Hormonal stimulation of Ca2+ and Mg2+ transport in the cortical thick ascending limb of Henle's loop of the mouse: evidence for a change in the paracellular pathway permeability

Paracellular Transport and Renal Tubule Calcium Handling: Emerging Roles in Kidney Stone Disease

The kidney plays a major role in maintenance of serum calcium concentration, which must be kept within a narrow range to avoid disruption of numerous physiologic processes that depend critically on the level of extracellular calcium, including cell signaling, bone structure, and muscle and nerve function. This defense of systemic calcium homeostasis comes, however, at the expense of the dumping of calcium into the kidney tissue and urine. Because of the large size and multivalency of the calcium ion, its salts are the least soluble among all the major cations in the body. The potential pathologic consequences of this are nephrocalcinosis and kidney stone disease. In this review, we discuss recent advances that have highlighted critical roles for the proximal tubule and thick ascending limb in renal calcium reabsorption, elucidated the molecular mechanisms for paracellular transport in these segments, and implicated disturbances in these processes in human disease.

Mechanisms of paracellular transport of magnesium in intestinal and renal epithelia

Magnesium is the fourth most abundant cation in the body. It plays a critical role in many biological processes, including the process of energy release. Paracellular transport of magnesium is mandatory for magnesium homeostasis. In addition to intestinal absorption that occurs in part across the paracellular pathway, magnesium is reabsorbed by the kidney tubule. The bulk of magnesium is reabsorbed through the paracellular pathway in the proximal tubule and the thick ascending limb of the loop of Henle. The finding that rare genetic diseases due to pathogenic variants in genes encoding specific claudins (CLDNs), proteins located at the tight junction that determine the selectivity and the permeability of the paracellular pathway, led to an awareness of their importance in magnesium homeostasis. Familial hypomagnesemia with hypercalciuria and nephrocalcinosis is caused by a loss of function of CLDN16 or CLDN19. Pathogenic CLDN10 variants cause HELIX syndrome, which is associated with a severe renal loss of sodium chloride and hypermagnesemia. The present review summarizes the current knowledge of the mechanisms and factors involved in paracellular magnesium permeability. The review also highlights some of the unresolved questions that need to be addressed.

Molecular mechanisms underlying paracellular calcium and magnesium reabsorption in the proximal tubule and thick ascending limb

Calcium and magnesium are the most abundant divalent cations in the body. The plasma level is controlled by coordinated interaction between intestinal absorption, reabsorption in the kidney, and, for calcium at least, bone storage and exchange. The kidney adjusts urinary excretion of these ions in response to alterations in their systemic concentration. Free ionized and anion-complexed calcium and magnesium are filtered at the glomerulus. The majority (i.e., >85%) of filtered divalent cations are reabsorbed via paracellular pathways from the proximal tubule and thick ascending limb (TAL) of the loop of Henle. Interestingly, the largest fraction of filtered calcium is reabsorbed from the proximal tubule (65%), while the largest fraction of filtered magnesium is reclaimed from the TAL (60%). The paracellular pathways mediating these fluxes are composed of tight junctional pores formed by claudins. In the proximal tubule, claudin-2 and claudin-12 confer calcium permeability, while the exact identity of the magnesium pore remains to be determined. Claudin-16 and claudin-19 contribute to the calcium and magnesium permeable pathway in the TAL. In this review, we discuss the data supporting these conclusions and speculate as to why there is greater fractional calcium reabsorption from the proximal tubule and greater fractional magnesium reabsorption from the TAL.

NHE3 in the thick ascending limb is required for sustained but not acute furosemide-induced urinary acidification

