Functions of Calcium Functions of calcium: Teeth Bones Activate enzymes. Neurotransmission. Blood coagulation
Normal values Plasma calcium: –Total Level: –Ionized (Free): –Non-ionized (bound): Plasma Phosphorus: [Ca++] X [Pi++] = constant
Body Distribution of Calcium and Phosphate There are three major pools of calcium in the body: Intracellular calcium: A large majority of calcium within cells is sequestered in mitochondria and endoplasmic reticulum. Intracellular free calcium concentrations fluctuate greatly, from roughly 100 nM to greater than 1 uM, due to release from cellular stores or influx from extracellular fluid. These fluctuations are integral to calcium's role in intracellular signaling, enzyme activation and muscle contractions. Calcium in blood and extracellular fluid: Roughly half of the calcium in blood is bound to proteins. The concentration of ionized calcium in this compartment is normally almost invariant at approximately 1 mM, or 10,000 times the basal concentration of free calcium within cells. Also, the concentration of phosphorus in blood is essentially identical to that of calcium. Bone calcium: A vast majority of body calcium is in bone. Within bone, 99% of the calcium is tied up in the mineral phase, but the remaining 1% is in a pool that can rapidly exchange with extracellular calcium.
Regulation of Calcium & Phosphorus The main regulators: –Vitamin D (Calcitriol). –Parathyroid Hormone. –Calcitonin
Vitamin D (Calcitriol) Bioactive vitamin D or calcitriol is a steroid hormone that has long been known for its important role in regulating body levels of calcium and phosphorus, and in mineralization of bone. More recently, receptors for vitamin D are present in a wide variety of cells, This hormone has biologic effects which extend far beyond control of mineral metabolism.
Structure and Synthesis The term vitamin D is, unfortunately, an imprecise term referring to one or more members of a group of steroid molecules. Vitamin D3, also known as cholecalciferol is generated in the skin of animals when light energy is absorbed by a precursor molecule 7- dehydrocholesterol. Vitamin D is thus not a true vitamin, because individuals with adequate exposure to sunlight do not require dietary supplementation. There are also dietary sources of vitamin D, including egg yolk, fish oil and a number of plants. The plant form of vitamin D is called vitamin D2 or ergosterol. However, natural diets typically do not contain adequate quantities of vitamin D, and exposure to sunlight or consumption of foodstuffs purposefully supplemented with vitamin D are necessary to prevent deficiencies. Vitamin D, as either D3 or D2 does not have significant biological activity. Rather, it must be metabolized within the body to the hormonally-active form known as 1,25-dihydroxycholecalciferol. This transformation occurs in two steps :
Structure and Synthesis Within the liver: cholecalciferal is hydroxylated to 25- hydroxycholecalciferol by the enzyme 25-hydroxylase. Within the kidney: 25-hydroxycholecalciferol serves as a substrate for 1-alpha-hydroxylase, yielding 1,25- dihydroxycholecalciferol, the biologically active form. Each of the forms of vitamin D is hydrophobic, and is transported in blood bound to carrier proteins. The major carrier is called, appropriately, vitamin D-binding protein. The half-life of 25-hydroxycholecalciferol is several weeks, while that of 1,25- dihydroxycholecalciferol is only a few hours.
Control of Vitamin D Synthesis Hepatic synthesis of 25-hydroxycholecalciferol is only loosely regulated, and blood levels of this molecule largely reflect the amount of amount of vitamin D produced in the skin or ingested. In contrast, the activity of 1-alpha-hydroxylase in the kidney is tightly regulated and serves as the major control point in production of the active hormone. The major inducer of 1-alpha- hydroxylase is Parathyroid hormone, it is also induced by low blood levels of phosphate.
Vitamin D deficiency The classical manifestations of vitamin D deficiency: is rickets, which is seen in children and results in bony deformaties including bowed long bones. Deficiency in adults leads to the disease osteomalacia. Both rickets and osteomalacia reflect impaired mineralization of newly synthesized bone matrix, and usually result from a combination of inadequate exposure to sunlight and decreased dietary intake of vitamin D. Causes: Vitamin D deficiency or insufficiency occurs in several other situations, which you might predict based on the synthetic pathway described above: Genetic defects in the vitamin D receptor: a number of different mutations have been identified in humans that lead to hereditary vitamin D resistance.Severe liver or kidney disease: this can interfere with generation of the biologically-active form of vitamin D.Insufficient exposure to sunlight: Elderly Vit D deficiency Children Adults Ricketes Osteomalacia
Histology of parathyroid gland The structure of a parathyroid gland is distinctly different from a thyroid gland. The cells that synthesize and secrete parathyroid hormone are arranged in rather dense cords or nests around abundant capillaries. The image below shows a section of a feline parathyroid gland on the left, associated with thyroid gland (note the follicles) on the right
Parathyroid Hormone Parathyroid hormone is the most important endocrine regulator of calcium and phosphorus concentration in extracellular fluid. This hormone is secreted from cells of the parathyroid glands and finds its major target cells in bone and kidney. Another hormone, parathyroid hormone-related protein, binds to the same receptor as parathyroid hormone and has major effects on development. Like most other protein hormones, parathyroid hormone is synthesized as a preprohormone. After intracellular processing, the mature hormone is packaged within the Golgi into secretory vesicles, then secreted into blood by exocytosis. Parathyroid hormone is secreted as a linear protein of 84 amino acids.
