calcium reduces serum phosphate [67]. These findings suggest that calcium directly affects renal Pi absorption. The mechanism responsible for this action is unknown. Calcium could modify renal Pi transport by directly altering luminal membrane Na–Pi cotransport. Raising luminal calcium enhanced phosphate absorption in proximal convoluted tubules [36]. Alternatively, calcium could modify the action of PTH on retrieval of apical membrane Na–Pi cotransporters. PTH-induced cAMP accumulation is inhibited by divalent cations [68].

Fig. 3. CaSR inhibition of PMCA activity. (A) Concentration-dependent inhibition of PMCA activity by basolateral addition of Ca2+ ; (B) gadolinium; and (C) neomycin to MDCK cells. Cells were grown on permeable filter supports and stimulated by basolateral addition of the indicated CaSR agonists. Membranes were prepared and PMCA activity determined enzymatically. Modified from [57].

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Thus, elevated Ca2+ could serve to antagonize the PTH response. Both the CaSR and NaPi cotransporter are located in apical brush-border membranes as well as in subapical vesicles [69]. This suggested the possibility that the CaSR might participate in modulating cell surface NaPi expression by changing the rate of extent of NaPi exocytosis or endocytosis [70,71]. There is also coördinated regulation of the expression and brush border localization of CaSR and NaPi-2 in rats following chronic exposure to a high phosphate diet or to acute PTH infusion [69]. It remains to be determined, however, whether the CaSR and the NaPi cotransporter are actually colocalized and whether the former regulates the activity of the latter. PTH promotes phosphaturia by stimulating the endocytotic retrieval of Na–Pi cotransporter-containing vesicles from the proximal tubular brush-border membrane followed by lysosomal degradation of the cotransporter [72,73]. Further, NaPi sequestration can be evoked not only by basolateral PTH stimulation, but also by luminal PTH addition [74]. We pursued these observations by determining whether luminal PTH inhibits Pi absorption by proximal tubules. PTH inhibits phosphate absorption by single perfused mouse proximal tubule S3 segments (Fig. 4A). Activation of the CaSR with gadolinium (Gd3+ ) blocked PTH inhibitable Pi absorption (Fig. 4B). Treatment of hypoparathyroidism with VitaminD and calcium supplementation decreases serum phosphate. CaSR-induced inhibition and/or removal of Na–Pi cotransporter from the apical membranes of the proximal tubule could contribute to this action. Indeed, Ca2+ -evoked stimulation of the CaSR promotes NaPi cotransporter retrieval from brush-border membranes [75]. Activation of the VitaminD 1-␣hydroxylase (1␣OHase) in proximal tubule cells is responsible for the formation of biologically active 1,25(OH)2 D3 . 1␣OHase activity and abundance are enhanced by PTH in response to a fall in systemic Ca2+ [76]. However, 1,25(OH)2 D3 production remains sensitive to serum Ca2+ in thyroparathyroidectomized rats [77], suggesting that Ca2+ could directly affect 1␣OHase activity. Indeed, direct effects of Ca2+ on proximal tubule cells regulate renal 1,25(OH)2 D3 production [78]. As extracellular calcium was elevated from 1 to 2 or 4 mM 1␣OHase activity decreased with a concomitant fall in 1,25(OH)2 D3 synthesis. This inhibitory action of calcium on 1,25(OH)2 D3 formation was not accompanied by a change in its inactivation to 24,25(OH)2 D3 [78]. Reducing extracellular calcium from 1 to 0.5 mM stimulated 1␣OHase activity and 1,25(OH)2 D3 formation. In this case, however, parallel increases in 24,25(OH)2 D3 generation were also noted. A preliminary report suggests that CaSR overexpression amplifies these responses and implicates the CaSR in mediating them [79]. Thus, proximal tubule CaSR activation may participate in the regulation of phosphate homeostasis by regulating phosphate transport and 1␣OHase activity. These actions could antagonize or limit the effects of PTH as a stimulator of 1,25(OH)2 D3 production and phosphate excretion.

