Calcium Metabolism, Homeostasis & Related Diseases 1

Objectives of Lectures By the end of these lectures, students should be able to: Recall Calcium Sources, Distributions & Functions Understand Organ & Endocrinal Homeostasis of Calcium with recognition of the roles of: - Vitamin D - Parathyroid Hormone (PTH) - Calcitonin Hormone Verify Main Causes of Hypocalcaemia & Hypercalcemia Recognize the biochemical and Clinical Interrelations of Metabolic diseases of bone: - Rickets & Osteomalacia - Osteoporosis 2

Components of Bones Bone is a specialized mineralized connective tissue containing: 1- Cellular Elements: - Osteoblasts (bone forming cells) - Osteoclasts (bone resorping cells) 2- Organic Matrix - Proteins: Type I Collagen Proteoglycan - Inorganic Minerals: Calcium & Phosphate - Others: small amount of hydroxide & carbonate Calcium is tightly regulated with phosphorous in the body

Calcium Calcium is the most abundant mineral in the body: about 1 kg in a 70 kg man ~ 99% of the body’s calcium is present in the bone where it is combined with phosphate Calcium is the most prevalent mineral in the body where a 70 Kg adult about 1Kg of calcium is present , where 99% is present in bone mainly, while 1% is present in tissues and body fluids

Dietary Sources of Calcium Rich: Milk, milk products as cheese & yoghurt Fair Legumes, vegetables

Biological Functions of Calcium Total Calcium Of the Body 99 % in bone ICF 1 % Blood Clotting Excitability of Nerve & Muscle Bone Formation Reservoir for ECF [Ca2+] Metabolic Regulation for Action of Hormones & Enzyme Activation ECF Physiologically, calcium is classified as either extracellular or intracellular. The skeleton is a major reservoir for providing calcium for both the exracellular and the intracellular pools

Calcium in Blood The ionized fraction is the biologically active fraction

Organ Calcium Homeostasis The concentration of calcium, phosphorous & magnesium in the plasma depend on ORGAN PHYSIOLOGY: Net effect of bone mineral deposition & resorption Intestinal absorption Renal excretion

Organ Calcium Homeostasis cont. The concentration of calcium, phosphorous & magnesium in the plasma depends on ORGAN PHYSIOLOGY: Net effect of bone mineral deposition & resorption Intestinal absorption Renal excretion

Blood [Ca2+] Endocrinal Calcium Homeostasis Parathyroid Hormone (PTH) Active Vitamin D (Calcitriol or 1,25 DHCC) Calcitonin Hormone Blood [Ca2+] is Regulated By:

Vitamin D Sources of Vitamin D A group of sterols with a hormone-like function. Sources of Vitamin D 1- Skin synthesis (On Exposure to Sun Lights): In the skin, 7 dehydrocholesterol is converted to vitamin D3 by exposure to sunlight 2- Diet: - Animal Source Cholecalciferol (Vitamins D3) - Plant Source: Ergocalciferol (Vitamin D2) Vitamin D2 & D3 are NOT biologically active Activation of cholecalciferol (vitamin D3) Cholecalciferol (Vitamin D3) is activated in the body to the biologically active form by two hydroxylations: first in the liver (at position 25) by 25 hydroxylase & then in the kidney at positions 1 by 1 α hydroxylase Active Vitamin D 1, 25 dihydroxycholecalciferol (Calcitriol or DHCC)

SOURCES OF VITAMIN D Vitamin D2 Vitamin D3 DIET VITAMINs D plant source DIET VITAMINs D Vitamin D3 animal source fatty fish Liver egg yolk Sun Rays 7-dehydrocholesterol In the skin

Vitamin D metabolism Cholecalciferol (Vitamin D3) is derived from 7-dehydrocholesterol in the skin by sunlight or supplied in the diet In liver: Cholecalciferol is converted to 25-hydroxycholecalciferol (25-HCC) by the enzyme 25 hydroxylase 25-hydroxycholecalciferol is the predominant form of vitamin D in blood 25-hydroxycholecalciferol is the main storage form of vitamin in the body In kidneys: The 1 α hydroxylase enzyme converts 25 hydroxycholecalciferol to 1,25-dihydroxycholecalciferol (1, 25 DHCC or Calcitriol) which is the biologically active form of vitamin D 13