Na+/H+ exchanger isoform 3 (NHE3) facilitates Na+ reabsorption and H+ secretion by the kidneys. Despite stronger NHE3 abundance in the thick ascending limb (TAL) compared with the S1 and S2 segments of the proximal tubule, the role of NHE3 in the TAL is poorly understood. To investigate the role of NHE3 in the TAL, we generated and phenotyped TAL-specific NHE3 knockout (NHE3TAL-KO) mice. Compared with control mice, NHE3TAL-KO mice did not show significant differences in body weight, blood pH, or plasma Na+, K+, or Cl- levels. Fluid intake trended to be higher and urine osmolality was significantly lower in NHE3TAL-KO mice. Despite a similar glomerular filtration rate, NHE3TAL-KO mice had a greater urinary K+-to-creatinine ratio. One proposed role of NHE3 relates to furosemide-induced urinary acidification. Acute bolus treatment with furosemide under anesthesia did not result in differences in the dose dependence of urinary flow rate, Cl- excretion, or maximal urinary acidification between genotypes; however, in contrast with control mice, urinary pH returned immediately toward baseline levels in NHE3TAL-KO mice. Chronic furosemide treatment reduced urine osmolality similarly in both genotypes but metabolic alkalosis, hypokalemia, and calciuresis were absent in NHE3TAL-KO mice. Compared with vehicle, chronic furosemide treatment resulted in greater Na+-K+-2Cl- abundance regardless of genotype. Na+-phosphate cotransporter 2a abundance was also greater in furosemide-treated control mice compared with vehicle treatment, an effect that was absent in NHE3TAL-KO mice. In summary, NHE3 in the TAL plays a role in the sustained acidification effect of furosemide. Consistent with this, long-term treatment with furosemide did not result in metabolic alkalosis in NHE3TAL-KO mice.NEW & NOTEWORTHY Na+/H+ exchanger isoform 3 (NHE3) is very abundant in the thick ascending limb (TAL) compared with the proximal tubule. Much has been learned about the role of NHE3 in the proximal tubule; however, the function of NHE3 in the TAL remains elusive. A novel mouse model that lacks NHE3 selectively in the TAL not only shows a phenotype under baseline conditions but also identifies that NHE3 is required for sustained but not acute furosemide-induced urinary acidification.

Mechanisms Underlying Calcium Nephrolithiasis

Nephrolithiasis is a worldwide problem with increasing prevalence, enormous costs, and significant morbidity. Calcium-containing kidney stones are by far the most common kidney stones encountered in clinical practice. Consequently, hypercalciuria is the greatest risk factor for kidney stone formation. Hypercalciuria can result from enhanced intestinal absorption, increased bone resorption, or altered renal tubular transport. Kidney stone formation is complex and driven by high concentrations of calcium-oxalate or calcium-phosphate in the urine. After discussing the mechanism mediating renal calcium salt precipitation, we review recent discoveries in renal tubular calcium transport from the proximal tubule, thick ascending limb, and distal convolution. Furthermore, we address how calcium is absorbed from the intestine and mobilized from bone. The effect of acidosis on bone calcium resorption and urinary calcium excretion is also considered. Although recent discoveries provide insight into these processes, much remains to be understood in order to provide improved therapies for hypercalciuria and prevent kidney stone formation. Expected final online publication date for the Annual Review of Physiology, Volume 84 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Medical Hypoparathyroidism

Hypoparathyroidism is a metabolic disorder characterized by hypocalcemia, hyperphosphatemia, and inadequate levels of or function of parathyroid hormone (PTH). The authors review the nonsurgical or medical causes of hypoparathyroidism. The most common of the nonsurgical causes is autoimmune destruction of the parathyroid. Magnesium deficiency or excess can cause a functional hypoparathyroidism. Genetic conditions result in hypoparathyroidism as part of a syndrome or in isolation. Pseudohypoparathyroidism reflects a resistance to PTH. Infiltrative, metastatic, radiation destruction, mineral deposition, or idiopathic are uncommon causes of hypoparathyroidism. This article reviews the causes of hypoparathyroidism and an approach to the evaluation of this condition.

Effects of phospho- and calciotropic hormones on electrolyte transport in the proximal tubule

Calcium and phosphate are critical for a myriad of physiological and cellular processes within the organism. Consequently, plasma levels of calcium and phosphate are tightly regulated. This occurs through the combined effects of the phospho- and calciotropic hormones, parathyroid hormone (PTH), active vitamin D ₃, and fibroblast growth factor 23 (FGF23). The organs central to this are the kidneys, intestine, and bone. In the kidney, the proximal tubule reabsorbs the majority of filtered calcium and phosphate, which amounts to more than 60% and 90%, respectively. The basic molecular mechanisms responsible for phosphate reclamation are well described, and emerging work is delineating the molecular identity of the paracellular shunt wherein calcium permeates the proximal tubular epithelium. Significant experimental work has delineated the molecular effects of PTH and FGF23 on these processes as well as their regulation of active vitamin D ₃ synthesis in this nephron segment. The integrative effects of both phospho- and calciotropic hormones on proximal tubular solute transport and subsequently whole body calcium-phosphate balance thus have been further complicated. Here, we first review the molecular mechanisms of calcium and phosphate reabsorption from the proximal tubule and how they are influenced by the phospho- and calciotropic hormones acting on this segment and then consider the implications on both renal calcium and phosphate handling as well as whole body mineral balance.

Ways of calcium reabsorption in the kidney

The role of the kidney in calcium homeostasis has been reshaped from a classic view in which the kidney was regulated by systemic calcitropic hormones such as vitamin D3 or parathyroid hormone to an organ actively taking part in the regulation of calcium handling. With the identification of the intrinsic renal calcium-sensing receptor feedback system, the regulation of paracellular calcium transport involving claudins, and new paracrine regulators such as klotho, the kidney has emerged as a crucial modulator not only of calciuria but also of calcium homeostasis. This review summarizes recent molecular and endocrine contributors to renal calcium handling and highlights the tight link between calcium and sodium reabsorption in the kidney.