Fig. 4. Effects of PTH and CaSR activation on proximal tubular phosphate transport. (A) Combined luminal and basolateral application of PTH inhibited phosphate absorption. Mouse proximal S3 tubules were microperfused by the Burg technique. 100 nM PTH(1–34) was added to lumen and bath and inhibited phosphate absorption by 50%. Phosphate absorption was corrected for fluid absorption, which did not change in response to PTH. ∗∗ P < 0.01. (B) CaSR activation blocks the inhibitory effect of PTH on phosphate absorption. 30 ␮M Gd3+ was included in the luminal perfusate during the experiments. 100 nM PTH(1–34) was added to basolateral bathing solution. The data show that luminal Gd3+ blocked PTH-suppressible phosphate absorption. Modified from [91].

3.3. Effects of CaSR on water reabsorption The CaSR is expressed along the entire collecting duct [80]. The collecting duct is the nephron site where the antidiuretic hormone vasopressin increases urine concentration by the osmotic abstraction of water. This effect of vasopressin is mediated by stimulating the insertion of aquaporin-2 water channels into apical membranes of collecting duct cells, thereby substantially increasing the osmotically driven

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absorption of water from the tubule lumen and resulting in urinary concentration [81]. Ca2+ and Mg2+ inhibit vasopressin action in collecting ducts [82,83]. CaSR activation in inner medullary collecting ducts may account for this inhibitory effect. Exposure of apical membranes of isolated perfused rat inner medullary collecting ducts to elevated Ca2+ or to neomycin inhibited vasopressin-induced water permeability by 30% [40]. This finding suggested that the CaSR serves to mitigate calcium precipitation and stone formation by blunting the urinary concentrating capacity [40]. Such an effect may represent the dynamic and integrated actions of apical CaSR activation in medullary thick limbs and basolateral CaSR activation in collecting ducts and contribute to or account for the reduced concentrating ability associated with hypercalcemic states [84,85]. Individuals with inactivating mutations of the CaSR are able to concentrate their urine normally despite their hypercalcemia [86]. Conversely, persons with activating mutations of the CaSR may develop symptoms of diminished urinary concentrating capacity at normal or even low levels of Ca2+ when treated with VitaminD and calcium supplementation [87]. These symptoms presumably result from hypersensitivity of renal CaSRs to the usual actions of elevated Ca2+ . 4. Summary Important advances in the structural aspects of mineral ion transport have provided opportunities to advance our understanding of the cellular mechanisms by which changes of extracellular calcium directly influence renal conservation of calcium and phosphate. At the same time, these advances raise an entirely new set of questions. Although the CaSR has been localized to apical plasma membranes in proximal tubules and collecting ducts, it is found on basolateral membranes of thick ascending limbs. At the present time it is not known how this polarized localization is determined. Similarly, it is clear that the CaSR is down-regulated in chronic renal disease but the mechanism responsible for this effect has not been uncovered [88,89]. Even the signaling mechanisms of the CaSR require considerable examination to understand how they behave in native cells and may be altered in disease. An even greater challenge will be to unravel how the various hormonal and CaSR signals are integrated to produce an integrated and coördinated responses to alterations in the extracellular milieu. Acknowledgements Original work from the authors’ lab was supported by grant DK-54171-18 from the National Institutes of Health. We are especially grateful to Dr. Dennis Brown for kindly providing the immunofluorescence image of CaSR localization in cortical thick limbs.

