Calcium Homeostasis: Parathyroid Hormone, Calcitonin and Vitamin D3

1. Calcium Homeostasis: Parathyroid Hormone, Calcitonin and Vitamin D3

2. Physiological Importance of Calcium • Ca salts in bone provide structural integrity of the skeleton. • Ca is the most abundant mineral in the body. • The amount of Ca is balance among intake, storage, and excretion. • This balance is controlled by transfer of Ca among 3 organs: intestine, bone, kidneys. • Ca ions in extracellular and cellular fluids is essential to normal function of a host of biochemical processes • Neuoromuscular excitability and signal transduction • Blood coagulation • Hormonal secretion • Enzymatic regulation • Neuron excitation

3. Intake of Calcium • About 1000 mg of Ca is ingested per day. • About 200 mg of this is absorbed into the body. • Absorption occurs in the small intestine, and requires vitamin D (stay tuned....)

4. Storage of Calcium • The primary site of storage is our bones (about 1000 grams). • Some calcium is stored within cells (endoplasmic reticulum and mitochondria). • Bone is produced by osteoblast cells which produce collagen, which is then mineralized by calcium and phosphate (hydroxyapatite). • Bone is remineralized (broken down) by osteoclasts, which secrete acid, causing the release of calcium and phosphate into the bloodstream. • There is constant exchange of calcium between bone and blood.

5. Excretion of Calcium • The major site of Ca excretion in the body is the kidneys. • The rate of Ca loss and reabsorption at the kidney can be regulated. • Regulation of absorption, storage, and excretion of Ca results in maintenance of calcium homeostasis.

6. Regulation of [Calcium] • The important role that calcium plays in so many processes dictates that its concentration, both extracellularly and intracellularly, be maintained within a very narrow range. • This is achieved by an elaborate system of controls

7. Regulation of Intracellular [Calcium] • Control of cellular Ca homeostasis is as carefully maintained as in extracellular fluids • [Ca2+]cyt is approximately 1/1000th of extracellular concentration • Stored in mitochondria and ER • “pump-leak” transport systems control [Ca2+]cyt • Calcium leaks into cytosolic compartment and is actively pumped into storage sites in organelles to shift it away from cytosolic pools.

9. Extracellular Calcium • When extracellular calcium falls below normal, the nervous system becomes progressively more excitable because of increase permeability of neuronal membranes to sodium. • Hyperexcitability causes tetanic contractions • Hypercalcemic tetany [Ca2+]cyt

10. Extracellular Calcium • Three definable fractions of calcium in serum: • Ionized calcium 50% • Protein-bound calcium 40% • 90% bound to albumin • Remainder bound to globulins • Calcium complexed to serum constituents 10% • Citrate and phosphate

11. Extracellular Calcium • Binding of calcium to albumin is pH dependent • Acute alkalosis increases calcium binding to protein and decreases ionized calcium • Patients who develop acute respiratory alkalosis have increased neural excitability and are prone to seizures due to low ionized calcium in the extracellular fluid which results in increased permeability to sodium ions

12. Calcium and Phosphorous • Ca is tightly regulated with P in the body. • P is an essential mineral necessary for ATP, cAMP 2nd messenger systems, and other roles

13. Calcium Turnover

14. Calcium in Blood and Bone • Ca2+ normally ranges from 8.5-10 mg/dL in the plasma. • The active free ionized Ca2+ is only about 48% 46% is bound to protein in a non-diffusible state while 6% is complexed to salt. • Only free, ionized Ca2+ is biologically active.

15. Phosphate Turnover

16. Phosphorous in Blood and Bone • PO4 normal plasma concentration is 3.0-4.5 mg/dL. 87% is diffusible, with 35% complexed to different ions and 52% ionized. • 13% is in a non-diffusible protein bound state. 85-90% is found in bone. • The rest is in ATP, cAMP, and proteins

17. Calcium and Bone • 99% of Ca is found in the bone. Most is found in hydroxyapatite crystals. Very little Ca2+ can be released from the bone– though it is the major reservoir of Ca2+ in the body.