Regulation of calcium reabsorption along the rat nephron: a modeling study

We expanded a mathematical model of transepithelial transport along the rat nephron to include the transport of Ca2+ and probe the impact of calcium-sensing mechanisms on Ca2+ reabsorption. The model nephron extends from the medullary thick ascending limb (mTAL) to the inner medullary collecting duct (IMCD). Our model reproduces several experimental findings, such as measurements of luminal Ca2+ concentrations in cortical tubules, and the effects of furosemide or deletion of the transient receptor potential channel vanilloid subtype 5 (TRPV5) on urinary Ca2+ excretion. In vitro microperfusion of rat TAL has demonstrated that activation of the calcium-sensing receptor CaSR lowers the TAL permeability to Ca2+, PCaTAL (Loupy A, Ramakrishnan SK, Wootla B, Chambrey R, de la Faille R, Bourgeois S, Bruneval P, Mandet C, Christensen EI, Faure H, Cheval L, Laghmani K, Collet C, Eladari D, Dodd RH, Ruat M, Houillier P. J Clin Invest 122: 3355, 2012). Our results suggest that this regulatory mechanism significantly impacts renal Ca2+ handling: when plasma Ca2+ concentration ([Ca2+]) is raised by 10%, the CaSR-mediated reduction in PCaTAL per se is predicted to enhance urinary Ca2+ excretion by ∼30%. If high [Ca2+] also induces renal outer medullary potassium (ROMK) inhibition, urinary Ca2+ excretion is further raised. In vitro, increases in luminal [Ca2+] have been shown to activate H+-ATPase pumps in the outer medullary CD and to lower the water permeability of IMCD. Our model suggests that if these responses exhibit the sigmoidal dependence on luminal [Ca2+] that is characteristic of CaSR, then the impact of elevated Ca2+ levels in the CD on urinary volume and pH remains limited. Finally, our model suggests that CaSR inhibitors could significantly reduce urinary Ca2+ excretion in hypoparathyroidism, thereby reducing the risk of calcium stone formation.

Mechanisms and regulation of renal magnesium transport

Magnesium's most important role is in the release of chemical energy. Although most magnesium is stored outside of the extracellular fluid compartment, the regulated value is blood magnesium concentration. Cellular magnesium and bone magnesium do not play a major role in the defense of blood magnesium concentration; the major role is played by the kidney, where the renal tubule matches the urinary magnesium excretion and the net entry of magnesium into the extracellular fluid. In the kidney, magnesium is reabsorbed in the proximal tubule, the thick ascending limb of the loop of Henle, and the distal convoluted tubule. Magnesium absorption is mainly paracellular in the proximal tubule and in the thick ascending limb of the loop of Henle, whereas it is transcellular in the distal convoluted tubule. Several hormones and extracellular magnesium itself alter the distal tubular handling of magnesium, but the hormone(s) regulating extracellular magnesium concentration remains unknown.

Paracellular calcium transport across renal and intestinal epithelia

Calcium (Ca(2+)) is a key constituent in a myriad of physiological processes from intracellular signalling to the mineralization of bone. As a consequence, Ca(2+) is maintained within narrow limits when circulating in plasma. This is accomplished via regulated interplay between intestinal absorption, renal tubular reabsorption, and exchange with bone. Many studies have focused on the highly regulated active transcellular transport pathways for Ca(2+) from the duodenum of the intestine and the distal nephron of the kidney. However, comparatively little work has examined the molecular constituents creating the paracellular shunt across intestinal and renal epithelium, the transport pathway responsible for the majority of transepithelial Ca(2+) flux. More specifically, passive paracellular Ca(2+) absorption occurs across the majority of the intestine in addition to the renal proximal tubule and thick ascending limb of Henle's loop. Importantly, recent studies demonstrated that Ca(2+) transport through the paracellular shunt is significantly regulated. Therefore, we have summarized the evidence for different modes of paracellular Ca(2+) flux across renal and intestinal epithelia and highlighted recent molecular insights into both the mechanism of secondarily active paracellular Ca(2+) movement and the identity of claudins that permit the passage of Ca(2+) through the tight junction of these epithelia.

Proximal tubular NHEs: sodium, protons and calcium?