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References [1] E.M. Brown, R.J. MacLeod, Extracellular calcium sensing and extracellular calcium signaling, Physiol. Rev. 81 (2001) 239– 297. [2] K. Kurokawa, The kidney and calcium homeostasis, Kidney Int. 45 (Suppl. 44) (1994) S97–S105. [3] D. Rouse, W.N. Suki, Renal control of extracellular calcium, Kidney Int. 38 (1990) 700–708. [4] K. Kurokawa, Calcium-regulating hormones and the kidney, Kidney Int. 32 (1987) 760–761. [5] M.F. Attie, J.R. Gill Jr., J.L. Stock, et al., Urinary calcium excretion in familial hypocalciuric hypercalcemia. Persistence of relative hypocalciuria after induction of hypoparathyroidism, J. Clin. Invest. 72 (1983) 667–676. [6] E.M. Brown, G. Gamba, D. Riccardi, et al., Cloning and characterization of an extracellular Ca2+ -sensing receptor from bovine parathyroid, Nature 366 (1993) 575–580. [7] E.M. Brown, M. Pollak, C.E. Seidman, et al., Calcium-ion-sensing cell-surface receptors, N. Engl. J. Med. 333 (1995) 234–240. [8] P.A. Friedman, F.A. Gesek, Cellular calcium transport in renal epithelia: measurement, mechanisms, and regulation, Physiol. Rev. 75 (1995) 429–471. [9] W.E. Lassiter, C.W. Gottschalk, M. Mylle, Micropuncture study of renal tubular reabsorption of calcium in normal rodents, Am. J. Physiol. 204 (1963) 771–775. [10] L.S. Costanzo, E.E. Windhager, Calcium and sodium transport by the distal convoluted tubule of the rat, Am. J. Physiol. 235 (1978) F492–F506. [11] R. Greger, F. Lang, H. Oberleithner, Distal site of calcium reabsorption in the rat nephron, Pflugers Arch. 374 (1978) 153–157. [12] D. Rouse, R.C.K. Ng, W.N. Suki, Calcium transport in the pars recta and thin descending limb of Henle of rabbit perfused in vitro, J. Clin. Invest. 65 (1980) 37–42. [13] K. Bomsztyk, J.P. George, F.S. Wright, Effects of luminal fluid anions on calcium transport by proximal tubule, Am. J. Physiol. 246 (1984) F600–F608. [14] W.N. Suki, D. Rouse, Hormonal regulation of calcium transport in thick ascending limb renal tubules, Am. J. Physiol. 241 (1981) F171–F174. [15] W.N. Suki, D. Rouse, R.C.K. Ng, et al., Calcium transport in the thick ascending limb of Henle. Heterogeneity of function in the medullary and cortical segments, J. Clin. Invest. 66 (1980) 1004–1009. [16] R. Vennekens, J.G. Hoenderop, J. Prenen, et al., Permeation and gating properties of the novel epithelial Ca2+ channel, J. Biol. Chem. 275 (2000) 3963–3969. [17] J.-B. Peng, X.-Z. Chen, U.V. Berger, et al., A rat kidney-specific calcium transporter in the distal nephron, J. Biol. Chem. 275 (2000) 28186–28194. [18] J. Loffing, D. Loffing-Cueni, V. Valderrabano, et al., Distribution of transcellular calcium and sodium transport pathways along mouse distal nephron, Am. J. Physiol. Renal Physiol. 281 (2001) F1021– F1027. [19] B. Nilius, R. Vennekens, J. Prenen, et al., Whole-cell and single channel monovalent cation currents through the novel rabbit epithelial Ca2+ channel ECaC, J. Physiol. 527 (2000) 239–248. [20] J.G.J. Hoenderop, A.W.C.M. van der Kemp, A. Hartog, et al., Molecular identification of the apical Ca2+ channel in 1, 25-dihydroxyvitamin D3 -responsive epithelia, J. Biol. Chem. 274 (1999) 8375–8378. [21] J.B. Peng, X.Z. Chen, U.V. Berger, et al., Molecular cloning and characterization of a channel-like transporter mediating intestinal calcium absorption, J. Biol. Chem. 274 (1999) 22739–22746. [22] D.H. Ellison, H. Velázquez, F.S. Wright, Thiazide-sensitive sodium chloride cotransport in early distal tubule, Am. J. Physiol. 253 (1987) F546–F554.

