Calcium Homeostasis

Vitamin D Biosynthesis

The activation of vitamin D is a two step process: -The first step involves the conversion of *vitamin D to 25-hydroxyvitamin D [25(OH)D]*. >This step occurs mainly in the liver and is essentially unregulated. >25(OH)D has very LITTLE biological activity. >It has a LONG circulatory half-life (about 15 days) and is the most ABUNDANT form of vitamin D in the blood. >25(OH)D is the metabolite that is generally measured when clinically assessing a *patient's vitamin D status*. -The second step involves the hydroxylation of 25(OH)D at carbon 1 to form *1,25-dihydroxyvitamin D [1,25(OH)2D].* >This process is highly regulated and occurs principally in the proximal tubules of the kidneys. >1,25(OH)2D is, by far, the most POTENT vitamin D metabolite. >The renal *1α-hydroxylase* that catalyzes its formation from 25(OH)D is a highly regulated enzyme. -Both hypocalcemia and increasing PTH levels have direct stimulatory effects on 1α-hydroxylase activity and mRNA expression. -In addition, dietary restriction of phosphate increases 1α-hydroxylase activity. -A DECREASE in 1α-hydroxylase activity occurs in response a RISE in plasma calcium levels and a FALL in plasma PTH levels. -In addition, 1,25(OH)2D has a NEGATIVE feedback effect on 1α-hydroxylase activity in the kidney. -24,25(OH)2D is the *second most abundant circulating metabolite of vitamin D.* >It is formed from 25(OH)D in a reaction catalyzed by *24-hydroxylase*. >24-Hydroxylase is a 1,25(OH)2D-inducible enzyme that is expressed in the kidney and 1,25(OH)2D target tissues. >Since 24,25(OH)2 has only *very weak biological activity*, its formation may be a means of LIMITING the exposure of tissues to excessive amounts of 1,25(OH)2D.

PTH Secretion Calcium-Sensing Receptors

The calcium-sensing receptor (CaSR): -The calcium sensing receptor is a G protein-coupled, plasma membrane receptor that is expressed in several different tissues including the parathyroid glands and calcitonin-secreting cells of the thyroid. -In chief cells of the parathyroid glands, CaSRs monitor minute-to-minute fluctuations in extracellular fluid Ca2+ and adjust the secretion of PTH accordingly. -When activated by increasing levels of ionic calcium in the extracellular fluid, CaSRs activate Gi and Gq proteins which INHIBIT adenylyl cyclase and activate phospholipase C, respectively. -CaSR activation also INCREASES intracellular Ca2+ by turning on pathways that open ligand-gated calcium channels in the endoplasmic reticulum and voltage-insensitive calcium channels in the plasma membrane. -The increase in intracellular calcium *decreases PTH secretion*. >This mechanism for regulating PTH release is uniquely different from that observed in other endocrine cells that use Ca2+ to regulate secretion. -High intracellular levels of Ca2+ in chief cells of the parathyroid also *suppress PTH synthesis*. -Effect of 1,25(OH)2D3 on PTH secretion: 1,25(OH)2D3, the active metabolite of vitamin D3 INHIBITS PTH secretion. >It acts primarily at the genomic level to SUPPRESS PTH gene expression.

Parathyroid Gland Histology

The principal physiological regulator of PTH secretion is the *concentration of ionic calcium in the EXTRACELLULAR fluid*. -*Chief cells* of the parathyroid glands are very SENSITIVE to fluctuations in extracellular fluid Ca2+ and respond by varying the rate of PTH secretion. -The sensitivity of these cells is such that a SMALL DEVIATION (2-3%) from the normal physiological concentration of Ca2+ in the extracellular fluid elicits a LARGE change in the rate of PTH release. -Chief cells secrete PTH continuously. -At normal plasma levels of Ca2+, the rate of PTH secretion is well below the half-maximal secretory rate.