Na⁺/H⁺ exchange activity in the apical membrane of the proximal tubule is fundamental to the reabsorption of Na⁺ and water from the filtrate. The role of this exchange process in bicarbonate reclamation and, consequently, the maintenance of acid-base homeostasis has been appreciated for at least half a century and remains a pillar of renal tubular physiology. More recently, apical Na⁺/H⁺ exchange, mediated by Na⁺/H⁺ exchanger isoform 3 (NHE3), has been implicated in proximal tubular reabsorption of Ca²⁺ and Ca²⁺ homeostasis in general. Overexpression of NHE3 increased paracellular Ca²⁺ flux in a proximal tubular cell model. Consistent with this observation, mice with genetic deletion of Nhe3 have a noticable renal Ca²⁺ leak. These mice also display decreased intestinal Ca²⁺ uptake and osteopenia. This review highlights the traditional roles of proximal tubular Na⁺/H⁺ exchange and summarizes recent novel findings implicating the predominant isoform, NHE3, in Ca²⁺ homeostasis.

PTH-independent regulation of blood calcium concentration by the calcium-sensing receptor

Tight regulation of calcium levels is required for many critical biological functions. The Ca2+-sensing receptor (CaSR) expressed by parathyroid cells controls blood calcium concentration by regulating parathyroid hormone (PTH) secretion. However, CaSR is also expressed in other organs, such as the kidney, but the importance of extraparathyroid CaSR in calcium metabolism remains unknown. Here, we investigated the role of extraparathyroid CaSR using thyroparathyroidectomized, PTH-supplemented rats. Chronic inhibition of CaSR selectively increased renal tubular calcium absorption and blood calcium concentration independent of PTH secretion change and without altering intestinal calcium absorption. CaSR inhibition increased blood calcium concentration in animals pretreated with a bisphosphonate, indicating that the increase did not result from release of bone calcium. Kidney CaSR was expressed primarily in the thick ascending limb of the loop of Henle (TAL). As measured by in vitro microperfusion of cortical TAL, CaSR inhibitors increased calcium reabsorption and paracellular pathway permeability but did not change NaCl reabsorption. We conclude that CaSR is a direct determinant of blood calcium concentration, independent of PTH, and modulates renal tubular calcium transport in the TAL via the permeability of the paracellular pathway. These findings suggest that CaSR inhibitors may provide a new specific treatment for disorders related to impaired PTH secretion, such as primary hypoparathyroidism.

PTH-independent regulation of blood calcium concentration by the calcium-sensing receptor

Tight regulation of calcium levels is required for many critical biological functions. The Ca2+-sensing receptor (CaSR) expressed by parathyroid cells controls blood calcium concentration by regulating parathyroid hormone (PTH) secretion. However, CaSR is also expressed in other organs, such as the kidney, but the importance of extraparathyroid CaSR in calcium metabolism remains unknown. Here, we investigated the role of extraparathyroid CaSR using thyroparathyroidectomized, PTH-supplemented rats. Chronic inhibition of CaSR selectively increased renal tubular calcium absorption and blood calcium concentration independent of PTH secretion change and without altering intestinal calcium absorption. CaSR inhibition increased blood calcium concentration in animals pretreated with a bisphosphonate, indicating that the increase did not result from release of bone calcium. Kidney CaSR was expressed primarily in the thick ascending limb of the loop of Henle (TAL). As measured by in vitro microperfusion of cortical TAL, CaSR inhibitors increased calcium reabsorption and paracellular pathway permeability but did not change NaCl reabsorption. We conclude that CaSR is a direct determinant of blood calcium concentration, independent of PTH, and modulates renal tubular calcium transport in the TAL via the permeability of the paracellular pathway. These findings suggest that CaSR inhibitors may provide a new specific treatment for disorders related to impaired PTH secretion, such as primary hypoparathyroidism.

Hereditary tubular transport disorders: implications for renal handling of Ca2+ and Mg2+

The kidney plays an important role in maintaining the systemic Ca2+ and Mg2+ balance. Thus the renal reabsorptive capacity of these cations can be amended to adapt to disturbances in plasma Ca2+ and Mg2+ concentrations. The reabsorption of Ca2+ and Mg2+ is driven by transport of other electrolytes, sometimes through selective channels and often supported by hormonal stimuli. It is, therefore, not surprising that monogenic disorders affecting such renal processes may impose a shift in, or even completely blunt, the reabsorptive capacity of these divalent cations within the kidney. Accordingly, in Dent's disease, a disorder with defective proximal tubular transport, hypercalciuria is frequently observed. Dysfunctional thick ascending limb transport in Bartter's syndrome, familial hypomagnesaemia with hypercalciuria and nephrocalcinosis, and diseases associated with Ca2+-sensing receptor defects, markedly change tubular transport of Ca2+ and Mg2+. In the distal convolutions, several proteins involved in Mg2+ transport have been identified [TRPM6 (transient receptor potential melastatin 6), proEGF (pro-epidermal growth factor) and FXYD2 (Na+/K+-ATPase gamma-subunit)]. In addition, conditions such as Gitelman's syndrome, distal renal tubular acidosis and pseudohypoaldosteronism type II, as well as a mitochondrial defect associated with hypomagnesaemia, all change the renal handling of divalent cations. These hereditary disorders have, in many cases, substantially increased our understanding of the complex transport processes in the kidney and their contribution to the regulation of overall Ca2+ and Mg2+ balance.

