Role of the Kidney in Calcium and Phosphorus Homeostasis

Acknowledgments

We acknowledge the coauthors of this chapter in the previous edition: Drs. Abhijeet Pal, Juhi Kumar, Craig B. Woda, Robert P. Woroniecki, and Susan E. Mulroney.

Introduction

Calcium and phosphate serve many complex and vital functions. They are key components of the cartilage and skeleton systems. Calcium is an important cofactor in many complex enzymatic reactions and a main messenger in signaling pathways in excitability of nerve and muscle, signal transduction, clotting of blood, and muscle contraction. Phosphate plays an important role in metabolic processes, including adenosine triphosphate (ATP) formation, and is a component of nucleosides, nucleotides, and phospholipids. Together with the bone and intestinal tract, the kidney plays a key role in maintaining serum calcium and phosphate levels as well as calcium and phosphate balance in the body.

Calcium

Calcium, which ranges from 1000 to 1200 g in adults, is the fifth most abundant element in the human body. It plays both a structural role as a constituent of the bone and tooth matrices and a functional role in processes as diverse as blood coagulation, regulation of endocrine and exocrine secretory activities, complement system activation, neuromuscular activity, intracellular adhesion, and signal transduction. A total of 99% of calcium is stored in bone and teeth, approximately 1% is found in the intracellular fluid (ICF), and 0.1% is in the extracellular fluid (ECF). Serum calcium concentration is normally maintained within very narrow ranges of 8.8 to 10.4 mg/dL. ^, Approximately 50% of the total calcium in plasma is ionized, 40% is bound to plasma proteins (albumin 80% to 90%, and globulins), and 10% is complexed to several anions including citrate, phosphate, bicarbonate, and sulfate. The ionized calcium is available for transport and cellular metabolism. Protein binding of calcium is affected by pH, the serum sodium concentration, and serum albumin concentration. Acidemia increases the percentages of ionized calcium, and alkalemia decreases the ionized calcium. Both hydrogen ions and calcium are bound to serum albumin; in the presence of alkalemia, bound hydrogen ions dissociate from albumin, phosphate, bicarbonate, citrate, and sulfate, freeing up the albumin to bind with more calcium leading to the decrease in ionized calcium. The increase in hydrogen in metabolic acidosis causes more hydrogen to bind to plasma proteins, phosphate, citrate, and sulfate, thereby displacing calcium, resulting in the increase of the plasma ionized calcium. ^, ^, For every 0.1 change in pH, ionized calcium changes by 0.12 mg/dL. Similarly, an increase in serum albumin concentration of 1 g/dL increases protein-bound calcium by 0.8 mg/dL and decreases ionized calcium in plasma. Hyponatremia increases protein-bound calcium. Extracellular Ca ²⁺ homeostasis is dependent on complex interactions among several hormones (parathyroid hormone [PTH], vitamin D, and calcitonin) and multiple organs (the gastrointestinal tract, bone, and kidney).

Within cells, calcium is mainly sequestered in the endoplasmic reticulum and mitochondria, or it is bound to cytoplasmic proteins including calmodulin and other calcium-binding proteins (CaBPs) and ionic ligands. The fraction of ionized Ca ²⁺ is four times lower in the intracellular than the extracellular compartment. ^, The large concentration gradient for calcium across the cell membrane is maintained by a calcium adenosine triphosphatase (ATPase) (PMCA1b) in all cells and by a 3Na-Ca exchanger (NCX)1 in some cells. Furthermore, the fraction of free intracellular Ca ²⁺ available for signaling and various cellular processes is approximately 10 ⁻⁴ fold lower than that present in the extracellular milieu.

During certain physiologic states, calcium requirements are greatly increased, such as in children during skeletal growth and during pregnancy and lactation. During lactation, plasma Ca ²⁺ levels can significantly drop because of Ca ²⁺ excretion in the milk, and during pregnancy, Ca ²⁺ transport from the mother to the fetus across the placenta affects the plasma Ca ²⁺ concentration. ^,

Calcium homeostasis depends on two factors, the total amount of calcium in the body and the distribution of calcium between bone and ECF. The calcium balance is determined by the net difference between the amount of calcium absorbed by the intestinal tract and the amount of calcium excreted by the kidneys, intestines, and sweat glands. Regulation of calcium excretion by the kidneys is one of the major ways that the body regulates ECF calcium.