Functions of Vitamin D Active vitamin D (1, 25 DHCC or Calcitriol) regulates calcium levels in the body (calcium homeostasis) Through: Increasing absorption of calcium by the intestine Minimizing loss of calcium by kidney Stimulating resorption of bone (when necessary) In the intestine: 1, 25-DHCC bind to vitamin D receptors forming a complex that binds to the vitamin D response element upstream of the transcription start site of vitamin D influenced genes causing enhancing gene transcription of genes of calcium transport from intestinal lumen (calcium absorption). In the bones: 1, 25-DHCC stimulates terminal differentiation of osteoclasts precursors to osteoclasts & also stimulates osteoblasts to influence osteoclasts to mobilize bone calcium 1, 25-DHCC does not directly affect mature osteoclastic physiology Deficiency of Active Vitamin D (1, 25-DHCC or Calcitriol) Defective Bone Mineralization (poor calcification) 14

Mechanism of Action of Vitamin D In the intestine: 1, 25-DHCC bind to vitamin D receptors forming a complex that binds to the vitamin D response element upstream of the transcription start site of vitamin D influenced genes causing enhancing gene transcription of genes of calcium transport from intestinal lumen (calcium absorption). In the bones: 1, 25-DHCC stimulates terminal differentiation of osteoclasts precursors to osteoclasts & also stimulates osteoblasts to influence osteoclasts to mobilize bone calcium 1, 25-DHCC does not directly affect mature osteoclastic physiology

Parathyroid hormone (PTH)

Functions of Parathyroid Hormone (PTH) The active hormone is secreted in response to al fall in plasma Ca2+ resulting in in Ca2+ increase in blood. On bone: PTH stimulates bone resorption by osteoclasts resulting in release of calcium ions from bones to blood in cases of hypocalcemia On kidney: 1- PTH increases reabsorption of calcium from kidney tubules. 2- PTH promotes activity of 1a hydroxylase of the kidney (with more hydroxylation of 25 hydroxycholecalciferol (25 HCC) to 1,25 DHCC (activation of vitamin D) which increases intestinal absorption of calcium So, action of PTH on intestine is indirect (via Vitamin D)

Role of Parathyroid Hormone (PTH) in Hypocalcemia is the principal acute regulator of blood [Ca2+] is a hypercalcemic hormone in case of hypocalcemia

Calcitonin Hormone Calcitonin hormone is : In cases of hypercalcemia, Secreted by the parafollicular or “C” cells of the thyroid gland Released in response to high blood calcium (hypercalcemia) In cases of hypercalcemia, Calcitonin Hormone   blood [Ca2+] by:  Osteoclast activity (preventing release of calcium to blood)  Renal reabsorption of calcium Net result of its action   blood calcium CALCITONIN IS THE ONLY HYPOCALCEMIC HORMONE

MAIN CAUSES of HYPERCALCEMIA Primary hyperparathyroidism: due to adenomas (single or multiple) of the parathyroid gld Blood PTH is high (or upper normal range *) Blood calcium is high & Blood phosphate is low Urine calcium & phosphorous are high (hypercalciuria & hyperphasphatruria) Tumors Humoral hypercalcemia of malignancy due to PTHrP (PTH related protein) released by some kinds of tumor cells. PTHrP is not responsive to negative feedback by calcium Hypervitaminosis D: Excessive intake of vitamin D Extrarenal hydroxylation of 25HCC as in granulmotaous diseases as sarcoidosis * Blood PTH in primary hyperparathyroidism may be INAPPROPRIATELY NORMAL as hypercalcemia tries to suppress NORMAL PARATHYROID TISSUES in an attempt to restore normal calcium levels.

MAIN CAUSES of HYPOCALCEMIA Hypoparathyroidism ( PTH) Vitamin D deficiencies Renal disease :low 1 a hydroxylase activity & by hyperphosphaturia Hypoalbuminemia: low blood albumin Nutritional calcium deficiency Intestinal disorders causing inadequate calcium or vit.D absorption

Metabolic Diseases of Bone: Rickets & Osteomalacia Rickets & osteomalacia are metabolic bone diseases occurring due to poor mineralization (calcium content) of bone Causes of poor calcification of bones: I. Vitamin D deficiency 1- Deficiency of sources of vitamin D3: BOTH: Nutrional Vitamin D deficiency (vitamin D3) Poor exposure to sun light 2- Impaired vitamin D metabolism: Renal Rickets: deficiency of 1 hydroxylase of the kidney Deficiency of parathyroid hormone : decrease activity of 1 α hydroxylase Genetic defects in vitamin D metabolism (defect in its activation) Genetic defects of vitamin D receptors or abnormal ligand binding II. Calcium deficiency (nutritional or defect in intestinal absorption) 22