Osteoblasts Create Bone

When the process of matrix formation is completed, between 50 and 70% of the mature osteoblasts undergo APOPTOSIS -Some of the remaining osteoblasts become the *lining cells* that cover the newly formed bone whereas others lose their bone forming properties and become *osteocytes*. -Lining cells remain on the bone surface and function to regulate the influx and efflux of mineral ions. >These cells also retain the capacity to re-differentiate into secretory osteoblasts in response to SPECIFIC stimuli. -Osteocytes remain quiescent (inactive) -Osteocytes communicate with osteoblast lining cells by means of protoplasmic processes that extend into canals called *canaliculi and function as mechanoreceptors.* -When bone formation is complete, the affected area enters a prolonged period of quiescence until the next remodeling cycle

What is the function of calcitonin?

-After complete thyroidectomy with removal of all calcitonin-secreting tissue, plasma [Ca2+] remains normal (provided the parathyroid glands are NOT injured). -Conversely, patients with a rare calcitonin-secreting tumor of the C-cells frequently have plasma calcitonin levels that are 50 to 100 times normal, yet they maintain normal plasma levels of Ca2+, vitamin D, and PTH. OVERALL no problem with excess/deficiency of calcitonin

Bone Remodeling

-Bone remodeling is an ongoing process of *bone breakdown and renewal* -Bone remodeling occurs: >To replace damaged and worn bone >In response to mechanical strain or disuse >In response to factors that mobilize calcium *Osteocytes act as mechanoreceptors that promote bone remodeling in response to mechanical strain.* -The unique composition of the extracellular matrix provides bone with the rigidity necessary for its functions while at the same time enabling it to resist a wide range of forces WITHOUT BREAKING -The rigidity of bone is due to the minerals impregnated in the matrix. >These minerals account for about two-thirds of the bone's weight. -Organic constituents: About one-third of bone weight is due to organic constituents & water. >The principal organic molecule in the matrix is *type I collagen.* >Highly organized collagen fibrils give bone tensile strength and serve as a framework for mineralization. -Bones play an important role in calcium homeostasis because they contain a substantial *calcium reserve.*

Calcitonin

-Does not appear to be a major regulator of calcium homeostasis in humans. -Release stimulated by ELEVATED CALCIUM levels and certain GI hormones such as GASTRIN. >Calcium levels above 9 mg/dL -Calcitonin *suppresses bone resorption* in osteoclasts by a cAMP-dependent mechanism. >Osteoclasts express calcitonin receptors -Circulating levels are normally LOW in humans. -Calcitonin can be used clinically to TREAT osteoporosis in certain circumstances -The net effect of calcitonin on calcium metabolism is to lower plasma calcium levels. >The primary action of calcitonin is to suppress bone resorption by osteoclasts >This action tends to lower blood levels of calcium and phosphate.

Vitamin D (In circulation)

-More than 99% of the vitamin D and vitamin D metabolites in circulation are bound to plasma proteins. -The principal carrier is an Alpha-GLOBULIN called vitamin D binding protein (*DBP*). -In the proximal tubule cells, the entire 25(OH)D-DBP complex is taken up from the glomerular filtrate by receptor-mediated endocytosis.

PTH stimulates...

-Movement of ionized calcium and phosphate from bone to ECF -Bone dissolving activity of osteoclasts -PTH tips bone remodelling in favor of resorption.

Causes of Vitamin D deficiency

-Obesity -Using too much sun block -Wearing too much clothing -Being outdoors too little -Being old Vitamin D deficiency raises health risks: -Elevated risk of cancer -Cardiovascular disease -Autoimmune disease -Multiple sclerosis -Osteoporosis

Actions of Vitamin D

-On Bone >Acts synergistically with PTH to promote the mobilization of calcium from bone >Indirect STIMULATORY effects on bone mineralization -On the Parathyroid Gland >Feedback SUPPRESSION of PTH release >Modulates chief cell response to calcium and vitamin D > *1,25(OH)2D3* has a *negative feedback effect on PTH secretion*. ->It does so, in part, by inhibiting the expression of the PTH gene. >1,25(OH)2D3 also modulates the response of the chief cells to calcium and vitamin D by inducing the expression of the genes for the *calcium sensing receptor (CaSR)* and the *vitamin D receptor*. -On the Kidney >Regulation of genes for the 1,25- and 24,25-hydroxylase enzymes >Stimulates synthesis of *calbindin* -On the Intestine >Stimulates ABSORPTION of Ca2+ and phosphate

