Calcium and Phosphate Regulation

Calcium and phosphate regulation in health and kidney disease Dr Mirna Vucak-Dzumhur Nephrologist Learning objectives • Describe the roles of calcium and phosphate in the body • Outline effector mechanisms involved in calcium and phosphate regulation (PTH, Vit D, FGF23) • Describe how renal diseases influence the calcium and phosphate axis, influencing skeletal and vascular physiology 2 1. • The roles of calcium and phosphate in the body • Calcium and phosphate homeostasis The roles of calcium and phosphate in the body Calcium role Phosphate role • • • • • • • Bone mineralisation • Cell signalling and enzyme activation • Energy storage in the form of ATP • Vital constituent of cell membranes, nicotinamide adenine dinucleotide (NAD), cAMP, cGMP • • Cell signalling Neural transmission Muscle function Blood coagulation Enzymatic co-factor Membrane (intercellular adhesion) and cytoskeletal functions Hormone secretion Bone mineralisation 4 Renal Control of Calcium, Phosphate, and Magnesium Homeostasis: Judith Blaine, Clin J Am Soc Nephrol 2014 Distribution of calcium and phosphate in the body Total body content Calcium 1000 g1200 g Phosphate 680 g % in skeleton % in soft tissue 99% 1% in IC and EC 85% 15% (14% IC, 1% EC) Renal Control of Calcium, Phosphate, and Magnesium Homeostasis: Judith Blaine, Clin J Am Soc Nephrol 2014 Distribution of calcium in plasma Calcium in the plasma: • The ultrafilterable calcium: – ionized + complexed fractions. • Plasma calcium concentration measured in the lab includes all of the plasma calcium (free and bound) – “total” – In general, measuring the total plasma calcium concentration is sufficient since changes in this parameter are usually associated with parallel changes in the ionized concentration • Exceptions: hypoalbuminemia, acid-base disorders and CKD -> • hence, use corrected calcium: – Corrected Ca = [0.8 x (normal albumin patient's albumin)] + serum Ca level 1. 40% bound to albumin 2. 15% complexed with citrate, sulfate or phosphate 3. 45% as ionized (or free) calcium (physiologically active) Renal Control of Calcium, Phosphate, and Magnesium Homeostasis: Judith Blaine, Clin J Am Soc Nephrol 2014 Hypocalcaemia Hypercalcaemia Plasma phosphorus • Plasma phosphorus exists in organic and inorganic forms: 1.Organic form • phospholipids • ester phosphates 2. Inorganic phosphates • ionized, circulating as HPO42- or H2PO4- in a ratio of 4:1 Tight control of plasma calcium and phosphate crucial for survival Small fraction of total body calcium and phosphate is located in plasma: • plasma concentrations of ionized calcium and inorganic phosphate are under hormonal control Organs regulating calcium and phosphate homeostasis: 1. GI tract: facilitates intestinal absorption 2. Skeleton: bone mineralization and resorption 3. Kidneys: renal excretion/reabsorption Effectors regulating these processes: 1. Vitamin D [1,25(OH)2D3] 2. PTH 3. FGF 23/ Klotho Calcium and phosphate homeostasis: Concerted interplay of new regulators R. Civitelli and K. Ziambaras J. Endocrinol. Invest. 34 (Suppl. to no. 7): 3-7, 2011 Calcium and phosphate flux between body compartments Total body content % in skeleton % in soft tissue Calcium 1000 g1200 g 99% 1% in IC and EC Phosphate 680 g 85% 15% (14% IC, 1% EC) Judith Blaine et al. CJASN doi:10.2215/CJN.09750913 Calcium and phosphate flux between body compartments 15% (14% IC, 1% EC) Exchangeable phosphorus pool: Intracellular phosphorus 70% Skeletal mineralisation front 29% Serum phosphorus <1% KA Hruska et al.: Hyperphosphatemia of CKD; Kidney International 2008 Calcium handling 1. Gastrointestinal calcium handling • Dietary calcium is absorbed by two mechanisms: 1. Active, transcellular pathway via the transient receptor potential vanilloid 6 (TRPV6) channel on apical membrane of duodenum and proximal jejunum 2. Paracellular calcium transport that occurs throughout the length of the intestine • Absorption of calcium and phosphate incomplete: – calcitriol required for intestinal calcium absorption – formation of insoluble salts (calcium phosphate and calcium oxalate) • Normal adult may ingest 1000 mg/day, of which ~ 400 to 500 mg may be absorbed • 300 mg of calcium lost into the stool via digestive secretions • Thus, of 1000 mg of ingested calcium, net absorption is only 100 to 200 mg • In the steady state, this quantity of calcium is excreted in the urine