236

J. Ba, P.A. Friedman / Cell Calcium 35 (2004) 229–237

[23] F.A. Gesek, B.A. Coutermarsh, P.A. Friedman, Mechanism of thiazide-stimulated calcium transport in distal convoluted tubule cells, J. Am. Soc. Nephrol. 2 (1991) 620. [24] P.A. Friedman, H.S. Tenenhouse, Renal handling of calcium and phosphorous, in: F.L. Coe, M.J. Favus (Eds.), Disorders of Bone and Mineral Metabolism, Lippincott Williams & Wilkins, Philadelphia, 2002, pp. 3–33. [25] H. Murer, B. Kaissling, I. Forster, et al., Cellular mechanisms in proximal tubular handling of phosphate, in: D.W. Seldin, G. Giebisch (Eds.), The Kidney. Physiology and Pathophysiology, vol. 2, Lippincott Williams & Wilkins, Philadelphia, 2000, pp. 1869–1884. [26] H. Murer, N. Hernando, I. Forster, et al., Proximal tubular phosphate reabsorption: molecular mechanisms, Physiol. Rev. 80 (2000) 1373– 1409. [27] L. Beck, A.C. Karaplis, N. Amizuka, et al., Targeted inactivation of Npt2 in mice leads to severe renal phosphate wasting, hypercalciuria, and skeletal abnormalities, Proc. Natl. Acad. Sci. U.S.A. 95 (1998) 5372–5377. [28] H. Murer, N. Hernando, I. Forster, et al., Molecular aspects in the regulation of renal inorganic phosphate reabsorption: the type IIa sodium/inorganic phosphate co-transporter as the key player, Curr. Opin. Nephrol. Hypertens. 10 (2001) 555–561. [29] H. Murer, I. Forster, N. Hernando, et al., Posttranscriptional regulation of the proximal tubule NaPi-II transporter in response to PTH and dietary Pi , Am. J. Physiol. 277 (1999) F676–F684. [30] W. Suki, E.D. Lederer, D. Rouse, Renal transport of calcium, magnesium, and phosphate, in: B.M. Brenner, F.C. Rector (Eds.), Brenner and Rector’s the kidney, vol. 1, Saunders, Philadelphia, 2000, pp. 520–574. [31] T. Shimada, S. Mizutani, T. Muto, et al., Cloning and characterization of FGF23 as a causative factor of tumor-induced osteomalacia, Proc. Natl. Acad. Sci. U.S.A. 98 (2001) 6500–6505. [32] R. Kumar, New insights into phosphate homeostasis: fibroblast growth factor 23 and frizzled-related protein-4 are phosphaturic factors derived from tumors associated with osteomalacia, Curr. Opin. Nephrol. Hypertens. 11 (2002) 547–553. [33] G.J. Strewler, FGF23, hypophosphatemia, and rickets: has phosphatonin been found? Proc. Natl. Acad. Sci. U.S.A. 98 (2001) 5945– 5946. [34] L.D. Quarles, FGF23, PHEX, and MEPE regulation of phosphate homeostasis and skeletal mineralization, Am. J. Physiol. Endocrinol. Metab. 285 (2003) E1–E9. [35] T. Berndt, T.A. Craig, A.E. Bowe, et al., Secreted frizzled-related protein 4 is a potent tumor-derived phosphaturic agent, J. Clin. Invest. 112 (2003) 785–794. [36] D. Rouse, W.N. Suki, Modulation of phosphate absorption by calcium in the rabbit proximal convoluted tubule, J. Clin. Invest. 76 (1985) 630–636. [37] D. Riccardi, W.S. Lee, K. Lee, et al., Localization of the extracellular Ca2+ -sensing receptor and PTH/PTHrP receptor in rat kidney, Am. J. Physiol. 271 (1996) F951–F956. [38] D. Riccardi, A.E. Hall, N. Chattopadhyay, et al., Localization of the extracellular Ca2+ polyvalent cation-sensing protein in rat kidney, Am. J. Physiol. Renal Physiol. 274 (1998) F611–F622. [39] R.R. Butters Jr., N. Chattopadhyay, P. Nielsen, et al., Cloning and characterization of a calcium-sensing receptor from the hypercalcemic New Zealand white rabbit reveals unaltered responsiveness to extracellular calcium, J. Bone Miner. Res. 12 (1997) 568– 579. [40] J.M. Sands, M. Naruse, M. Baum, et al., Apical extracellular calcium/polyvalent cation-sensing receptor regulates vasopressin-elicited water permeability in rat kidney inner medullary collecting duct, J. Clin. Invest. 99 (1997) 1399–1405. [41] T.X. Yang, S. Hassan, Y.N.G. Huang, et al., Expression of PTHrP, PTH/PTHrP receptor, and Ca2+ -sensing receptor mRNAs along the rat nephron, Am. J. Physiol. 272 (1997) F751–F758.