PTH-Administration to a PTH-Deficient Person

-The figure shows the response of a PTH-deficient subject to PTH administration. -The top panel shows changes in serum levels of calcium and phosphate. -The arrow between the 2 broken lines represents the period of PTH administration. -The two panels with bar graphs depict changes in the concentration of urine calcium and C-telopeptides (CTX). >C-teleopeptides are a measure of the breakdown of Type 1 collagen from bone. Phosphorus normal range 3.0-4.5 mg/dl

A normal free ionized calcium level in the blood is maintained primarily by:

A. GI absorption of calcium B. exchange between bone and ECF C. an anterior pituitary hormone D. thyroid hormones E. the supply of iodine in the diet --- B

A two-day-old infant experiences an episode of tetany. The ER resident orders stat serum ionized calcium and PTH levels. The baby's calcium is below the normal range and his PTH is elevated. Of the following, the most likely cause is:

A. autoimmune destruction of his parathyroid glands B. A hypersecreting parathyroid tumor C. A PTH receptor defect - A PTH receptor defect: NOT responsive -Should see elevated calcium c

Osteomalacia (Picture)

After epiphyseal plate closed (Adults)

Vitamin D Enhances Intestinal Absorption of Calcium

Calcium is absorbed along the entire length of the small intestine by simple diffusion between the intestinal cells (paracellular route) and by a transcellular, active transport mechanism in the duodenum and proximal jejunum. -When the amount of calcium in the diet is LOW, more calcium is ABSORBED by ACTIVE transport in the duodenum and proximal jejunum because this mechanism is more efficient and the gradient that drives passive diffusion is low. -When dietary calcium is HIGH, more calcium is ABSORBED by PASSIVE transport because the active transporters become saturated and because chyme spends much more time in the ileum than in other regions of the small intestine. -At an average daily intake of 1000 mg, approximately 35% of dietary calcium is absorbed >However: in diets that are LOW in calcium, the fraction of dietary calcium that is absorbed is GREATER The transcellular pathway -Ca2+ enters into the intestinal cell at the apical surface by two distinct calcium channels (TRPV5 &TRPV6). -On entering the cell, Ca2+ is immediately bound by high affinity calcium-binding proteins such as *calbindin* and shuttled to the basolateral surface of the cell where it is extruded primarily by a calcium ATPase. -By binding Ca2+, *calbindin protects* the cell against the cytotoxic effects of elevated Ca2+ levels and maintains a gradient for Ca2+ influx.

Forms of Ca2+ in the Blood

Calcium is involved in numerous physiological and biochemical processes: signal transduction, hormone release, neurotransmitter release, muscle contraction, blood coagulation, enzyme activation, and the maintenance of membrane stability. -The normal concentration of calcium in human plasma is between *8.4 and 10.2 mg/dl*. -The approximate distribution of plasma calcium is 50% as free ionized (Ca2+). >This is the biologically ACTIVE form of calcium. -40% bound to plasma proteins. >This fraction is pH sensitive. -10% in the form of soluble complexes with citrate, phosphate, or other compounds. *NORMAL [Ca2+] = 8.4-10.2 mg/dL*

Hyperparathyroidism

Clinical features of PTH excess -Bone lesions: direct action of PTH >Enhanced resorption induced by PTH. -Effects due to hypercalcemia >Formation of kidney stones >Duodenal ulcers: Due to stimulatory effects of calcium on gastric acid secretion >Deposition of calcium in soft tissues >DECREASED membrane excitability >Tiredness/Lethargy >Muscle weakness >Constipation >Depression of nervous system activity

Regulation of the Genes That Code for 1α-Hydroxylase and 24-Hydroxylase in Epithelial Cells of the Proximal Tubule