Physiologic Effects of Parathyroid Hormone If calcium ion concentrations in extracellular fluid fall below normal, bring them back within the normal range. In conjunction with increasing calcium concentration, the concentration of phosphate ion in blood is reduced. Parathyroid hormone accomplishes its job by stimulating at least three processes: Mobilization of calcium from bone: –stimulate osteoclasts to reabsorb bone mineral, liberating calcium into blood. Enhancing absorption of calcium from the small intestine: –Facilitating calcium absorption from the small intestine would clearly serve to elevate blood levels of calcium. Parathyroid hormone stimulates this process, but indirectly by stimulating production of the active form of vitamin D in the kidney.vitamin D – Vitamin D induces synthesis of a calcium-binding protein in intestinal epithelial cells that facilitates efficient absorption of calcium into blood. Suppression of calcium loss in urine: –In addition to stimulating fluxes of calcium into blood from bone and intestine, parathyroid hormone puts a brake on excretion of calcium in urine, thus conserving calcium in blood. This effect is mediated by stimulating tubular reabsorption of calcium. –Another effect of parathyroid hormone on the kidney is to stimulate loss of phosphate ions in urine.
Control of Parathyroid Hormone Secretion Parathyroid hormone is released in response to low extracellular concentrations of free calcium. Changes in blood phosphate concentration can be associated with changes in parathyroid hormone secretion, but this appears to be an indirect effect and phosphate per se is not a significant regulator of this hormone. When calcium concentrations fall below the normal range, there is a steep increase in secretion of parathyroid hormone. Low levels of the hormone are secreted even when blood calcium levels are high. The figure to the right depicts parathyroid hormone release from cells cultured in vitro in differing concentrations of calcium. The parathyroid cell monitors extracellular free calcium concentration via an integral membrane protein that functions as a calcium-sensing receptor. calcium-sensing receptor.
Calcitonin Calcitonin is a hormone known to participate in calcium and phosphorus metabolism. In mammals, the major source of calcitonin is from the parafollicular or C cells in the thyroid gland, but it is also synthesized in a wide variety of other tissues, including the lung and intestinal tract. In birds, fish and amphibians, calcitonin is secreted from the ultimobrachial glands. Calcitonin is a 32 amino acid peptide cleaved from a larger prohormone. It contains a single disulfide bond, which causes the amino terminus to assume the shape of a ring. Alternative splicing of the calcitonin pre-mRNA can yield a mRNA encoding calcitonin gene-related peptide; that peptide appears to function in the nervous and vascular systems. The calcitonin receptor has been cloned and shown to be a member of the seven-transmembrane, G protein- coupled receptor family.
Physiologic Effects of Calcitonin A large and diverse set of effects has been attributed to calcitonin, but in many cases, these were seen in response to pharmacologic doses of the hormone, and their physiologic relevance is suspect. It seems clear however, that calcitonin plays a role in calcium and phosphorus metabolism. In particular, calcitonin has the ability to decrease blood calcium levels at least in part by effects on two well- studied target organs: Bone: Calcitonin suppresses resorption of bone by inhibiting the activity of osteoclasts, a cell type that "digests" bone matrix, releasing calcium and phosphorus into blood. Kidney: Calcium and phosphorus are prevented from being lost in urine by reabsorption in the kidney tubules. Calcitonin inhibits tubular reabsorption of these two ions, leading to increased rates of their loss in urine.
Three organs participate in supplying calcium to blood and removing it from blood when necessary: The small intestine is the site where dietary calcium is absorbed. Importantly, efficient absorption of calcium in the small intestine is dependent on expression of a calcium- binding protein in epithelial cells. Bone serves as a vast reservoir of calcium. Stimulating net resorption of bone mineral releases calcium and phosphate into blood, and suppressing this effect allows calcium to be deposited in bone. The kidney is critically important in calcium homeostasis. Under normal blood calcium concentrations, almost all of the calcium that enters glomerular filtrate is reabsorbed from the tubular system back into blood, which preserves blood calcium levels. If tubular reabsorption of calcium decreases, calcium is lost by excretion into urine.