Physiology of epithelial Ca2+ and Mg2+ transport

Ca2+ and Mg2+ are essential ions in a wide variety of cellular processes and form a major constituent of bone. It is, therefore, essential that the balance of these ions is strictly maintained. In the last decade, major breakthrough discoveries have vastly expanded our knowledge of the mechanisms underlying epithelial Ca2+ and Mg2+ transport. The genetic defects underlying various disorders with altered Ca2+ and/or Mg2+ handling have been determined. Recently, this yielded the molecular identification of TRPM6 as the gatekeeper of epithelial Mg2+ transport. Furthermore, expression cloning strategies have elucidated two novel members of the transient receptor potential family, TRPV5 and TRPV6, as pivotal ion channels determining transcellular Ca2+ transport. These two channels are regulated by a variety of factors, some historically strongly linked to Ca2+ homeostasis, others identified in a more serendipitous manner. Herein we review the processes of epithelial Ca2+ and Mg2+ transport, the molecular mechanisms involved, and the various forms of regulation.

Recent advances in physiological calcium homeostasis

A constant extracellular Ca2+ concentration is required for numerous physiological functions at tissue and cellular levels. This suggests that minor changes in Ca2+ will be corrected by appropriate homeostatic systems. The system regulating Ca2+ homeostasis involves several organs and hormones. The former are mainly the kidneys, skeleton, intestine and the parathyroid glands. The latter comprise, amongst others, the parathyroid hormone, vitamin D and calcitonin. Progress has recently been made in the identification and characterisation of Ca2+ transport proteins CaT1 and ECaC and this has provided new insights into the molecular mechanisms of Ca2+ transport in cells. The G-protein coupled calcium-sensing receptor, responsible for the exquisite ability of the parathyroid gland to respond to small changes in serum Ca2+ concentration was discovered about a decade ago. Research has focussed on the molecular mechanisms determining the serum levels of 1,25(OH)2D3, and on the transcriptional activity of the vitamin D receptor. The aim of recent work has been to elucidate the mechanisms and the intracellular signalling pathways by which parathyroid hormone, vitamin D and calcitonin affect Ca2+ homeostasis. This article summarises recent advances in the understanding and the molecular basis of physiological Ca2+ homeostasis.

Calcium absorption across epithelia

Ca(2+) is an essential ion in all organisms, where it plays a crucial role in processes ranging from the formation and maintenance of the skeleton to the temporal and spatial regulation of neuronal function. The Ca(2+) balance is maintained by the concerted action of three organ systems, including the gastrointestinal tract, bone, and kidney. An adult ingests on average 1 g Ca(2+) daily from which 0.35 g is absorbed in the small intestine by a mechanism that is controlled primarily by the calciotropic hormones. To maintain the Ca(2+) balance, the kidney must excrete the same amount of Ca(2+) that the small intestine absorbs. This is accomplished by a combination of filtration of Ca(2+) across the glomeruli and subsequent reabsorption of the filtered Ca(2+) along the renal tubules. Bone turnover is a continuous process involving both resorption of existing bone and deposition of new bone. The above-mentioned Ca(2+) fluxes are stimulated by the synergistic actions of active vitamin D (1,25-dihydroxyvitamin D(3)) and parathyroid hormone. Until recently, the mechanism by which Ca(2+) enter the absorptive epithelia was unknown. A major breakthrough in completing the molecular details of these pathways was the identification of the epithelial Ca(2+) channel family consisting of two members: TRPV5 and TRPV6. Functional analysis indicated that these Ca(2+) channels constitute the rate-limiting step in Ca(2+)-transporting epithelia. They form the prime target for hormonal control of the active Ca(2+) flux from the intestinal lumen or urine space to the blood compartment. This review describes the characteristics of epithelial Ca(2+) transport in general and highlights in particular the distinctive features and the physiological relevance of the new epithelial Ca(2+) channels accumulating in a comprehensive model for epithelial Ca(2+) absorption.