[42] W.H. Wang, M. Lu, S.C. Hebert, Cytochrome P-450 metabolites mediate extracellular Ca2+ -induced inhibition of apical K+ channels in the TAL, Am. J. Physiol. 271 (1996) C103–C111. [43] D. Chabardes, M. Imbert, A. Clique, et al., PTH sensitive adenyl cyclase activity in different segments of the rabbit nephron, Pflugers Arch. 354 (1975) 229–239. [44] M.G. Brunette, D. Chabardes, M. Imbert-Teboul, et al., Hormone-sensitive adenylate cyclase along the nephron of genetically hypophosphatemic mice, Kidney Int. 15 (1979) 357–369. [45] F. Morel, A. Doucet, Hormonal control of kidney functions at the cell level, Physiol. Rev. 66 (1986) 377–468. [46] H. Nonoguchi, A. Owada, N. Kobayashi, et al., Immunohistochemical localization of V2 vasopressin receptor along the nephron and functional role of luminal V2 receptor in terminal inner medullary collecting ducts, J. Clin. Invest. 96 (1995) 1768–1778. [47] K. Takaichi, K. Kurokawa, High Ca2+ inhibits peptide hormone-dependent cAMP production specifically in thick ascending limbs of Henle, Miner. Electrolyte Metab. 12 (1986) 342–346. [48] G.A. Quamme, Control of magnesium transport in the thick ascending limb, Am. J. Physiol. 256 (1989) F197–F210. [49] E. Desfleurs, M. Wittner, S. Simeone, et al., Calcium-sensing receptor: regulation of electrolyte transport in the thick ascending limb of Henle’s loop, Kidney Blood Press Res. 21 (1998) 401–412. [50] H.I. Motoyama, P.A. Friedman, Calcium-sensing receptor regulation of PTH-dependent calcium absorption by mouse cortical ascending limbs, Am. J. Physiol. Renal Physiol. 283 (2002) F399–F406. [51] O. Kifor, R. Diaz, R. Butters, et al., The Ca2+ -sensing receptor (CaR) activates phospholipases C, A2 , and D in bovine parathyroid and CaR-transfected, human embryonic kidney (HEK293) cells, J. Bone Miner. Res. 12 (1997) 715–725. [52] M.E. Handlogten, C. Huang, N. Shiraishi, et al, The Ca2+ -sensing receptor activates cytosolic phospholipase A2 via a Gq ␣-dependent, ERK-independent pathway, J. Biol. Chem. 276 (2001) 13941–13948. [53] C. Huang, M.E. Handlogten, R.T. Miller, Parallel activation of phosphatidylinositol 4-kinase and phospholipase C by the extracellular calcium-sensing receptor, J. Biol. Chem. 277 (2002) 20293–20300. [54] P.A. Friedman, B.A. Coutermarsh, S.M. Kennedy, et al., Parathyroid hormone stimulation of calcium transport is mediated by dual signaling mechanisms involving protein kinase A and protein kinase C, Endocrinology 137 (1996) 13–20. [55] C.E. Magyar, K.E. White, R. Rojas, et al., Plasma membrane Ca2+ -ATPase and NCX1 Na+ /Ca2+ exchanger expression in distal convoluted tubule cells, Am. J. Physiol. Renal Physiol. 283 (2002) F29–F40. [56] J.G. Hoenderop, B. Nilius, R.J. Bindels, Molecular mechanism of active Ca2+ reabsorption in the distal nephron, Annu. Rev. Physiol. 64 (2002) 529–549. [57] K.A. Blankenship, J.J. Williams, M.S. Lawrence, et al., The calcium-sensing receptor regulates calcium absorption in MDCK cells by inhibition of PMCA, Am. J. Physiol. Renal Physiol. 280 (2001) F815–F822. [58] S.N. Kip, E.E. Strehler, Characterization of plasma membrane Ca2+ ATPases and their contribution to transcellular Ca2+ flux in MDCK epithelial cells, Am. J. Physiol. Renal Physiol. 284 (2002) F122–F132. [59] M. Bai, S. Quinn, S. Trivedi, et al., Expression and characterization of inactivating and activating mutations in the human Ca0 2+ -sensing receptor, J. Biol. Chem. 271 (1996) 19537–19545. [60] S.H.S. Pearce, E.M. Brown, Disorders of calcium ion sensing, J. Clin. Endocrinol. Metab. 81 (1996) 2030–2035. [61] M.R. Pollak, E.M. Brown, Y.-H.W. Chou, et al., Mutations in the human Ca2+ -sensing receptor gene cause familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism, Cell 75 (1993) 1297–1303. [62] M.R. Pollak, E.M. Brown, H.L. Estep, et al., Autosomal dominant hypocalcaemia caused by a Ca2+ -sensing receptor gene mutation, Nat. Genet. 8 (1994) 303–307.