The balance of calcium in bone depends on relative rates of bone formation and resorption. Children are in positive bone balance (formation > resorption), which ensures healthy skeletal growth. Healthy young adults are in neutral bone balance (formation = resorption) and have achieved peak bone mass. Elderly individuals are typically in negative bone balance (formation < resorption), which leads to age-related bone loss. Factors that promote positive bone balance in adults include exercise, anabolic and antiresorptive drugs, and conditions that promote bone formation over bone resorption (e.g., “hungry bone” syndrome, osteoblastic prostate cancer). On the other hand, immobilization, weightlessness, and sex steroid deficiency, among others, produce negative bone balance.

An adult ingests the average amount of 1000 mg of calcium daily from which 200 mg is absorbed in the small intestine through an active, carrier-mediated transport mechanism stimulated by calcitriol, the active metabolite of vitamin D3, 1,25(OH)2, which is produced in the proximal tubule of the kidneys. When calcitriol levels rise, the intestine is able to absorb up to 600 mg of calcium a day. To maintain calcium balance, the kidney must excrete the same amount of calcium that the small intestine absorbs 200 mg a day ( Fig. 99.1 ). The kidneys do this by filtration of calcium across the glomeruli and reabsorption along the renal tubules. The identified epithelial Ca ²⁺ channels, transient receptor potential vanilloid (TRPV) 5 and TRPV6, are the rate-limiting step in Ca ²⁺ -transporting cells. TRPV5 acts primarily as a gatekeeper of epithelial Ca ²⁺ transport in the kidney, whereas TRPV6 is the main Ca ²⁺ influx pathway in the small intestine. They form the main target for action of hormones to control active Ca ²⁺ movement from the intestinal lumen or urine space to the blood compartment. The second factor controlling calcium homeostasis is the distribution of calcium between bone and the ECF, which is regulated by PTH and calcitriol. Other factors including pH and extracellular Ca ²⁺ have been shown to influence the calcium movement across epithelia. ^, ^, ^, ^,

Fig. 99.1, Overview of calcium homeostasis. PTH , Parathyroid hormone.

Renal Handling of Calcium

The kidney contributes to the maintenance of Ca ²⁺ homeostasis by regulating Ca ²⁺ reabsorption ( Figs. 99.2, 99.3, and 99.4 ). Clearance studies in humans and animals have shown that, if the filtered load of Ca ²⁺ is increased (by infusing Ca ²⁺ ), absolute calcium reabsorption increases, as does urinary Ca ²⁺ excretion. Sodium (Na ⁺ ) and Ca ²⁺ excretion often increases or decreases in parallel. The relationship between Na ⁺ and Ca ²⁺ reabsorption is maintained during various conditions that ultimately alter Ca ²⁺ excretion, including the use of furosemide and thiazide diuretics, metabolic acidosis and metabolic alkalosis, phosphate depletion, PTH administration, and volume depletion or repletion.

Fig. 99.2, Model of calcium absorption by the thick ascending limb of Henle. Calcium absorption is via both an active transcellular pathway and by a passive paracellular pathway. Only transport pathways relevant to calcium absorption are shown. Basal absorption is passive and is driven by the ambient electrochemical gradient for calcium. The apical Na + -K + -2Cl − co-transporter and the renal outer medullary potassium K + channel generate the “driving force” for paracellular cation transport. The apical Na + -K + -2Cl − co-transporter mediates apical absorption of Na, K, and Cl. The apical renal outer medullary K channel mediates apical recycling of K back to the tubular lumen and generates lumen-positive voltage. Cl channel Kb mediates Cl exit through the basolateral membrane. Here Na + -K + -ATPase also mediates Na exit through the basolateral membrane and generates the Na gradient for Na absorption. ATP , Adenosine triphosphate; CaSR , calcium-sensing receptor.

Fig. 99.3, Model of calcium absorption by distal convoluted tubules. Calcium entry across the plasma membrane proceeds through calcium channels with basolateral exit occurring through a combination of the plasma membrane ATPase and Na + -Ca + exchanger. Calcium absorption is entirely transcellular. ATP , Adenosine triphosphate; CaSR , calcium-sensing receptor.

Fig. 99.4, Mechanism of epithelial calcium (Ca 2+ ) transport. Entry of Ca 2+ is facilitated by the apical Ca 2+ channel (transient receptor potential vanilloid [TRPV]5/6) . In the cell, Ca 2+ binds to calbindin-D and diffuses through the cytosol to the basolateral membrane. There Ca 2+ is extruded via a Na + -Ca 2+ exchanger (NCX1) and a Ca 2+ -ATPase (PMCA1b) .