Expression of the 1α-hydroxylase gene is increased in response to: -High PTH -Hypocalcemia via CaSR pathway -Hypophosphatemia -1,25(OH)2D down regulates 1α- hydroxylase gene expression 24-hydroxylase expression is increased in response to: -High levels of 1,25(OH)2D >1,25(OH)2D3 modulates its own formation in the proximal tubules by INHIBITING 1α-hydroxylase activity and STIMULATING 24-hydroxlase activity as described earlier. The administration of 1,25(OH)2D3 can also stimulate *calbindin synthesis and permissive effects on PTH mediated renal calcium reabsorption in distal tubules*

PTH Clearance

Intact PTH has a very SHORT half-life of about 2-4 minutes. -Enzymes in the liver and kidney CLEAVE PTH and release C-terminal PTH fragments back into circulation. - These C-terminal fragments have a MUCH longer half-life than intact PTH and are CLEARED principally by the kidneys.

Phosphorus Homeostasis

Note that most ingested phosphorus is absorbed and eventually eliminated from the body via the URINE. Picture: Fluxes of phosphorus (mg/d) are shown in bold font. -Total phosphorus content in each compartment is shown in regular font.

Actions of Vitamin D on Ca2+ and PO4 Transport

Regulation of Ca2+ uptake: Vitamin D3 increases Ca2+ absorption by the small intestine by stimulating transport via the transcellular pathway. -It acts primarily at the genomic level to increase the synthesis of luminal membrane calcium channels and calbindin Phosphate transport -The absorption of phosphate occurs in the small intestine primarily by a *transcellular mechanism* involving a sodium-phosphate (Na+/Pi) cotransporter on the apical surface of the intestinal cell -The energy expended for this process is utilized by the Na+/K+-ATPases that maintains the sodium gradient. -*Vitamin D3 enhances phosphate absorption by this mechanism.* -Although Pi absorption by the small intestine is very efficient, the process can be further ENHANCED by 1,25(OH)2D3 stimulation.

Rickets: Vitamin D therapy

Vitamin D deficiency -Impairs GI absorption of calcium resulting high PTH causes *bone demineralization*, bones become deformable >In children, result is rickets >In adults, result is called osteomalacia L: Before therapy R: After therapy

Calcitonin Secretion by...

*C Cells of the Thyroid Gland* Polypeptide secreted by the parafollicular cells ("C" cells) of the thyroid -Half-life of 5 to 10 minutes.

Calcitonin: Effects on bone remodeling

*Opposite of PTH* -Calcitonin decreases movement of ionized calcium from bone fluid to ECF -Inhibits activity of osteoclasts = DECREASES bone resorption -No significant clinical findings associated with calcitonin excess or deficiency

Hypoparathyroidism

*PTH Deficiency* The clinical features of hypoparathyroidism are due to *hypocalcemia.* -Neuronal membranes are more *permeable and excitable*: TETANY results from the spontaneous discharge of peripheral nerves -Tetany in the HAND usually occurs before tetany in other parts of the body: usually occurs when serum calcium falls to 6 mg/dl (8.4-10.2 mg/dl is normal) -The major causes are autoimmunity and the inadvertent removal of glands at surgery. -Pseudohypoparathyroidism: rare familial disorder characterized by target tissue RESISTANCE to PTH. Picture: *Trousseau sign* of latent tetany is a medical sign observed in patients with low calcium.