J. Ba, P.A. Friedman / Cell Calcium 35 (2004) 229–237 [63] J.J. Anderson, R.V. Talmage, The effect of calcium infusion and calcitonin on plasma phosphate in sham-operated and thyroparathyroidectomized dogs, Endocrinology 93 (1973) 1222–1226. [64] C. Amiel, H. Kuntziger, S. Couette, et al., Evidence for a parathyroid hormone-independent calcium modulation of phosphate transport along the nephron, J. Clin. Invest. 57 (1976) 256–263. [65] A.R. Lavender, T.N. Pullman, Changes in inorganic phosphate excretion induced by renal arterial infusion of calcium, Am. J. Physiol 205 (1963) 1025–1032. [66] H. Rasmussen, C. Anast, C. Arnaud, Thyrocalcitonin, EGTA, and urinary electrolyte excretion, J. Clin. Invest. 46 (1967) 746–752. [67] D. Rouse, S. Sessoms, B.J. Stinebaugh, et al., The effect of hypocalcemia on renal bicarbonate absorption, Miner. Electrolyte Metab. 10 (1984) 31–35. [68] R.S. Mathias, E.M. Brown, Divalent cations modulate PTHdependent 3 ,5 -cyclic adenosine monophosphate production in renal proximal tubular cells, Endocrinology 128 (1991) 3005–3012. [69] D. Riccardi, M. Traebert, D.T. Ward, et al., Dietary phosphate and parathyroid hormone alter the expression of the calcium-sensing receptor (CaR) and the Na+ -dependent Pi transporter (NaPi-2) in the rat proximal tubule, Pflugers Arch. 441 (2000) 379–387. [70] H. Murer, J. Biber, Control of proximal tubular apical Na/Pi cotransport, Exp. Nephrol. 4 (1996) 201–204. [71] H. Murer, I. Forster, H. Hilfiker, et al., Cellular/molecular control of renal Na/Pi -cotransport, Kidney Int. Suppl. 65 (1998) S2–S10. [72] I. Keusch, M. Traebert, M. Lotscher, et al., Parathyroid hormone and dietary phosphate provoke a lysosomal routing of the proximal tubular Na/Pi-cotransporter type II, Kidney Int. 54 (1998) 1224–1232. [73] M.F. Pfister, I. Ruf, G. Stange, et al., Parathyroid hormone leads to the lysosomal degradation of the renal type II Na/Pi cotransporter, Proc. Natl. Acad. Sci. U.S.A. 95 (1998) 1909–1914. [74] M. Traebert, H. Völkl, J. Biber, et al., Luminal and contraluminal action of 1–34 and 3–34 PTH peptides on renal type IIa Na–Pi cotransporter, Am. J. Physiol. Renal Physiol. 278 (2000) F792–F798. [75] I.C. Forster, M. Traebert, M. Jankowski, et al., Protein kinase C activators induce membrane retrieval of type II Na+ -phosphate cotransporters expressed in Xenopus oocytes, J. Physiol. (Lond.) 517 (Pt 2) (1999) 327–340. [76] D. Zehnder, M. Hewison, The renal function of 25-hydroxyvitamin D3 -1␣-hydroxylase, Mol. Cell. Endocrinol. 151 (1999) 213–220. [77] U. Trechsel, J.A. Eisman, J.A. Fischer, et al., Calciumdependent, parathyroid hormone-independent regulation of 1,25dihydroxyvitamin D, Am. J. Physiol. Endocrinol. Metab. 239 (1980) E119–E124. [78] R. Bland, E.A. Walker, S.V. Hughes, et al., Constitutive expression of 25-hydroxyvitamin D3 -1␣-hydroxylase in a transformed human