Summary of Hormonal Control Systems Maintaining normal blood calcium and phosphorus concentrations is managed through the concerted action of three hormones that control fluxes of calcium in and out of blood and extracellular fluid: Parathyroid hormone serves to increase blood concentrations of calcium. : Parathyroid hormone preserves blood calcium by several major effects: –Stimulates production of the biologically-active form of vitamin D within the kidney. –Facilitates mobilization of calcium and phosphate from bone. To prevent detrimental increases in phosphate, parathyroid hormone also has a potent effect on the kidney to eliminate phosphate (phosphaturic effect). –Maximizes tubular reabsorption of calcium within the kidney. This activity results in minimal losses of calcium in urine. Vitamin D acts also to increase blood concentrations of calcium. It is generated through the activity of parathyroid hormone within the kidney. Far and away the most important effect of vitamin D is to facilitate absorption of calcium from the small intestine. In concert with parathyroid hormone, vitamin D also enhances fluxes of calcium out of bone. Calcitonin is a hormone that functions to reduce blood calcium levels. It is secreted in response to hypercalcemia and has at least two effects: –Suppression of renal tubular reabsorption of calcium. In other words, calcitonin enhances excretion of calcium into urine. –Inhibition of bone resorption, which would minimize fluxes of calcium from bone into blood. –Although calcitonin has significant calcium-lowing effects in some species, it appears to have a minimal influence on blood calcium levels in humans.
Disorders of Calcium metabolism Hypocalcemia refers to low blood calcium concentration. Clinical signs of this disorder reflect increased neuromuscular excitability and include muscle spasms, tetany and cardiac dysfunction. Hypercalcemia indicates a concentration of blood calcium higher than normal. The normal concentration of calcium and phosphate in blood and extracellular fluid is near the saturation point; elevations can lead to diffuse precipitation of calcium phosphate in tissues, leading to widespread organ dysfunction and damage. Preventing hypercalcemia and hypocalcemia is largely the result of robust endocrine control systems.
Disorders of parathyroid gland Hyperparathyroidism Hypoparathyroidism
Excessive secretion of parathyroid hormone is seen in two forms: Primary hyperparathyroidism is the result of parathyroid gland disease, most commonly due to a parathyroid tumor (adenoma) which secretes the hormone without proper regulation. Common manifestations of this disorder are chronic elevations of blood calcium concentration (hypercalcemia), kidney stones and decalcification of bone. Secondary hyperparathyroidism is the situation where disease outside of the parathyroid gland leads to excessive secretion of parathyroid hormone. A common cause of this disorder is kidney disease - if the kidneys are unable to reabsorb calcium, blood calcium levels will fall, stimulating continual secretion of parathyroid hormone to maintain normal calcium levels in blood. Secondary hyperparathyroidism can also result from inadequate nutrition - for example, diets that are deficient in calcium or vitamin D, or which contain excessive phosphorus (e.g. all meat diets for carnivores). A prominent effect of secondary hyperparathyroidism is decalcification of bone, leading to pathologic fractures or "rubber bones".
Hyperparathyroidism, continued There is no doubt that chronic secretion or continuous infusion of parathyroid hormone leads to decalcification of bone and loss of bone mass. However, in certain situations, treatment with parathyroid hormone can actually stimulate an increase in bone mass and bone strength. This seemingly paradoxical effect occurs when the hormone is administered in pulses (e.g. by once daily injection), and such treatment appears to be an effective therapy for diseases such as osteoporosis.
Hypoparathyroidism Inadequate production of parathyroid hormone - hypoparathyroidism - typically results in decreased concentrations of calcium and increased concentrations of phosphorus in blood. Common causes of this disorder include surgical removal of the parathyroid glands and disease processes that lead to destruction of parathyroid glands. The resulting hypocalcemia often leads to tetany and convulsions, and can be acutely life-threatening. Treatment focuses on restoring normal blood calcium concentrations by calcium infusions, oral calcium supplements and vitamin D therapy.
Tetany Causes: Hypocalcemia due to e.g. vit D deficiency, hypoparathyroidism, etc.. Manifestations: Manifest tetany: –Occurs when plasma calcium <7mg/dl –Convulsions, laryngeal spasm. Latent Tetany: –Occurs when plasma calcium <9-7mg/dl –Chovestek’s sign –Trousseaus' sign. –Hyperreflexia.