Calcium homeostasis

Calcium homeostasis and role of parathyroid hormone (PTH), vitamin D and Calcitonin: Calcium balance is influenced by dietary intake, intestinal absorption, renal excretion and bone remodeling. Blood calcium levels are not a good indicator of body calcium stores as more than 99% of calcium is present in bones. Apart from bone formation, calcium is essential in varied body functions like extra- and intracellular signaling, nerve impulse transmission and muscle contraction. Serum ionized calcium is metabolically active form and its levels are tightly regulated in the bloodstream. Main calcium-binding proteins include albumin and globulin in serum and calmodulin and other calcium-binding proteins in the cell. Children are in positive bone balance (formation > resorption), healthy adults in neutral bone balance (formation = resorption) and elderly in negative bone balance (formation < resorption).

PTH: PTH is secreted by the chief cells of the parathyroid gland. The cell membrane of the parathyroid gland has a Ca++ sensing receptor (CaSR) which is a GPCR linked to Gq. When the plasma Ca++ increases, calcium binds to the CaSR activating phospholipase C leading to increased intracellular Ca++, which in turn inhibits the release of PTH. Conversely, decreased serum Ca++ will cause decreased calcium binding to the CaSR which stimulates PTH release. Hypermagnesemia inhibits PTH release while hypomagnesemia stimulates PTH release. However, severe hypomagnesemia will cause decreased production of PTH.

PTH receptor: The PTH receptor or PTHr is located on the renal PCT and DCT and osteoblasts in bone. It is a G protein-coupled receptor with seven membrane-spanning helices. Ligands can activate at least two second messenger signaling systems in PTHr - the adenylyl cyclase/protein kinase A pathway and the phospholipase C/protein kinase C pathway. Both PTH and PTHrp or PTH related peptide bind to the same receptor.

PTH effects: In response to low blood calcium levels, PTH is secreted into the circulation and then acts primarily on the kidney and bone, where it binds to cells expressing the PTHr. In kidney, PTH directly stimulates the reabsorption of calcium at the DCT and it stimulates the activity of 1α-hydroxylase and thereby stimulates the synthesis of 1,25(OH)2D3 or Vit D. PTH also inhibits the reabsorption of phosphate at the PCT by inhibiting Na± phosphate cotransport by Npt2. PTH raises cAMP level in the urine. In bone, PTH can induce a rapid release of calcium from the bone matrix, but it also mediates longer-term changes in calcium metabolism by acting directly on osteoblasts and indirectly on osteoclasts, the bone-resorbing cells. PTH action on osteoblasts leads to changes in the synthesis and/or activity of several proteins, including osteoclast-differentiating factor also known as TRANCE (tumor necrosis factor-related activation-induced cytokine), RANKL or osteoprotegerin (OPG) ligand. PTHr are located on osteoblasts. PTH stimulates the production of RANKL and inhibits the expression of OPG. RANKL binds to its receptor RANK on hematopoietic precursors of osteoclasts and stimulates their differentiation and survival. The action of RANKL is blocked by OPG which functions as a soluble decoy receptor. PTH increases the expressions of some bone-resorbing cytokines such as OPG ligand or OPGL, M-CSF, and TRAIL in osteoblasts.

Calcitonin: Calcitonin is produced by the parafollicular or C cells of the thyroid. Calcitonin signals intracellularly by binding to its receptor, the CTR, on the plasma membrane of effector cells. Calcitonin binding to the CTR activates multiple G‐protein mediated signaling pathways including cyclic adenosine monophosphate (cAMP)/protein kinase A pathway or protein kinase C pathways or mitogen‐activated protein kinase (MAPK) pathway.

Calcitonin is released in response to hypercalcemia. Calcitonin inhibits osteoclast activity and bone resorption. Calcitonin interferes with cytoskeletal structures involved in osteoclast adhesion like actin and integrins and also inhibits the NF‐κB ligand (RANKL). Calcitonin inhibits renal reabsorption of calcium. Like PTH, calcitonin increases the synthesis of Vit D and stimulates the transcription of the 1α‐hydroxylase gene in the proximal tubule of the kidney. Circulating calcitonin levels are elevated during pregnancy and lactation protecting the maternal skeleton from excessive resorption during lactation.