The calcium available for glomerular filtration is approximately 60% of the plasma calcium, consisting of the ionized calcium fraction and the amount with anions. Normally, 99% of the filtered calcium is reabsorbed by the renal tubules. Filtered calcium is reabsorbed throughout the nephron by various active and passive processes. The proximal tubule reabsorbs approximately 50% to 60% of the filtered calcium. The loop of Henle, mainly the cortical portion of the thick ascending limb (TAL), reabsorbs 15%. The remaining 10% to 15% is reabsorbed by the distal segments of the nephron. ^, ^, The distal cortical nephron plays an important role in maintaining calcium excretion based on physiologic needs. Approximately 1% of the filtered load of Ca ²⁺ is excreted in the urine.

In the proximal tubules, Ca ²⁺ is mainly reabsorbed via the passive paracellular pathway, which accounts for approximately 80% of calcium reabsorption in this segment of the nephron. This paracellular pathway is driven by solvent drag, the lumen positive transepithelial voltage across the second haft of the proximal tubule, and by a favorable concentration gradient of calcium. Both are established by transcellular sodium and water reabsorption in the first haft of the proximal tubule. The rate of transport depends on the magnitude of the electrochemical gradient, the Ca ²⁺ permeability coefficient, the delivery of Ca ²⁺ to the transport site, and the rate of Ca ²⁺ extrusion from the interstitium. A small but significant component of active calcium transport is observed in the proximal tubules. The active transport of calcium proceeds in a two-step process, with calcium entry from the tubular fluid across the apical membrane and exit through the basolateral membrane. This active transport is generally considered to constitute 10% to 15% of total proximal tubule calcium reabsorption, and it is mainly regulated by PTH and calcitonin. ^, ^, ^,

In the TAL of Henle , calcium reabsorption is primarily also via the paracellular pathway and occurs at the tight junctions (see Fig. 99.2 ). Calcium reabsorption and sodium reabsorption parallel each other, along the proximal tubule. Calcium reabsorption is secondary to sodium reabsorption, generating a lumen-positive transepithelial voltage. The apical Na ⁺ -K ⁺ -2Cl ⁻ co-transporter (NKCC2) and the renal outer medullary potassium K1 (ROMK) channel generate the “driving force” for paracellular cation transport. Calciotropic hormones, such as PTH and calcitonin, stimulate active calcium absorption in cortical TALs. Inhibition of NKCC2 co-transporter by loop diuretics or in Bartter syndrome decreases the transepithelial voltage, thus diminishing passive calcium absorption. Loop diuretics inhibit sodium reabsorption by inhibiting NKCC2, which reduces the magnitude of the lumen positive transepithelial voltage. This action inhibits the calcium reabsorption and increases urine calcium excretion. Loop diuretics are used to increase renal calcium excretion in patients with hypercalcemia. Calcium transport in the TAL of the loop of Henle is also influenced by the calcium-sensing receptor (CaSR), which is localized in the basolateral membrane. An acute inhibition of the CaSR does not alter NaCl reabsorption or the transepithelial potential difference but increases the permeability to calcium in the paracellular pathway. The tight junction in the TAL expresses several claudins (claudin-14, claudin-16, and claudin-19). A normal expression of claudin-16 and claudin-19 is required for a normal absorption of divalent cations in this tubular segment. Claudin-16 (previously termed paracellin-1 ) is a claudin family tight junction protein that plays a critical role in calcium and magnesium reabsorption. Loss-of-function mutations in claudin-16 result in the syndrome of familial hypomagnesemic hypercalciuria and nephrocalcinosis. This disorder is characterized by enhanced excretion of calcium and magnesium due to a decrease in the passive reabsorption of these ions by the paracellular route. Treatment with cinacalcet (a calcimimetic that lowers the amount of PTH) increases the abundance of claudin-14 messenger RNA (mRNA), and in cell culture models, overexpression of claudin-14 decreases the paracellular permeability to calcium.