Vitamin D endocrine system and the intestine:Sylvia Christakos; BoneKEy Reports (2014) Calcium handling 2. Bone calcium handling • Ca++ and phosphate exists in bone as hydroxyapatite (Ca10[PO4]6[OH]2) • Bone is a calcium reservoir involved in maintaining a normal plasma ionized calcium concentration: – 1% of bone Ca++ freely exchangeable with IC/EC fluid – depends on the activity of osteoblasts and osteoclasts – regulated by many hormones and proteins, including PTH and calcitriol A Systems Approach to Understanding Bone Cell Interactions in Health and Disease: P. Pivonka, P.R. Buenzli and C.R. Dunstan; Cell interaction, 2012 Calcium handling 3. Renal calcium handling • Ionized (free) and complexed Ca++ filtered by the glomerulus: • 97 - 99% reabsorbed: – PCT: ~70% passively with Na and H2O reabsorption – TALH: ~ 20% passively – DCT and CD: ~10 - 15% actively through a transcellular mechanism • responsible for physiologic Ca++ regulation and dysregulation Judith Blaine et al. CJASN doi:10.2215/CJN.09750913 Calcium transport in the TALH and distal nephron K-> <-ROMK Phosphate handling and regulation 1. Phosphate handling in the gut NaPi2b regulated by: dietary phosphate and 1.25 D3 • Much of body phosphate exists in bone as hydroxyapatite (Ca10[PO4]6[OH]2) • In the small intestine, absorption of dietary phosphate occurs via: 1. sodium-dependent (active) pathway • NaPi-IIb transport protein (NaPi2b) during fasting or low phosphate diet 2. sodium-independent (passive) pathways (may involve vitamin D regulation): • robust absorption of dietary phosphate in patients vitamin D deficient or in advanced CKD Judith Blaine et al. CJASN doi:10.2215/CJN.09750913 Phosphate handling and regulation 2. Phosphate handling in the kidney • with normal serum phosphate and adequate phosphate intake: – most of the filtered phosphate reabsorbed by proximal tubule • type IIa sodium phosphate cotransporter (Npt2a) • type IIc sodium phosphate cotransporter (Npt2c) • type III sodium phosphate cotransporter (Pit-2) 3% 85% 2% 10% – 10 to 20% is excreted in the urine – little, if any, phosphate transport occurs beyond proximal tubule • In CKD, reabsorption could be reduced from 85% to 15% Judith Blaine et al. CJASN doi:10.2215/CJN.09750913 2. Effector mechanism involved in calcium and phosphate regulation Effector mechanism involved in calcium and phosphate regulation Plasma concentrations of ionized calcium and inorganic phosphate are under hormonal control • Calcium balance regulated by: – parathyroid hormone (PTH) and calcitriol (1,25dihydroxyvitamin D) • Phosphorous balance regulated by: – PTH, calcitriol, fibroblast growth factor 23 (FGF-23) and its cofactor, Klotho • The physiologic roles of other hormones (such as calcitonin and estrogens) in the regulation of calcium and phosphate balance are incompletely understood 1. PTH • PTH is a polypeptide secreted by the parathyroid glands in response to decreases in plasma ionized calcium • Calcium regulates synthesis, release and degradation of PTH • Calcium is sensed by the calcium-sensing receptor (CaSR) on parathyroid cells and leads to the appropriate changes in PTH secretion • PTH response to hypocalcemia: ●Seconds to minutes – exocytosis of PTH from secretory vesicles into EC fluid ●Minutes to one hour – reduction in the intracellular degradation of PTH ●Hours to days – increase in PTH gene expression ●Days to weeks – proliferation of parathyroid cells (also stimulated by low serum calcitriol concentrations) N-terminal 1. PTH 2017 UpToDate 2. Vitamin D3 (cholecalciferol) a. Vitamin D3 (cholecalciferol) • fat-soluble steroid obtained through synthesis in the skin from 7-dehydrocholesterol in the presence of UV light b. 1.25 dihydroxyvitamin D (most active form of Vitamin D PTH FGF23 Pi calcitriol Effect of vitamin D on calcium 1. Increased intestinal absorption - 1. increased expression of TRPV6, calbindin and PMCA1b 2. Bone resorption - increases calcium release from skeletal stores – by binding to osteoblasts and osteocytes – increasing levels of pyrophosphate, a potent mineralization inhibitor 3. Renal reabsorption – increases reabsorption of calcium in the DCT and CD by increasing expression of TRPV5, calbindinD28k and PMCA1b Ca++ 2. 