PTHrP

*PTH-Related Protein* -PTHrP is similar enough in structure to PTH that it can bind to and activate the PTH receptor -PTHrP is produced by certain normal tissues and some tumors -PTHrP is the most *common humoral cause of hypercalcemia in malignancy* >Humoral hypercalcemia of malignancy (HHM) is a syndrome associated with a variety of malignant tumors. >It is caused when tumors release humoral agents into systemic circulation that promote bone resorption. >PTHrP is the most common mediator of HHM. -The N-terminal ends of PTHrP and PTH exhibit some sequence homology and conformational similarity which enable them both to *activate the same receptor* >Otherwise these two molecules are very distinct structures. -Functionally: PTH is a systemic hormone produced by a well defined endocrine gland. -PTHrP is produced locally in a wide variety of fetal and adult TISSUES >In these tissues, PTHrP functions as a paracrine or autocrine regulator >The PTHrP gene product also undergoes extensive posttranslational processing to produce several products with distinctive biological activities. >Gene knockout experiments indicate that PTHrP regulates the proliferation and differentiation of certain embryonic tissues such as developing bone. >During fetal development, PTHrP also regulates Ca2+ flux through the placenta from maternal to fetal circulation. -In the adult, PTHrP is expressed in a number of different tissues: Skin, smooth muscle, and neurons of the CNS where it acts locally. -In smooth muscle for example: the local action of PTHrP involves inducing RELAXATION when the muscle is stretched.

The Structures of Vitamin D3 and Vitamin D2

*Vitamin D3* is a lipid soluble vitamin formed from 7-dehydrocholesterol in the skin by a photolytic reaction induced by UV light. -Vitamin D3 is also obtained from the food we eat. *Vitamin D2 (ergocalciferol)* is formed from a PLANT sterol called *ergosterol* by ultraviolet irradiation. Both vitamin D3 and vitamin D2 are biologically INACTIVE and need to be chemically modified by enzymes located in the *liver and kidneys* to become ACTIVE. Note that vitamin D3 and D2 differ only by a double bond between carbons 22 and 23 and a methyl group at position 24.

Anatomy of Parathyroid Glands

-Circulating levels of Ca2+ are closely regulated >In humans, the principal regulators of calcium homeostasis are *parathyroid hormone (PTH) and vitamin D* -Parathyroid Glands: PTH is secreted by the chief cells of the parathyroid glands. >Typically: humans have four parathyroid glands attached to the posterior surface of the thyroid. >Intact PTH is a polypeptide hormone composed of 84 amino acids. ->The N-terminal region of this peptide is particularly IMPORTANT in determining biological activity. ->It is possible to remove a substantial portion of the C-terminal region WITHOUT appreciably affecting the biological activity of the molecule. >Synthetic N-terminal fragments containing amino acids 1-34 can induce all the known actions of intact PTH.

Prevention of osteoporosis

-Getting adequate calcium and Vitamin D throughout life -Doing weight-bearing exercise -Not smoking -Drinking fewer cola soft drinks: Affects bone density -Less caffeine: >Slightly lowers GI calcium absorption >Increases renal calcium excretion

Skin pigmentation affects Vitamin D production

-If your shadow is longer than your height, the sunlight is too weak to cause Vitamin D production in your skin. -*15 minute full-body exposure* of pale skin at mid-day in summer produces ~ 10,000 IU Vit D!

Changes in Total Body Calcium as a Function of Age

-Modeling of bone continues through early adulthood UNTIL peak bone size and mineral density are achieved. -In young adults, the skeleton is in a HOMEOSTATIC state in which the rate of *new bone formation matches the rate of bone resorption*. -Starting with the 4th decade of life: Less bone is produced than resorbed during periods of bone remodeling. >Hence, there is a continued gradual DECLINE in bone mass with increasing age in both genders. >In women: There is also a 5 to 10 year period of ACCELERATED bone loss after the *onset of menopause that corresponds to the decline in estrogen production.* In this graph total body calcium is used as an index of bone mass

Osteoclasts Dissolve Bone

-Osteoclasts are large MULTINUCLEATED cells formed by the fusion of mononucleated precursors from the monocyte-macrophage lineage. -Osteoclast precursors originate in the bone marrow. -They migrate via the circulatory system to specific locations in bone intended for RESORPTION. -When activated: osteoclasts attach to bone by means of plasma membrane proteins called β-integrins. >This effectively seals off an area of bone surface. -The plasma membrane of the osteoclast in contact with the sealed area forms an elaborate plasma membrane structure called the ruffled border which secretes HYDROGEN IONS onto the bone surface by means of membrane-associated H+-ATPases. -The LOW pH dissolves the hydroxyapatite crystal in the bone matrix. -At the same time, proteases secreted by the osteoclast digest the matrix proteins. -When resorption is complete, osteoclasts undergo APOPTOSIS.