In the distal convoluted tubules (DCTs) , where the voltage in the tubule lumen is electrically negative, calcium reabsorption is entirely active because calcium is reabsorbed against its electrochemical gradient ( Figs. 99.3 and 99.4 ). The distal tubule reabsorbs calcium exclusively via the transcellular route. This active process can be divided into three steps. The first step requires calcium influx across the apical membrane by calcium permeable ion channels, the TRPV5. The second step is the diffusion of calcium through the cytosol. Inside the cell, calcium binds to calbindin-D28k and shuttles it through the cytosol towards the basolateral membrane, where calcium is extruded via the NCX1 and the plasma membrane calcium-ATPase (PMCA)1b, which is the final step in this process. The calcium and sodium excretions do not change in parallel because the reabsorption of Na ⁺ and Ca ²⁺ in the distal tubule is independent and is differentially regulated. ^, ^, Calciotropic hormones such as PTH and calcitonin stimulate calcium absorption. Calcitriol [1,25(OH)2D] stimulates calcium absorption through the activation of nuclear transcription factors. Inhibition of the apical NaCl co-transporter by thiazide diuretics or in Gitelman syndrome indirectly stimulates calcium absorption. Thiazide diuresis inhibits Na reabsorption but stimulates calcium reabsorption in this renal segment. Thiazide inhibits the entry of NaCl into the cell and causes the membrane potential to hyperpolarize. This hyperpolarization in turn activates the TRPV5 channel to increase calcium flux into cells. Thiazide is given for patients with kidney stones to reduce urinary calcium excretion. ^,

Transient Receptor Potential Vanilloid 5

Ca ²⁺ movement across TRPV5 is controlled in multiple ways. TRPV5 gene expression is regulated by calciotropic hormones such as vitamin D3 and PTH. ^, Channel activity is modulated by intracellular Ca ²⁺ by feedback inhibition. TRPV5 is also controlled by mobilization of the channel towards the plasma membrane. The hormone α-klotho expressed in the renal distal tubule enhances TRPV5 activity via a novel mechanism modifying its glycosylation status and entrapping the channel at the cell surface, resulting in a prolonged expression of TRPV5 at the plasma membrane. ^, The immunosuppressant tacrolimus is shown to decrease activity of TRPV5 and calbindin-28K, causing hypercalciuria. Extracellular pH determines the cell surface expression of TRPV5 via a unique mechanism. Extracellular alkalinization causes a pool of TRPV5-containing vesicles to be rapidly recruited to the cell surface without collapsing into the plasma membrane, resulting in increased TRPV5 activity. In contrast, extracellular acidification causes the vesicles to be retrieved from the plasma membrane, resulting in decreased TRPV5 activity. This effect could explain the molecular basis of acidosis-induced calciuresis. In the distal tubules, intracellular calcium concentration is increased by osmosensitive, nonselective ion channels such as TRPV4, especially under hypotonic stimulus.

Calcium Buffering Within the Cell

The vitamin D–dependent CaBPs named calbindins are expressed in cells that are challenged by a high Ca ²⁺ influx such as in brain, bone, teeth, inner ear, placenta, mammary gland, kidney, and intestine. In these tissues, CaBPs (i.e., CaBP9K and CaBP28K) are a key component in cellular Ca ²⁺ handling (see Fig. 99.4 ). In the kidney, calbindin-D (CaBP28K) facilitates Ca ²⁺ diffusion from the luminal Ca ²⁺ entry side of the cell to the basolateral side, where Ca ²⁺ is extruded into the extracellular compartment. CaBP28K provides protection against toxic high intracellular Ca ²⁺ levels by buffering the cytosolic Ca ²⁺ concentration during high Ca ²⁺ influx. Studies using protein-binding analysis, subcellular fractionation, and evanescent-field microscopy have shown that CaBP28K translocates towards the plasma membrane and directly associates with TRPV5 at a low Ca ²⁺ concentration. CaBP28K tightly buffers the flux of Ca ²⁺ entering the cell via TRPV5, facilitating high Ca ²⁺ transport rates by preventing channel inactivation. Therefore CaBP28K acts in calcium-transporting epithelia as a dynamic Ca ²⁺ buffer, regulating Ca ²⁺ in close vicinity to the TRPV5 pore by direct association with the channel. Calbindins are regulated by calciotropic hormones including vitamin D, estrogens, PTH, and dietary calcium.

CaBP28K was originally identified as a high-capacity Ca ²⁺ buffer with Ca ²⁺ affinities fitting the classic properties of a Ca ²⁺ buffer. However, sequential Ca ²⁺ binding and conformational changes suggested that CaBP28K can act as a Ca ²⁺ sensor controlling downstream cellular processes.