The human paracellin-1 gene (hPCLN-1): renal epithelial cell-specific expression and regulation

Tubular reabsorption of Mg2+ is mediated by the tight junction protein paracellin-1, which is encoded by the gene PCLN-1 (CLDN16) and exclusively expressed in the kidney. Tubular Mg2+ reclamation is modulated by many hormones and factors. The aim of this study was to define regulatory elements essential for renal tubular cell-specific expression of human PCLN-1 (hPCLN-1) and to explore the effect of Mg2+ transport modulators on the paracellin-1 gene promoter. Endogenous paracellin-1 mRNA and protein were detected in renal cell lines opossom kidney (OK), HEK293, and MDCT, but not in the fibroblast cell line NIH3T3. A 7.5-kb hPCLN-1 5'-flanking DNA sequence along with seven 5'-deletion products were cloned into luciferase reporter vectors and transiently transfected into the renal and nonrenal cells. The highest levels of luciferase activity resulted from transfection of a 5'-flanking 2.5-kb fragment (pJ2M). This activity was maximal in OK cells, was orientation dependent, and was absent in NIH3T3 cells. Mg2+ deprivation significantly increased pJ2M-driven activity in transfected OK cells, whereas Mg2+ load decreased it compared with conditions of normal Mg2+. Deletion analysis along with electrophoretic mobility-shift assay demonstrated that OK cells contain nuclear proteins, which bind a 70-bp region between -1633 and -1703 of major functional significance. Deleting this 70-bp segment, which contains a single peroxisome proliferator-response element (PPRE), or mutating the PPRE, caused a 60% reduction in luciferase activity. Stimulating the 70-bp sequence with 1,25(OH)2 vitamin D decreased luciferase activity by 52%. This effect of 1,25(OH)2 vitamin D was abolished in the absence of PPRE or in the presence of mutated PPRE. We conclude that the PPRE within this 70-bp DNA region may play a key role in the cell-specific and regulatory activity of the hPCLN-1 promoter. Ambient Mg2+ concentration and 1,25(OH)2 vitamin D may modulate paracellular, paracellin-1-mediated, Mg2+ transport at the transcriptional level. 1,25(OH)2 vitamin D exerts its activity on the hPCLN-1 promoter likely via the PPRE site.

Paracellin-1 is critical for magnesium and calcium reabsorption in the human thick ascending limb of Henle

BACKGROUND

A new protein, named paracellin 1 (PCLN-1), expressed in human thick ascending limb (TAL) tight junctions, possibly plays a critical role in the control of magnesium and calcium reabsorption, since mutations of PCLN-1 are present in the hypomagnesemia hypercalciuria syndrome (HHS). However, no functional experiments have demonstrated that TAL magnesium and calcium reabsorption were actually impaired in patients with HHS.

METHODS

Genetic studies were performed in the kindred of two unrelated patients with HHS. Renal magnesium and calcium reabsorption in TAL were analyzed in one homozygous affected patient of each family, one patient with extrarenal hypomagnesemia (ERH), and two control subjects (CSs).

RESULTS

We found two yet undescribed mutations of PCLN-1 (Gly 162 Val, Ala 139 Val). In patients with HHS, renal magnesium and calcium reabsorptions were impaired as expected; NaCl renal conservation during NaCl deprivation and NaCl tubular reabsorption in diluting segment were intact. Furosemide infusion in CS markedly increased NaCl, Mg, and Ca urinary excretion rates. In HHS patients, furosemide similarly increased NaCl excretion, but failed to increase Mg and Ca excretion. Acute MgCl(2) infusion in CS and ERH patient provoked a dramatic increase in urinary calcium excretion without change in NaCl excretion. When combined with MgCl(2) infusion, furosemide infusion remained able to induce normal natriuretic response, but was unable to increase urinary magnesium and calcium excretion further. In HHS patients, calciuric response to MgCl(2) infusion was blunted.

CONCLUSION

This study is the first to our knowledge to demonstrate that homozygous mutations of PCLN-1 result in a selective defect in paracellular Mg and Ca reabsorption in the TAL, with intact NaCl reabsorption ability at this site. In addition, the study supports a selective physiological effect of basolateral Mg(2+) and Ca(2+) concentration on TAL divalent cation paracellular permeability, that is, PCLN-1 activity.