3. Effector mechanisms involved in phosphate homeostasis 1. PTH affects Pi on several levels in “steady state” • • • Kidneys: decreased renal reabsorption of Pi and increased phosphaturia -> Pi Bones: increased calcium and phosphate resorption from the bone -> Pi GIT: increased resorption by increasing calcitriol (indirect action)-> Pi 2. FGF-23 and Klotho (cofactor) • • Kidneys: decreased renal reabsorption of Pi and increased phosphaturia ->Pi reduction of 1.25(OH)2D, indirectly reducing phosphate absorption in the gut and reabsorption in the PCT -> Pi 3. 1,25(OH)2D • GIT: increases phosphate by absorption in the gut and reabsorption in the PCT -> Pi 2015 American Society of Nephrology Fibroblast growth factor 23 • FGF-23 is a circulating peptide and phosphaturic agent: – secreted by osteocytes and osteoblasts in response to calcitriol, phosphate, PTH and calcium – primary function to maintain normal serum Pi concentration by: • reducing renal Pi reabsorption: – inhibition of NaPi2a and NaPi2c cotransporter abundance in PCT • reducing intestinal Pi absorption by decreasing calcitriol production via inhibition of 1-alpha-hydroxylase • FGF-23 also suppresses PTH secretion, through klotho • Klotho, a transmembrane protein produced by osteocytes and kidneys, required for FGF-23 receptor activation – Klotho expression declines early in the course of CKD and progressively with decreasing GFR – Reduction in klotho temporally coincides with the rise in FGF-23 – Decrease in klotho expression on hyperplastic parathyroid glands may contribute to resistance and impaired parathyroid suppression by FGF-23 2017 UpToDate 3. Abnormalities of PTH, calcium, phosphorus, Vitamin D and FGF23 with CKD progression Pathophysiology • Complex, involving a number of feedback loops between kidney, bone, intestine and vasculature • Main goal of this system is maintenance of calcium and phosphorus balance, often at the expense of abnormalities in other components of the system • Most elements of CKD associated mineral and bone disorder usually present when GFR falls <40 mL/min – some components may be observed earlier in the course of CKD 2017 UpToDate Pathophysiology The main abnormalities in mineral balance with the progression of CKD: ●Phosphate retention ● Decreased 1,25-dihydroxyvitamin D (calcitriol) ● Decreased free ionized calcium concentration ● Increased fibroblast growth factor 23 (FGF-23) ● Reduced expression of vitamin D receptors (VDRs), calcium-sensing receptors (CaSRs), FGF 23 receptors and klotho in the parathyroid glands 2017 UpToDate Phosphate retention and hyperphosphatemia Disordered bone remodeling New phosphate reservoir once Pi excretion fails Failure of excretion KA Hruska et al.: Hyperphosphatemia of CKD; Kidney International 2008 Decreased calcitriol activity when eGFR <80 mL/min/1.73 m2 In early CKD (Stage 2), calcitriol is low due to increase in FGF-23 • FGF-23 inhibits 1-alpha-hydroxylase , promote 24-alphahydroxylase • physiologic adaptation to enhance phosphaturia In advanced CKD, high Pi and loss of renal mass contribute to the decline in calcitriol synthesis • Phosphate retention directly inhibits 1-alpha-hydroxylase activity Effect of low calcitriol concentrations on PTH secretion: • Indirect effect: hypocalcaemia (decreased intestinal calcium absorption and calcium release from bone) • Direct effect: calcitriol acts on the VDR in parathyroid gland to suppress PTH 2017 UpToDate Hypocalcemia and calcium-sensing receptor – Hypocalcemia: • due to decreased calcitriol concentration • due to resistance to the calcemic actions of PTH on bone • due to phosphate retention – Number of CaSRs reduced in hypertrophied parathyroid glands results in inadequate suppression of PTH secretion by calcium ** * * 2017 UpToDate Fibroblast growth factor 23 off-target effects Phosphate and FGF-23 Harald Ju¨ppner: Kidney International (2011) 79 (Suppl 121), S24–S27 Feedback loops between calcium, phosphate, PTH, FGF23, Klotho Secondary HPT: low Ca++ and 1.25 D3, elevated Pi and FGF23 Tertiary HPT elevated Ca++, Pi and FGF23, low 1.25D3 PTH not suppressible by Ca++ and 1.25D3 • Markedly elevated PTH associated with hypercalcemia in ESKD: – severe parathyroid hyperplasia with autonomous secretion of PTH – results from prolonged stimulation of parathyroid cell growth – decreased expression of CaSR and VDRs – nodular parathyroid glands do not undergo involution despite resolution of some of triggering mechanisms – medical therapy often fails; parathyroidectomy required 2HPT 3HPT Tertiary HPT: Case 1 • 64 year old, female • ESKD due to diabetic