Inside the cell, calbindin directly interacts with a calmodulin-binding isoleucine-glutamine motif present in the C-tail of TRPV5 and TRPV6 channels. Deletion of the first eight amino acids of the isoleucine-glutamine motif in the carboxyl terminus tail of the Ca ²⁺ - channel subunit a1C eliminates Ca ²⁺ - dependent inactivation of voltage-gated, L-type Ca ²⁺ channels. Ca ²⁺ - dependent interaction also occurs between calbindin and a novel site in the C-terminal domain of the 1A subunit of P/Q-type Ca ²⁺ channels (calbindin-binding domain). In the presence of low concentrations of intracellular Ca ²⁺ chelators, Ca ²⁺ influx through P/Q-type channels enhances channel inactivation, increases recovery from inactivation, and produces a long-lasting facilitation of the Ca ²⁺ current. Ca ²⁺ “shuttling” across the intracellular organelles such as mitochondria or endoplasmic reticulum depends on a Ca ²⁺ electrochemical gradient, driven by internally negative membrane potential across the inner mitochondrial or endoplasmic reticular membrane.

Calcium Transport Across the Basolateral Membrane

The extrusion of Ca ²⁺ across the basolateral membrane into the interstitium occurs against an electrochemical gradient and, as such, is an energy-dependent process. Transport is primarily mediated by two calcium transporters located at the basolateral membrane: NCX and the Ca ²⁺ - ATPase (PMCA).

Na ⁺ -Ca ²⁺ Exchanger

Three genes for NCX, known as NCX1, NCX2, and NCX3, have been identified in mammals. In the kidney the expression of this transporter is restricted to the distal part of the nephron, where it predominantly localizes along the basolateral membrane. ^, NCX1 is widely distributed in many different mammalian tissues, whereas NCX2 and NCX3 are expressed only in brain and skeletal muscle. ^, It has been demonstrated that NCX1 is the primary extrusion mechanism, whereas only a minor amount of Ca ²⁺ in the distal tubular cells is extruded by the plasma Ca ²⁺ pump. It has also been shown that targeted deletion of NCX1 results in NCX1 -null embryos that do not have a spontaneously beating heart and die in utero. ^, Studies in oocytes and mammalian cell systems show that NCX is regulated by several factors, including the membrane potential, protein kinase C (PKC) activation, protons, nucleotides, and calciotropic hormones. PTH markedly stimulates Ca ²⁺ reabsorption in the distal part of the nephron, primarily by augmenting NCX1 activity via a cyclic adenosine monophosphate (cAMP)-mediated mechanism.

Plasma Membrane Ca ²⁺ -Adenosine Triphosphatase

PMCA belongs to a class of P-type, ion-motive ATPase proteins with molecular weights ranging from 120,000 to 140,000 Da. PMCAs are high-affinity Ca ²⁺ efflux pumps present in almost all cells and are responsible for the maintenance and resetting of the resting intracellular Ca ²⁺ levels. Four different isoforms—PMCA1 to A4—are encoded by separate genes. PMCAs are a universal system for the extrusion of Ca ²⁺ in cells. In the kidney, PMCAs are present in all nephron segments, with highest expression in the basolateral membrane of cells lining the distal part of the nephron. The DCT possesses the highest Ca ²⁺ -ATPase activity and exhibits the strongest immunocytochemical reactivity for PMCA protein expression.

In humans, PMCAs are found along the DCT and, in contrast to other species, along the cortical collecting duct. Studies suggest that PMCA1 and PMCA4 are the major isoforms expressed in the kidney, whereas PMCA2 and PMCA3 are more tissue specific. PMCA1 and PMCA4 are involved in the maintenance of cellular Ca ²⁺ homeostasis. PMCA4b plays a significant role in basolateral Ca ²⁺ extrusion. PMCA1b is the predominant isoform and abundantly expressed in the small intestine, where NCX1 is expressed at a low level. It is the major Ca ²⁺ extrusion mechanism in intestinal Ca ²⁺ absorption. PMCA also plays a role in cytosolic acidification, which in turn, acts a signal for activation of the sarcoplasmic reticulum Ca ²⁺ - ATPase and other cellular functions in muscle. Limited data are available regarding the regulation of PMCA by hormones or signaling mechanisms. Several studies indicate that PMCA is positively regulated by 1,25-dihydroxyvitamin D3, [1,25(OH)2D3], in the intestine to increase Ca ²⁺ absorption. In addition, several potassium (K ⁺ )-dependent Na ⁺ -Ca ²⁺ exchangers (NCKXs) have been described. ^, Northern blot analysis demonstrated that some isoforms (i.e., NCKX4 and NCKX6) of this family are expressed in epithelia, including small intestine and kidney. ^,