18. Structure of Bones Haversian canals within lamellae

19. Calcium Turnover in Bones • 80% of bone is mass consists of cortical bone– for example: dense concentric layers of appendicular skeleton (long bones) • 20% of bone mass consists of trabecular bone– bridges of bone spicules of the axial skeleton (skull, ribs, vertebrae, pelvis) • Trabecular bone has 5 X greater surface area, though comprises lesser mass. • Because of greater accessibility trabecular bone is more important to calcium turnover

20. Bones • 99% of the Calcium in our bodies is found in our bones which serve as a reservoir for Ca2+ storage. • 10% of total adult bone mass turns over each year during remodeling process • During growth rate of bone formation exceeds resporption and skeletal mass increases. • Linear growth occurs at epiphyseal plates. • Increase in width occurs at periosteum • Once adult bone mass is achieved equal rates of formation and resorption maintain bone mass until age of about 30 years when rate of resportion begins to exceed formation and bone mass slowly decreases.

21. Types of Bone Cells • There are 3 major types of bone cells: Osteoblasts are the differentiated bone forming cells and secrete bone matrix on which Ca2+ and PO43- precipitate. • Osteocytes, the mature bone cells are enclosed in bone matrix. • Osteoclasts is a large multinucleated cell derived from monocytes whose function is to resorb bone. Inorganic bone is composed of hydroxyapatite and organic matrix is composed primarily of collagen.

22. Bone Formation • Active osteoblasts synthesize and extrude collagen • Collagen fibrils form arrays of an organic matrix called the osetoid. • Calcium phosphate is deposited in the osteoid and becomes mineralized • Mineralization is combination of CaPO4, OH-, and H3CO3– hydroxyapatite.

23. Mineralization • Requires adequate Calcium and phosphate • Dependent on Vitamin D • Alkaline phosphatase and osteocalcin play roles in bone formation • Their plasma levels are indicators of osteoblast activity.

24. Canaliculi • Within each bone unit is a minute fluid-containing channel called the canaliculi. • Canaliculi traverse the mineralized bone. • Interior osteocytes remain connected to surface cells via syncytial cell processes. • This process permits transfer of calcium from enormous surface area of the interior to extracellular fluid.

25. Bones cells

26. Control of Bone Formation and Resorption • Bone resorption of Ca2+ by two mechanims: osteocytic osteolysis is a rapid and transient effect and osteoclasitc resorption which is slow and sustained. • Both are stimulated by PTH. CaPO4 precipitates out of solution id its solubility is exceeded. The solubility is defined by the equilibrium equation: Ksp = [Ca2+]3[PO43-]2. • In the absence of hormonal regulation plasma Ca2+ is maintained at 6-7 mg/dL by this equilibrium.

27. Osteocytic Osteolysis • Transfer of calcium from canaliculi to extracellular fluid via activity of osteocytes. • Does not decrease bone mass. • Removes calcium from most recently formed crystals • Happens quickly.

28. Bone Resorption • Does not merely extract calcium, it destroys entire matrix of bone and diminishes bone mass. • Cell responsible for resorption is the osteoclast.

29. Bone Remodeling • Endocrine signals to resting osteoblasts generate paracrine signals to osteoclasts and precursors. • Osteoclasts resorb and area of mineralized bone. • Local macrophages clean up debris. • Process reverses when osteoblasts and precursors are recruited to site and generate new matrix. • New matrix is minearilzed. • New bone replaces previously resorbed bone.

30. Osteoclasts and Ca2+ Resorption

31. Calcium, Bones and Osteoporosis • The total bone mass of humans peaks at 25-35 years of age. • Men have more bone mass than women. • A gradual decline occurs in both genders with aging, but women undergo an accelerated loss of bone due to increased resorption during perimenopause. • Bone resorption exceeds formation.

32. Calcium, Bones and Osteoporosis • Reduced bone density and mass: osteoporosis • Susceptibility to fracture. • Earlier in life for women than men but eventually both genders succumb. • Reduced risk: • Calcium in the diet • habitual exercise • avoidance of smoking and alcohol intake • avoid drinking carbonated soft drinks

33. Vertebrae of 40- vs. 92-year-old women Note the marked loss of trabeculae with preservation of cortex.

34. Hormonal Control of Bones

35. Hormonal Control of Ca2+ • Three principal hormones regulate Ca2+ and three organs that function in Ca2+ homeostasis. • Parathyroid hormone (PTH), 1,25-dihydroxy Vitamin D3 (Vitamin D3), and Calcitonin, regulate Ca2+ resorption, reabsorption, absorption and excretion from the bone, kidney and intestine. In addition, many other hormones effect bone formation and resorption.