PTH Effects on Kidneys

-PTH increases *renal reabsorption* of calcium -Increases EXCRETION of phosphate >prevents precipitation of calcium phosphate -> "METASTATIC calcification" >allows separation of control of calcium and phosphate >The control of plasma calcium is dominant over control of plasma phosphate. -The effects of PTH on kidney function are responsible for the minute-to-minute homeostatic adjustments of calcium levels in extracellular fluid. -When the concentration of calcium in extracellular fluid falls, PTH acts on tubule cells of the distal nephron to INCREASE calcium reabsorption from the glomerular filtrate. -PTH acts on proximal tubule cells to promote phosphate EXCRETION and induce 1α-renal hydroxylase expression. -Circulating phosphate is mostly ionized and in the form of phosphoric acid: it is referred to as inorganic phosphate (Pi). -About 10% is protein bound and 6% is in the form of soluble complexes with cations. -Circulating Ca2+ and Pi are derived from the diet and the skeleton. -The principal organs involved in Ca2+ and Pi homeostasis are the: small intestines, kidneys, and skeleton. -PTH increases calcium levels: Mobilizing calcium from bone and renal calcium reabsorption -*PTH induces activation of Vitamin D* >Renal synthesis of vitamin D promotes GI calcium absorption

Parathyroid Hormone (PTH)

-PTH is secreted by chief cells of the parathyroid gland -84 amino acid long peptide -Intraglandular degradation is a mechanism for controlling the amount of intact PTH released by chief cells -PTH biosynthesis begins with the formation of prepro-PTH. -Removal of the "pre" segment occurs by enzymatic cleavage as the nascent peptide enters the cisternum of the endoplasmic reticulum. -Subsequently, enzymes in the trans-Golgi network remove the "pro" segment prior to the packaging of mature PTH (1-84) into secretory granules. -A unique feature of chief cells is that they UTILIZE intraglandular degradation of stored hormone as a mechanism for regulating the quantity of intact PTH being released. -Proteolytic enzymes in the secretory granule are able to degrade intact PTH to INACTIVE C-terminal fragments. -In hypercalcemic states, there is a decrease in the secretion of intact PTH and an increase in the release of inactive C-terminal fragments. -In hypocalcemia, the chief cells release mostly intact PTH >Less release of inactive C-terminal fragments

Vitamin D Deficiency

A vitamin D deficiency can cause a metabolic bone disorder that is characterized by *impaired bone mineralization* that can result from: -Inadequate exposure to sunlight -A lack of vitamin D in the diet -An inability to absorb vitamin D due to a malabsorption syndrome -Steatorrhea: fat in stools >Vitamin D is a fat soluble vitamin Some diseases of Vitamin D Deficiency: -*Rickets*: Occurs in GROWING bone (children) >Most common cause: a vitamin D deficiency -*Osteomalacia*: after epiphyseal plate closed (Adults) >Most common cause: a *malabsorption syndrome* >Other causes: ->Defective 25-hydroxylation by liver ->Defective 1,25 hydroxylation by kidneys *The defective activation of vitamin D by hepatic or renal hydroxylases or a mutated VDR can have the same consequences as a vitamin D deficiency.*

A 60 year old type 1 diabetic patient presents for a routine checkup. His labs results show decreased calcium, increased phosphate, and increased PTH. What is the most likely explanation for this?

A) osteosarcoma B) chronic renal disease C) poor diet D) pituitary adenoma --- Diabetics are prone to renal failure. When this happens, vitamin D is not converted from 25-OH-D3 to 1,25-OH-D3, leading to decreased calcium absorption from the gut as well as hyperphosphatemia. PTH increases in response to the low calcium and increased phosphate.