These exchangers are found in various tissues, suggesting a key role in regulating intracellular Ca ²⁺ homeostasis in mammalian cells. Ca ²⁺ extrusion from the cell is a Na ⁺ -Ca ²⁺ countertransport driven by the Na ⁺ concentration gradient across the basolateral membrane maintained by Na ⁺ -K ⁺ -ATPase. Ca ²⁺ removal by this exchanger is slowed when extracellular Na ⁺ concentration is diminished or when Na ⁺ -K ⁺ -ATPase is inhibited with ouabain.

Regulation of Calcium Transport

The factors relating to calcium homeostasis are summarized in the Table 99.1 .

Table 99.1

Factors of Renal Regulation of Calcium. ^,

Data from Koeppen BM, Stanton BA. Regulation of calcium and phosphate homeostasis. In; Koeppen BM, Stanton BA, eds. Renal Physiology. 6th ed. Philadelphia: Elsevier; 2019:138–150; Blaine J, Chonchol M, Levi M. Renal control of calcium, phosphate, and magnesium homeostasis. Clin J Am Soc Nephrol. 2015;10(7):1257–1272.

Increase Calcium Reabsorption	Decrease Calcium Reabsorption
Hyperparathyroidism Calcitriol Hypocalcemia Volume contraction Metabolic alkalosis Hyperphosphatemia Thiazides	Hypoparathyroidism Low calcitriol Hypercalcemia Extracellular fluid excess Metabolic acidosis Hypophosphatemia Loop diuretics: furosemide

Parathyroid Hormone

The parathyroid glands play an important role in maintaining the extracellular Ca ²⁺ concentration by stimulating calcium absorption. PTH is a primary hormone responsible for maintaining Ca ²⁺ hemostasis and the most powerful control on renal Ca ²⁺ excretion. They have the capacity to sense changes in the level of blood Ca ²⁺ from its normal level via the CaSR. In response to low blood Ca ²⁺ levels, PTH is secreted into the circulation and then acts primarily on kidney and bone, where it activates the PTH–PTH-related peptide (PTHrP) receptor. PTH acts to increase the plasma calcium concentration by three ways: stimulated bone resorption, enhanced intestinal calcium and phosphate absorption by promoting the kidney 1,25(OH)2D3, and increased renal calcium absorption by the TAL of the Henle loop and the distal tubule. PTH secretion is regulated by a transcriptional and posttranscriptional level dependent on the extracellular level of calcium. PTH gene transcription is increased by hypocalcemia, glucocorticoids, and estrogen. Hypercalcemia can increase the intracellular degradation of PTH. Hypercalcemia leads to a reduction in PTH levels, which suppresses Ca ²⁺ reabsorption by the distal tubule.

In the kidney, PTH augments active renal absorption by stimulating transcellular Ca ²⁺ reabsorption via a slow genomic pathway and a rapid nongenomic pathway. The rapid pathway is via activation of adenylyl cyclase, accumulation of cAMP, and stimulation of protein kinase A (PKA), resulting in an increase in the cytosolic concentration and transcellular Ca ²⁺ transport. PKA inhibitors reduce the PTH-mediated rise of cytosolic Ca ²⁺ , which suggests that cAMP-dependent phosphorylation is an essential step in short-term PTH stimulation. In addition to cAMP, PKC is also involved in the short-term PTH response. The slow pathway results in an elevated expression of the Ca ²⁺ transport proteins TRPV5, calbindin-D28K, NCX1, and PMCA1b. The PTH receptor directly enhances the tubular Ca ²⁺ reabsorption, and it stimulates the activity of 1α-hydroxylase, thereby increasing the 1,25(OH)2 D3–dependent absorption of Ca ²⁺ from the intestine. PTH binds to two types of receptors: PTHR1, which also binds the PTH-PTHrP, and PTHR2, which binds only PTH. The PTHR1 receptor is the predominant receptor found in the kidney, and it is expressed in glomeruli and all tubule segments, except the TAL and collecting duct. The PTHR2 receptor is distributed primarily in brain, lung, pancreas, and vasculature (including the vascular pole of the glomerulus). Signal transduction for both PTH and PTHrP is via adenylate cyclase and phospholipase C. ^, PTH stimulates active Ca ²⁺ reabsorption in the distal part of the nephron. PTH increases transepithelial Ca ²⁺ transport via a dual signaling mechanism involving PKA- and PKC-dependent processes. ^, PTH has various actions such as membrane insertion of apical Ca ²⁺ channels, opening of basolateral chloride channels resulting in cellular hyperpolarization, and modulation of PMCA activity.