Vitamin D

Synthesis: Vitamin D₃, also known as cholecalciferol is generated in the skin from 7-dehydrocholesterol after exposure to sunlight. The plant form of vitamin D is called vitamin D₂ or ergocalciferol. Vitamin D, as either D₃ or D₂, does not have significant biological activity. The active form of vit D is called 1,25-dihydroxycholecalciferol or 1,25-dihydroxy vit D which is a steroid hormone. It is produced by hydroxylation at C-25 in the liver to form 25-hydroxyvitamin D followed by a second hydroxylation at C-1 in the kidney to form the active form of vitamin D called 1,25 dihydroxy vitamin D. CYP2R1 is the liver microsomal enzyme involved in vitamin D synthesis. 25(OH) vitD is transported to the kidney where it is internalized by megalin, a transmembrane protein that acts as a surface receptor for Vit D binding protein. In the proximal renal tubule, 25(OH)D3 is hydroxylated by 1α hydroxylase (CYP27B1) resulting in the formation of 1,25(OH)2D3. A serum concentration of 25-hydroxyvitamin D is the best indicator of vitamin D status as it is the major circulating form of the vitamin in the body. The half life of 25 (OH)D3 is several weeks, while that of 1,25-dihydroxy vit D is only a few hours. Vit D is hydroxylated at C-24 to form 24,25 dihydroxy vit D which is not physiologically active.

Vit D receptor: The genomic actions of 1,25(OH)2D3 are mediated by the vitamin D receptor (VDR). 1,25(OH)2D3-occupied VDR heterodimerizes with the retinoid X receptor and together with co-regulatory proteins interacts with vitamin D response elements in and around target genes and mediates their transcription. VDR is expressed in all segments of the small and large intestine. Vitamin D receptors are present in most cells in the body.

Actions of vit D: Vit D induces the expression of the intestinal apical membrane calcium channel TRPV6, the calcium-binding protein calbindin and the plasma membrane Ca ATPase, PMCA1b. TRPV6 is an apical brush border channel in the enterocytes that is involved in Vit D dependent absorption of Ca++. Calbindin facilitates translocation of calcium through the enterocyte and buffers calcium preventing toxic levels of calcium from accumulating in the cell. CaATPase pumps Ca++ across the basolateral membrane into the bloodstream. Thereby, 1,25(OH)2D3 exerts its control in the intestine on calcium entry, calcium binding and basolateral extrusion. Vit D also stimulates the intestinal absorption of phosphate and magnesium ions. In the distal tubule of the kidney, similar to the intestine, 1,25(OH)2D3 regulates the transcellular transport process by inducing calbindins and epithelial calcium channel TRPV5 which facilitates apical calcium entry. It increases the absorption of both calcium and phosphate in the renal tubules. Vit D promotes osteoclast activity, helping in bone resorption to increase blood Ca++ levels. At the same time, it promotes bone mineralization and formation of osteoid and new bone. Overall Vit D influences bone remodelling, promoting the resorption of old bone while stimulating new bone formation. Vit D induces the synthesis of osteocalcin. Osteocalcin is a protein hormone secreted by osteoblasts. It is involved with bone turnover and energy, lipid and carbohydrate metabolism. Lower osteocalcin levels are seen in metabolic syndrome and insulin resistance. Vitamin D has potent effects on the growth and differentiation of many types of cells including immune cells.

Regulation of Vit D: The activity of 1-alpha-hydroxylase in the kidney is tightly regulated and serves as the major control point in production of active Vit D. The major inducer of 1-alpha-hydroxylase is PTH. It is also induced by hypocalcemia and hypophosphatemia. When need for Ca++ is less the inactive 24,25 dihydroxy vit D form is high while when the need for Ca++ is higher like in hypocalcemia 1,25 dihydroxy vit D is preferentially synthesized.