Which organ is the site of production of biologically active vitamin D3?

A) skin B) liver C) kidney D) pancreas ---- Vitamin D3 is produced by the photoconversion of 7-dehydrocholestrol (7-DHC) in keratinocytes. Next 7-DHC it is converted to 25-hydroxyvitamin D3 in the ER of hepatocytes in the liver. Vit D3 does not become biologically active until it reaches the proximal tubule in a nephron where it is converted to 1,25-dihydroxyvitamin D3. (C)

Deficiency of PTH secretion results in low plasma ionized calcium. Which of the following is likely occurring?

A. Bone resorption is increased B. Renal reabsorption of Ca is reduced C. Intestinal absorption of Ca is increased ---- B (NO PTH to do the actions needed)

Decreased renal excretion of calcium is directly caused by elevation of...

A. Calcitonin B. 1,25-(OH)2 Vitamin D3 C. PTH ----- PTH -Calcitonin removes -B stimulates but indirect for stimulus

Hypocalcemia will lead to:

A. Decreased PTH secretion B. Increased bone resorption C. Increased calcitonin secretion ---- Bone resorption is the process by which osteoclasts break down bone and release the minerals, resulting in a transfer of calcium from bone fluid to the blood (b)

A healthy 20-yr-old man regularly eats a diet containing 1200 mg of calcium. This amount is triple the amount he needs. The excess calcium will be:

A. incorporated in his bones B. excreted in his urine C. excreted in his feces D. bound to albumin in his blood ------ B and C

Hypercalcemia in an otherwise healthy individual leads to:

A. increased synthesis of PTH B. increased bone resorption C. decreased intestinal calcium absorption D. increased synthesis of 24,25-(OH)2-Vitamin D3 ---- C

Effects of Acid-Base Disturbances on the Ionized Ca2+ Concentration in Blood

Acidemia increases ionized Ca2+ Alkalemia decreases ionized Ca2+ Acid-base disorders: -Acid-base disorders can lead to changes in the ionized calcium concentration. -An elevation in extracellular pH (alkalemia) increases the *binding of calcium to albumin*, thereby LOWERING the serum ionized calcium concentration. >The fall in ionized calcium with acute *respiratory alkalosis* is approximately 0.16 mg/dL (0.04 mmol/L or 0.08 meq/L) for each 0.1 unit increase in pH. >Thus, acute respiratory alkalosis, as in the *hyperventilation syndrome*, can induce symptoms of hypocalcemia, including cramps, paresthesias, tetany, and seizures although the alkalosis is likely to be of primary importance. >The same relationship is true in vitro when the pH is changed in specimens of whole blood or serum. >There is also a significant fall in the ionized calcium concentration in chronic respiratory alkalosis. >However, the fall in ionized calcium in this setting is not due to increased calcium binding, since the renal adaptation lowers the serum bicarbonate concentration and minimizes the rise in extracellular pH. ->The hypocalcemia in this setting is due both to relative hypoparathyroidism and to renal resistance to PTH, with resultant *hypercalciuria*. -In chronic metabolic acidosis, the *increase in ionized calcium due to less albumin binding may not be recognized by measurement of total calcium concentrations.* >In one study, the total serum calcium underestimated the diagnosis of hypercalcemia in incident renal transplant recipients. ->This was explained primarily by the high prevalence of metabolic acidosis in these patients. >The binding of calcium to albumin that is induced by an elevation in extracellular pH may be important in patients with severe chronic kidney disease who often have both hypocalcemia and metabolic acidosis, which will tend to RAISE the ionized calcium concentration. >Treatment of the metabolic acidosis with bicarbonate therapy or dialysis can lower the ionized calcium concentration, which may exacerbate preexisting hypocalcemia and precipitate symptoms such as TETANY.