PTH activates dihydropyridine-sensitive channels responsible for Ca ²⁺ entry. Once inserted or activated, these dihydropyridine-sensitive channels could mediate Ca ²⁺ entry into Ca ²⁺ -transporting epithelial cells. It is thought that dihydropyridine-sensitive Ca ²⁺ channels play a role in signal transduction processes to maintain the cellular Ca ²⁺ homeostasis. PTH affects renal Ca ²⁺ handling through the regulation of the expression of the active renal Ca ²⁺ transport proteins, including the epithelial Ca ²⁺ channel TRPV5. PTH also stimulates the PMCA activity by increasing the affinity for Ca ²⁺ in the distal tubule, unlike 1,25-(OH)2D3 that does not directly affect the basolateral membrane PMCA activity.

In bone, PTH can induce a rapid release of Ca ²⁺ from the bone matrix and can also mediate long-term changes in Ca ²⁺ metabolism by acting directly on the bone-forming osteoblasts and indirectly on bone-resorbing osteoclasts by increasing their number and activity.

Calcium-Sensing Receptor

CaSR was discovered in 1993. It is a plasma membrane–bound G protein–coupled receptor found in many tissues, including the parathyroid gland, thyroid, kidney, intestine, bone, bone marrow, brain, skin, pancreas, lung, and heart. CaSR plays a critical role in Ca ²⁺ homeostasis by inducing changes in PTH secretion and renal Ca ²⁺ reabsorption in response to variations in the extracellular concentration of Ca ²⁺ . CaSR is not specific for calcium alone; it responds to other cations such as magnesium, aluminum, and gadolinium, but it has the highest affinity for calcium. The human CaSR is encoded by six exons of the CaSR gene located on chromosome 3q13.3–21. The receptor is expressed abundantly in the parathyroid glands and, to less extent, along the length of the kidney tubule. Activation of the CaSR on the parathyroid gland cells by elevated serum Ca ²⁺ leads to activation of many secondary messengers, resulting in inhibition of PTH synthesis and the production of calcitriol by proximal tubule. Moreover, the reduction of PTH secretion also contributes to calcitriol production because calcitriol is stimulated by PTH. A decrease in serum calcium has the opposite effect on PTH and calcitriol. Activation of the CaSR in response to hypercalcemia produces several different effects in the renal tubules. In the TAL, an increase in serum calcium activates CaSR, which inhibits calcium absorption and thereby increases calcium excretion. This occurs due to a decrease in Na ⁺ , K ⁺ , and Cl ⁻ reabsorption accompanied by a decrease in K ⁺ transfer across a specific potassium channel at the apical membrane. This causes a decrease in the lumen positive voltage in the tubular lumen, leading to decreased calcium and magnesium reabsorption and increased distal delivery. In the distal tubule, CaSR stimulation on the basolateral membrane results in inhibition of calcium transport. In the medullary collecting duct epithelium, CaSRs are found on the apical side. At that location, they reduce the insertion of aquaporin-2 water channels when the luminal calcium concentration is high, decreasing water reabsorption and thereby decreasing calcium concentration, and calcium crystal formation in the urine. Inactivating mutations in the CaSR gene are associated with familial hypocalciuric hypercalcemia, neonatal severe hyperparathyroidism, and autosomal dominant hypocalcemia. Hypocalciuria is caused by enhanced calcium reabsorption in the TAL and distal tubule due to elevated PTH levels and defective CaSR regulation of calcium transport in the kidneys. Autosomal dominant hypoparathyroidism results from an activating mutation in CaSR. Hypercalciuria is caused by decreased PTH levels and defective CaSR-regulated calcium transport in the kidneys. In contrast, activating mutations of the CaSR result in a Bartter-like syndrome (Bartter syndrome type V) characterized by calcium, magnesium, sodium, potassium, chloride, and water wasting.