Leucokinin and the modulation of the shunt pathway in Malpighian tubules

Transepithelial secretion in Malpighian tubules of the yellow fever mosquito (Aedes aegypti) is mediated by active transport of Na(+) and K(+) through principal cells and passive Cl(-) transport through the shunt. Permeation through the shunt was assessed by measuring transepithelial halide diffusion potentials in isolated perfused Malpighian tubules, after first inhibiting active transport with dinitrophenol. Diffusion potentials were small under control conditions, revealing Eisenman selectivity sequence I (I(-)>Br(-)>Cl(-)>F(-)) which is the halide mobility sequence in free solution. Accordingly, electrical field strengths of the shunt are small, selecting halides for passage on the basis of hydrated size. Leucokinin-VIII (LK-VIII) significantly increased the shunt conductance from 57.1 µS/cm to 250.0 µS/cm. In parallel, the shunt selectivity sequence shifted to Eisenman sequence III (Br(-)>Cl(-)>I(-)>F(-)), revealing increased electrical field strengths in the shunt, now capable of selecting small, dehydrated halides for passage. High concentrations of peritubular F(-) (142.5 mM) duplicated the effects of LK-VIII on shunt conductance and selectivity, suggesting a role for G-protein. In the presence of LK-VIII (or F(-)), coulombic interactions between the shunt and I(-) and F(-) may be strong enough to cause binding, thereby blocking the passage of Cl(-). Thus, LK-VIII increases both shunt conductance and selectivity, presumably via G-protein.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

Physiology of renal sodium transport

A wealth of studies performed with a spectrum of methods spanning simple clearance studies to the molecular identification of ion transporters has increased our understanding of how approximately 1.7 kg of NaCl and 180 L of H2O are absorbed by renal tubules in man and how the urinary excretion is fine-tuned to meet homeostatic requirements. This review will summarize our current understanding. In the proximal nephron, approximately 60 to 70% of the filtered Na+ and H2O is absorbed together with approximately 90% of the filtered HCO3-. The exact quantities are determined by many regulatory factors, such as glomerulotubular balance, angiotensin II, endothelin, sympathetic innervation, parathyroid hormone, dopamine, acid base status and others. The essential components of absorption are luminal membrane Na+/H+ exchange and the basolateral (Na+ + K+)-ATPase. In the thick ascending limb of the loop of Henle, 20 to 30% of the filtered NaCl is absorbed via Na+2Cl-K+ cotransport driven by the basolateral (Na+ + K+)-ATPase. No H2O is absorbed at this nephron site. The transport rate is determined by the Na+ load and by several hormones and neurotransmitters, including prostaglandins, parathyroid hormone, glucagon, calcitonin, arginine vasopressin (AVP), and adrenaline. In the distal tubule, some 5 to 10% of the filtered load is absorbed via Na+Cl- cotransport in the luminal membrane driven by the basolateral (Na+ + K+)-ATPase. The rate of transport is again determined by the delivered load and by several hormones and neurotransmitters. One of the tasks of the collecting duct is to control the absorption of approximately 10 to 15% of the filtered H2O, regulated by AVP, and just a few percent of the filtered Na+, controlled by aldosterone and natriuretic hormone. The water absorption proceeds through the luminal membrane via aquaporin 2 and through the basolateral membrane via aquaporin 3 channels and is driven by the osmotic gradient built up by the counter current concentrating system. The Na+ absorption occurs via Na+ channels present in the luminal membrane driven by the basolateral (Na+ + K+)-ATPase. With no pharmacological interference, urinary excretion of Na+ can vary between less than 0.1% and no more than 3% of the filtered load, and that of H2O can vary between 0.3 and 15%.

Overexpression of protein kinase C-delta increases tight junction permeability in LLC-PK1 epithelia

The Ca2+-independent delta-isoform of protein kinase C (PKC-delta) was overexpressed in LLC-PK1 epithelia and placed under control of a tetracycline-responsive expression system. In the absence of tetracycline, the exogenous PKC-delta is expressed. Western immunoblots show that the overexpressed PKC-delta is found in the cytosolic, membrane-associated, and Triton-insoluble fractions. Overexpression of PKC-delta produced subconfluent and confluent epithelial morphologies similar to that observed on exposure of wild-type cells to the phorbol ester 12-O-tetradecanoylphorbol-13-acetate. Transepithelial electrical resistance (RT) in cell sheets overexpressing PKC-delta was only 20% of that in cell sheets incubated in the presence of tetracycline, in which the amount of PKC-delta and RT were similar to those in LLC-PK1 parental cell sheets. Overexpression of PKC-delta also elicited a significant increase in transepithelial flux of D-[14C]mannitol and a radiolabeled 2 x 10(6)-molecular-weight dextran, suggesting with the RT decrease that overexpression increased paracellular, tight junctional permeability. Electron microscopy showed that PKC-delta overexpression results in a multilayered cell sheet, the tight junctions of which are almost uniformly permeable to ruthenium red. Freeze-fracture electron microscopy indicates that overexpression of PKC-delta results in a more disorganized arrangement of tight junctional strands. As with LLC-PK1 cell sheets treated with 12-O-tetradecanoylphorbol-13-acetate, the reduced RT, increased D-mannitol flux, and tight junctional leakiness to ruthenium red that are seen with PKC-delta overexpression suggest the involvement of PKC-delta in regulation of tight junctional permeability.