[79]

[80]

[81]

[82]

[83]

[84]

[85]

[86]

[87]

[88]

[89]

[90]

[91]

237

proximal tubule cell line: evidence for direct regulation of vitamin D metabolism by calcium, Endocrinology 140 (1999) 2027–2034. R. Bland, D. Zehnder, S.V. Hughes, et al., Modification of calcium-sensing receptor expression modulates 25-hydroxylase D3-1␣-hydroxylase activity in a human proximal tubule cell line, J. Am. Soc. Nephrol. 13 (2002) 28A. D. Riccardi, J. Park, W.-S. Lee, et al., Cloning and functional expression of a rat kidney extracellular calcium/polyvalent cation-sensing receptor, Proc. Natl. Acad. Sci. U.S.A. 92 (1995) 131–135. S. Nielsen, J. Fror, M.A. Knepper, Renal aquaporins: key roles in water balance and water balance disorders, Curr. Opin. Nephrol. Hypertens. 7 (1998) 509–516. M.A. Dillingham, B.S. Dixon, R.J. Anderson, Calcium modulates vasopressin effect in rabbit cortical collecting tubule, Am. J. Physiol. Renal Physiol. 252 (1987) F115–F121. S.M. Jones, G. Frindt, E.E. Windhager, Effect of peritubular [Ca] or ionomycin on hydrosmotic response of CCTs to ADH or cAMP, Am. J. Physiol. Renal Physiol. 254 (1988) F240–F253. L. Wibell, P.A. Dahlberg, A. Karlsson, Hyperparathyroidism associated with distal tubular dysfunction but intact reabsorption of protein in the proximal tubules, Acta Med. Scand. 206 (1979) 507–510. N. Chattopadhyay, A. Mithal, E.M. Brown, The calcium-sensing receptor: a window into the physiology and pathophysiology of mineral ion metabolism, Endocrine Rev. 17 (1996) 289–307. S.J. Marx, M.F. Attie, J.L. Stock, et al., Maximal urine-concentrating ability: familial hypocalciuric hypercalcemia versus typical primary hyperparathyroidism, J. Clin. Endocrinol. Metab. 52 (1981) 736– 740. S.H. Pearce, C. Williamson, O. Kifor, et al., A familial syndrome of hypocalcemia with hypercalciuria due to mutations in the calcium-sensing receptor, N. Engl. J. Med. 335 (1996) 1115–1122. R.S. Mathias, H.T. Nguyen, M.Y. Zhang, et al., Reduced expression of the renal calcium-sensing receptor in rats with experimental chronic renal insufficiency, J. Am. Soc. Nephrol. 9 (1998) 2067–2074. S. Yano, T. Sugimoto, T. Tsukamoto, et al., Association of decreased calcium-sensing receptor expression with proliferation of parathyroid cells in secondary hyperparathyroidism, Kidney Int. 58 (2000) 1980– 1986. W.H. Chang, S. Pratt, T.H. Chen, et al., Coupling of calcium receptors to inositol phosphate and cyclic AMP generation in mammalian cells and Xenopus laevis oocytes and immunodetection of receptor protein by region-specific antipeptide antisera, J. Bone Miner. Res. 13 (1998) 570–580. J. Ba, D. Brown, P.A. Friedman, Calcium-sensing receptor regulation of PTH-inhibitable proximal tubule phosphate transport, Am. J. Physiol. Renal Physiol. 285 (2003) F1233–F1243.