nephropathy • On HD 7 years: usually dialysing 4.5 h, three times a week • Main symptoms: back and bone pain, reduced mobility, muscle ache and weakness, itchiness • Results: • EUC: (pre-dialysis) potassium 6.1 mmo/L, urea 24.8 mmol/L (RR 2.5-6.5), calcium corr. 2.72 mmol/L (2.15-2.55), phosphate 2.9 mmol/L(0.75-1.5), ALP 568 U/L (30-110), PTH 272 pmol/L (1.6-7.5), 25 hydroxyviatmin D 12 nmol/L. Tertiary HPT (A and B) (PA) hands: • note the radial margins of the proximal and middle phalanges bilaterally are frayed, irregular and lace-like (arrows) owing to characteristic subperiosteal resorption • note the brown tumour (small arrowhead) and osteolysis of the distal phalanges (large arrowheads) 2016 UpToDate Tertiary HPT Skull radiograph shows the typical "salt and pepper" appearance caused by osteitis fibrosa cystica (A). Skull radiograph of same patient six months after removal of the patient's parathyroid adenoma (B). The bones have returned to normal. 2016 UpToDate Tertiary HPT: Brown tumours of pelvic bones: Findings on CT • The CT scan of pelvis shows multiple lytic lesions in the pelvis (white arrows) consistent with brown tumours 2016 UpToDate LS X-ray Case 1 • This woman has tertiary hyperparathyroidism, due to long standing CKD and inadequate dialysis. • Biochem: hypercalcemia, hyperphosphatemia, high alkaline phosphatase (high bone turnover and probably osteomalacia due to low vitamin D), hyperparathyroidism and very low vitamin D levels. • She has symptoms, biochemical and radiological evidence of tertiary hyperparathyroidism. • If no contraindication, surgery would be the best option. Case 2: • 56 year old, 3 months post renal transplant for ESKD due to IgA nephropathy • Haemodialysis for 5 years prior to transplant • Non-compliance with dialysis and phosphate binders • Blood results post-transplant 3 months: creatinine 105 umol/L(60-110), corr calcium 2.9 mmol/L(2.15-2.55), phosphate 0.36 mmol/L(0.75-1.5), PTH 36 pmol/L(1-6.8), (pre-transplant 150 pmol/L), ALP 150 U/L(30-110). 1.25 dihydroxyvitamin D 230 pmol/L (60-158). • BMD: Osteoporosis Case 2: • Patient has persistent post transplant hyperparathyroidism slow to resolve • As his renal function has been established due to functioning renal allograft, in context of hyperparathyroidism and elevated 1.25 dihydroxyvitamin D, calcium has been reabsorbed in the distal tubule and absorption in the GI tract has been enhanced, causing hypercalcaemia, while there has been phosphaturia due to persisting HPT and elevated FGF23 Vascular calcifications Extensive vascular calcifications Calciphylaxis – calcific uremic arteriolopathy (CUA) 2017 UpToDate Calciphylaxis - calcific uremic arteriolopathy (CUA) • Arterial/ arteriolar medial calcification, leading to ischaemic changes • Most commonly in ESKD • Clinical manifestations: livedo reticularis and/or violaceous, painful, plaque–like subcutaneous nodules, ischemic/necrotic ulcers • Diagnosis: skin biopsy -> arteriolar occlusion and calcification in the absence of vasculitic change 2017 UpToDate Summary • In progressive CKD, homeostasis is centred on keeping calcium and phosphate in normal range at the expense of other components of homeostatic mechanism • First change: klotho reduction and FGF23 rise ~ eGFR 70 ml/min • PTH rising with eGFR ~60 ml/min • Calcitriol reduced with eGFR ~70 ml/min (subclinically, calcium low and phosphate rising) • Clinically, calcium low and phosphate rising when eGFR <20 ml/min (“decompensation” of homeostatic mechanism) and evidence of deleterious effects of compensatory mechanisms • Calcitriol reduction due to: 1. rising FGF23 initially, 2. reduced functional tissues, 3. hyperphosphatemia. Together ->hypocalcemia and worsening HPT • Hyperphosphatemia directly influences osteoblastic transformation of vascular smooth muscle cells and contribute to cardiovascular calcification and arterial stiffness->increased CV morbidity and mortality Thank you! References: 1. Judith Blaine: Renal Control of Calcium, Phosphate, and Magnesium Homeostasis; Clin J Am Soc Nephrol. 2015 2. R. Civitelli: Calcium and phosphate homeostasis: Concerted interplay of new regulators; J. Endocrinol 2011 3. UpToDate 4. Brenner and Rector's The Kidney 5. Harald Juppner: Phosphate and FGF-23; Kidney International (2011) 6. P. Pivonka: A Systems Approach to Understanding Bone Cell Interactions in Health and Disease: Cell interaction, 2012