Rickets & Osteomalacia Metabolic Diseases of Bones RICKETS Normal formation of the collagen matrix BUT Incomplete mineralization (poor calcification) Soft Bones CLINICALLY: Bone Deformity OSTEOMALACIA Demineralization (poor calcification) of preexisting bones with CLINICALLY: More Susceptibility to Fracture Rickets & Osteomalacia

Renal Rickets Renal Osteodystrophy In Chronic Renal Failure Low activity of Renal 1a-Hydroxylase Decreased ability to form the active form of vitamin D (1, 25 DHCC will be low) Treatment: 1,25 DHCC (Calcitriol)

Laboratory Investigations for the Diagnosis of Rickets & Osteomalacia Investigations to confirm the diagnosis of rickets:  Blood levels of 25-hydroxycholecalciferol (25 HCC) Blood calcium, (hypocalcemia)  Blood Alkaline phosphatase (ALP) Investigations to diagnose the cause of rickets: Kidney function tests (KFT) Blood 1, 25 dihydroxycholecalciferol (1, 25 DHCC) Blood PTH Others i.e. molecular genetics (if indicated) 25

Metabolic Diseases of Bone: Osteoporosis Most prevalent metabolic bone disease in adults It means reduction in bone mass per unit volume i.e. bone matrix composition is normal, but it is reduced Typically silent (without symptoms) until it leads to fracture at a degree of trauma that would not have caused a fracture in a non-osteoprotic skeleton. Fractures are called fragility or osteoporotic fractures) Most affected: vertebral compression (may be asymptomatic) & hip fractures (requires surgery in most cases) Post-menopausal women lose more bone mass than men (primary osteoporosis) Osteoporosis diagnosis is by dual energy x-ray absorpitometry (DXA) scan Lab diagnosis: not conclusive 26

Metabolic Diseases of Bone Osteoporosis 27

Secondary Osteoporosis Risk Factors Secondary osteoporosis may be caused by reduced bone mass with increased consequent risk of fractures Risk Factors for osteoporosis: Advanced age (esp. in females Certain Drugs Family history of osteoporosis or fractures Immobilization Smoking Excess alcohol intake Cushing’s syndrome Long term glucocorticoids therapy Hyperparathyroidism Hyperthyroidism Vitamin D disorders Certain malignancies In these cases, DXA is highly recommended to evaluate bone density 28

Case Study-1 A 27 years old man presents to his physician 3 weeks after his thyroid surgically removed for a thyroid cancer. However, since he went home from the hospital, he noticed painful, involuntary muscular cramping. He also felt numbness and tingling around his mouth & in his hands and feet. His parents said that he was irritable for the last 2 weeks. He is on levothyroxine medication. On examination He has a well-healing thyroidectomy scar & no palpable masses in the thyroid bed. Blood pressure cuff inflated above the systolic pressure induces involuntary muscular contracture in the ipsilateral hand after 60 seconds (Trousseau`s sign) Tapping on the face interior to the ears cause twitching in the ipsilateral corner of the mouth (Chevostek`s sign) Lab Investigations: Calcium: 5.6 mg/dl (N: 8.5 – 10.2) Albumin: 4.1 g/dl (N: 3.5 – 4.8) PTH: < 1 pg/ml (N: N: 11 – 54)

DIAGNOSIS of Case-1 The parathyroid glands were removed during thyroidectomy  PTH undetectable Hypocalcemia Clinical Manifestations of hypocalcemia (increased reflexes & muscular cramping)

Case Study-2 A 6-year old girl is brought to a pediatrician by her parents They reported that her height is not progressing as they think it should (or like it did for her 8 year old sister & her legs look bowed. She takes no medications Family history: Some cousins has the same problem Lower Lim X-Ray: Bowing of long bones Generalized demineralizations Clinical Chemistry Lab Investigations: Calcium: 7.2 mg/dl (N: 8.5 – 10.2) Albumin: 4.1 g/dl (N: 3.5 – 4.8) PTH: 866 pg/dl (N: 11 – 54) 25 HCC: 35 ng/dl (N: 20- 57) 1, 25 DHCC: less than 1 pg/ml (N: 20 – 75)

Pseudohyperparathyroidism DIAGNOSIS of Case-2 Pseudohyperparathyroidism  In which there is genetic mutations in the stimulatory G-protein IN ACTIVE G Protein No activation of adenylate cyclase NO cAMP NO EFFECT OF PTH HYPOCALCEMIA INCREASE OF PTH (HYPERPARATHYROIDISM WITH HYPOCALCEMIA) TO BE CONFIRMED by MOLECULAR GENETIC ANALYSIS