Renal magnesium handling: new insights in understanding old problems

Recent research has provided new concepts in our understanding of renal magnesium handling. Although the majority of the filtered magnesium is reabsorbed within the loop of Henle, it is now recognized that the distal tubule also plays an important role in magnesium conservation. Magnesium absorption within the cTAL segment of the loop is passive and dependent on the transepithelial voltage. Magnesium transport in the DCT is active and transcellular in nature. Many of the hormonal (PTH, calcitonin, glucagon, AVP) and nonhormonal (magnesium-restriction, acid-base changes, potassium-depletion) influences that affect magnesium transport within the cTAL similarly alter magnesium absorption within the DCT. However, the cellular mechanisms are different. Actions within the loop affect either the transepithelial voltage or the paracellular permeability. Influences acting in the DCT involve changes in active transcellular transport either Mg2+ entry across the apical membrane or Mg2+ exit from the basolateral side. These transport processes are fruitful areas for future research. An additional regulatory control has recently been recognized that involves an extracellular Ca2+/Mg(2+)-sensing receptor. This receptor is present in the basolateral membrane of the TAL and DCT and modulates magnesium and calcium conservation with elevation in plasma divalent cation concentration. Further studies are warranted to determine the physiological role of the Ca2+/Mg(2+)-sensing receptor, but activating and inactivating mutations have been described that result in renal magnesium-wasting and hypermagnesemia, respectively. All of these receptor-mediated controls change calcium absorption in addition to magnesium transport. Selective magnesium control is through intrinsic control of Mg2+ entry into distal tubule cells. The cellular mechanisms that intrinsically regulate magnesium transport have yet to be described. Familial diseases associated with renal magnesium-wasting provide a unique opportunity to study these intrinsic controls. Loop diuretics such as furosemide increase magnesium excretion by virtue of its effects on the transepithelial voltage thereby inhibiting passive magnesium absorption. Distally acting diuretics, like amiloride and chlorothiazide, enhance Mg2+ entry into DCT cells. Amiloride may be used as a magnesium-conserving diuretic whereas chlorothiazide may lead to potassium-depletion that compromises renal magnesium absorption. Patients with Bartter's and Gitelman's syndromes, diseases of salt transport in the loop and distal tubule, respectively, are associated with disturbances in renal magnesium handling. These may provide useful lessons in understanding segmental control of magnesium reabsorption. Metabolic acidosis diminishes magnesium absorption in MDCT cells by protonation of the Mg2+ entry pathway. Metabolic alkalosis increases magnesium permeability across the cTAL paracellular pathway and stimulates Mg2+ entry into DCT cells. Again, these changes are likely due to protonation of charges along the paracellular pathway of the cTAL and the putative Mg2+ channel of the DCT. Cellular potassium-depletion diminishes the voltage-dependent magnesium absorption in the TAL and Mg2+ entry into MDCT cells. However, the relationship between potassium and magnesium balance is far from clear. For instance, magnesium-wasting is more commonly found in patients with Gitelman's disease than Bartter's but both have hypokalemia. Further studies are needed to sort out these discrepancies. Phosphate deficiency also decreases Mg2+ uptake in distal cells but it apparently does so by mechanisms other than those observed in potassium depletion. Accordingly, potassium depletion, phosphate deficiency, and metabolic acidosis may be additive. The means by which cellular potassium and phosphate alter magnesium handling are unclear. Research in the nineties has increased our understanding of renal magnesium transport and regulation, but there are many in

Ca2+, Mg2+ and K+ transport in the cortical and medullary thick ascending limb of the rat nephron: influence of transepithelial voltage

Isolated segments of rat cortical (cTAL) and medullary (mTAL) thick ascending limbs were microperfused and the transepithelial net fluxes (JX) were determined by measuring the composition of the collected fluid with an electron microprobe. When perfused with symmetrical solutions both segments showed similar JNa and JCl and lumen-positive transepithelial voltage (Vte = 7-8 mV). JMg, JCa and JK were not significantly different from zero. When perfused with asymmetrical solutions (lumen 50 mM, bath 150 mM NaCl), the mean Vte were 23 mV and 17 mV in the cTAL and mTAL respectively; this rise was accompanied by significant increases in JMg and JCa in the cTAL, but not in the mTAL, and a marked increase in JK in both segments. It is concluded that, in the rat, divalent cations can be reabsorbed in the cTAL, and K+ can be reabsorbed in the cTAL and mTAL. The transport is voltage-dependent. The mTAL can reabsorb neither Mg2+ nor Ca2+, whatever Vte.