Vitamin D

The physiologic actions of vitamin D are mediated by its metabolite 1,25(OH)2D3 as the most active form of vitamin D, which is formed by 1-hydroxylation of 25(OH)D produced by the liver via 1α-hydroxylase (CYP24B1) in proximal tubules. There is 24α-hydroxylase, which forms 24,25-dihydroxyvitamin D as an inactive metabolite. Serum 25(OH)D, which is the precursor form of the biologically active vitamin D, is the best indicator of overall vitamin storage. The 1,25(OH)2D3 enters the circulation and acts upon the target organ receptors to maintain calcium hemostasis and bone health. Both 25(OH)D and 1,25(OH)2D3 are carried in the circulation by vitamin D–binding protein (DBP). In small intestine, 1,25(OH)2D3 enhances calcium absorption. An intact 1,25(OH)2D3 receptor system is important for PTH-induced osteoclastogenesis. Calcium and phosphate are released from mature osteoclasts to maintain the two minerals in serum. Rickets due to gene defects in FGD23 decreases the conversion of 25(OH)D to 1,25(OH)2D.

In the kidney, the most important effect of 1,25(OH)2D3 is tight control of its own homeostasis through simultaneous suppression of 1α-hydroxylase and stimulation of 24α-hydroxylase. The 1,25(OH)2D3 increases renal calcium absorption. Oral 1,25 (OH)2D3 is used to suppresses PTH in patients with chronic kidney disease (CKD) and end-stage kidney disease. The 25(OH)D inhibits PTH but requires higher levels. In the kidneys, vitamin D receptor (VDR) is expressed in epithelia that play a role in Ca ²⁺ reabsorption. Microarray analysis of 1,25(OH)2D3 has shown that TRPV6 is one of the most highly vitamin D–responsive genes. In the cell, 1,25(OH)2D3 can be inactivated by mitochondrial 24-hydroxylase (CYP24A1) or bind to the VDR in the cytoplasm. Once the VDR binds its ligand, the VDR translocates to the nucleus. The classic action of vitamin D is to stimulate transcription by binding to nuclear receptors. It binds with VDR in a ligand-dependent manner. Degradation of 1,25(OH)2D3 is believed to occur in the kidneys from side cleavage and oxidation by CYP24A1 to form 24,25(OH)2D. The CYP24B1 to produce 1,25(OH)2D synthesis is regulated by low calcium, low phosphate, increased PTH, estrogen, prolactin, growth hormones (GHs), fibroblast growth factor (FGF)23, and 1,25(OH)2 itself.

Calcitonin

Calcitonin is a 32–amino acid peptide secreted by the parafollicular cells of the thyroid gland. It is released in response to hypercalcemia and lowers serum calcium by various mechanisms. Its main action for lowering calcium is by inhibiting bone resorption. Calcitonin receptors are found on osteoclasts. The signal transduction pathways for calcitonin receptors are adenylate cyclase–PKA and phospholipase C–PKC via linking with G proteins. In the kidney, calcitonin increases the urinary excretion of calcium and phosphorus, independent of PTH. Calcitonin has not been shown to affect intestinal calcium absorption, but it may decrease phosphorus absorption. Calcitonin is a major regulator of renal 1,25-hydroxylase gene expression.

Plasma Calcium Concentration

Hypercalcemia

Hypercalcemia results in an increase in Ca ²⁺ excretion caused by a net increase in the filtered load and a decrease in tubular reabsorption. Hypercalcemia, in the presence of intact parathyroid glands, decreases the glomerular ultrafiltration coefficient (Kf) and causes renal vasoconstriction, which tends to offset the increase in filtered load. Both together cause a decline in glomerular filtration rate (GFR). ^, Hypercalcemia also causes a decline in the tubule reabsorption of Ca ²⁺ by PTH-independent and PTH-dependent mechanisms. ^, This effect is mediated by stimulation of the CaSR, which inhibits the apical K ⁺ channel and K ⁺ recycling, necessary for the activity of the Na ⁺ - K ⁺ -2Cl ⁻ transporter. ^, Decreased activity of the transporter decreases the lumen-positive potential difference and thus Na ⁺ , Ca ²⁺ , and Mg ²⁺ reabsorption. In addition, a decrease in intestinal Ca ²⁺ absorption is brought about by diminished synthesis of 1,25(OH)2D3.

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