36. Vitamin D • Vitamin D, after its activation to the hormone 1,25-dihydroxy Vitamin D3 is a principal regulator of Ca2+. • Vitamin D increases Ca2+ absorption from the intestine and Ca2+ resorption from the bone .

37. Synthesis of Vitamin D • Humans acquire vitamin D from two sources. • Vitamin D is produced in the skin by ultraviolet radiation and ingested in the diet. • Vitamin D is not a classic hormone because it is not produce and secreted by an endocrine “gland.” Nor is it a true “vitamin” since it can be synthesized de novo. • Vitamin D is a true hormone that acts on distant target cells to evoke responses after binding to high affinity receptors

38. Synthesis of Vitamin D • Vitamin D3 synthesis occurs in keratinocytes in the skin. • 7-dehydrocholesterol is photoconverted to previtamin D3, then spontaneously converts to vitamin D3. • Previtamin D3 will become degraded by over exposure to UV light and thus is not overproduced. • Also 1,25-dihydroxy-D (the end product of vitamin D synthesis) feeds back to inhibit its production.

39. Synthesis of Vitamin D • PTH stimulates vitamin D synthesis. In the winter or if exposure to sunlight is limited (indoor jobs!), then dietary vitamin D is essential. • Vitamin D itself is inactive, it requires modification to the active metabolite, 1,25-dihydroxy-D. • The first hydroxylation reaction takes place in the liver yielding 25-hydroxy D. • Then 25-hydroxy D is transported to the kidney where the second hydroxylation reaction takes place.

40. Synthesis of Vitamin D • The mitochondrial P450 enzyme 1a-hydroxylase converts it to 1,25-dihydroxy-D, the most potent metabolite of Vitamin D. • The 1a-hydroxylase enzyme is the point of regulation of D synthesis. • Feedback regulation by 1,25-dihydroxy D inhibits this enzyme. • PTH stimulates 1a-hydroxylase and increases 1,25-dihydroxy D.

41. Synthesis of Vitamin D • 25-OH-D3 is also hydroxylated in the 24 position which inactivates it. • If excess 1,25-(OH)2-D is produced, it can also by 24-hydroxylated to remove it. • Phosphate inhibits 1a-hydroxylase and decreased levels of PO4 stimulate 1a-hydroxylase activity

42. Regulation of Vitamin D Metabolism • PTH increases 1-hydroxylase activity, increasing production of active form. • This increases calcium absorption from the intestines, increases calcium release from bone, and decreases loss of calcium through the kidney. • As a result, PTH secretion decreases, decreasing 1-hydroxylase activity (negative feedback). • Low phosphate concentrations also increase 1-hydroxylase activity (vitamin D increases phosphate reabsorption from the urine).

43. Regulation of Vitamin D by PTH and Phosphate Levels PTH 1-hydroxylase 25-hydroxycholecalciferol 1,25-dihydroxycholecalciferol increase phosphate resorption Low phosphate

44. Synthesis of Vitamin D

45. Vitamin D • Vitamin D is a lipid soluble hormone that binds to a typical nuclear receptor, analogous to steroid hormones. • Because it is lipid soluble, it travels in the blood bound to hydroxylated a-globulin. • There are many target genes for Vitamin D.

47. Vitamin D action • The main action of 1,25-(OH)2-D is to stimulate absorption of Ca2+ from the intestine. • 1,25-(OH)2-D induces the production of calcium binding proteins which sequester Ca2+, buffer high Ca2+ concentrations that arise during initial absorption and allow Ca2+ to be absorbed against a high Ca2+ gradient

48. Vitamin D promotes intestinal calcium absorption • Vitamin D acts via steroid hormone like receptor to increase transcriptional and translational activity • One gene product is calcium-binding protein (CaBP) • CaBP facilitates calcium uptake by intestinal cells

49. Clinical correlate • Vitamin D-dependent rickets type II • Mutation in 1,25-(OH)2-D receptor • Disorder characterized by impaired intestinal calcium absorption • Results in rickets or osteomalacia despite increased levels of 1,25-(OH)2-D in circulation

50. Vitamin D Actions on Bones • Another important target for 1,25-(OH)2-D is the bone. • Osteoblasts, but not osteoclasts have vitamin D receptors. • 1,25-(OH)2-D acts on osteoblasts which produce a paracrine signal that activates osteoclasts to resorb Ca++ from the bone matrix. • 1,25-(OH)2-D also stimulates osteocytic osteolysis.