PTH Secretion in Response to Changes in Serum Levels of Ionic Calcium

Because chief cells store PTH, they are able to *respond immediately* to an ACUTE DECLINE in calcium by releasing preformed PTH. -Moreover: within minutes of a fall in Ca2+ levels, there is a DECREASE in the intraglandular degradation of PTH. -Consequently: MORE of the PTH being released is intact. -If hypocalcemia continues for hours or days, *PTH gene transcription and PTH mRNA stability increase.* -During longer periods of hypocalcemia: There is also an increase in the rate of chief cell PROLIFERATION. >This further increases the overall secretory capacity of the parathyroid glands. -The resultant rise in the extracellular fluid concentration of Ca2+ caused by the actions of PTH on target tissues has a *direct negative feedback effect* on PTH secretion by the parathyroid glands. Chief cells also respond to *overt hypercalcemia by markedly reducing PTH secretion*. -Intraglandular PTH breakdown increases and there is an *increase in the ratio of inactive C-terminal PTH fragments to intact PTH* in the secretory product of chief cells when extracellular fluid Ca2+ levels are elevated. -Moreover: there is a DECREASE in the transcription of the PTH gene and stability of PTH mRNA. >In spite of this, PTH secretion is NOT TOTALLY suppressed by hypercalcemia because there is a *basal rate* of PTH secretion (representing about 5% of the maximal rate) that appears to be UNAFFECTED by calcium levels of extracellular fluid.

Activation of Gene Transcription by Vitamin D

Most of the classical actions of vitamin D are mediated by a high affinity *vitamin D receptor (VDR)* that functions as a ligand activated transcription factor. -The vitamin D metabolite with the *highest affinity for the VDR is 1,25-(OH)2D3*. -When 1,25(OH)2D3 enters the nucleus, it binds a VDR. -The receptor undergoes a conformational change that enables it to form a heterodimer with an unliganded *retinoid X receptor (RXR). * -The vitamin D-VDR/RXR complex binds to a vitamin D Response Element in the regulatory region of a target gene. -When bound to the response element, the vitamin D-VDR/RXR complex ENHANCES gene transcription by recruiting coactivator proteins that link it to the basal transcription machinery. -In certain target genes, such as the PTH gene in chief cells and the 1-α hydroxylase gene in the proximal tubule cells, the vitamin D-VDR/RXR complex REPRESSES gene transcription by attracting CO-REPRESSOR proteins when it binds to the vitamin D response element. *Intestinal epithelium* -1,25(OH)2D3 ENHANCES calcium absorption from the lumen of the small intestine. -Transport efficiency is particularly dramatic in the DUODENUM where vitamin D receptor concentration is greatest. -1,25(OH)2D3 promotes Ca2+ uptake by inducing the expression of novel epithelial calcium channels that are inserted into the luminal membrane. -1,25(OH)2D3 also increases the expression of *calbindin which facilitates the movement of Ca2+ through the cell*. -Because adequate GI absorption of Ca2+ and phosphate are necessary for bone formation, the most apparent consequence of *vitamin D deficiency is a decrease in bone mineralization.* *Bone* -1,25(OH)2D3 is essential for normal skeletal *mineralization*. -It promotes mineralization primarily by ensuring calcium and phosphate availability by virtue of its actions the absorption of these minerals by the intestine. -1,25(OH)2D3 also promotes osteoclastogenesis in bone. >It does so, in part, by inducing the expression of RANKL in osteoblasts and stromal cells. >In addition, 1,25(OH)2D3 promotes osteoclastogenesis by suppressing the synthesis of osteoprotegerin (OPG).

Osteoblasts and Bone Remodeling

Osteoblasts initiate bone resorption by: -Recruiting osteoclast progenitor cells -Promoting the differentiation and activation of osteoclasts Osteoblasts secrete a new matrix (osteoid) -Fate of osteoblasts following matrix formation >To become lining cells >To differentiate into osteocytes >To undergo apoptosis -Mineralization >*Hydroxyapatite* (Ca10[PO4]6[OH]2) is the principal bone crystal. >Other salts and elements foreign to bone can conjugate to bone crystals. -Bone Turnover >Normal healthy bone undergoes continuous remodeling throughout the life of the individual. >Remodeling occurs in localized groups of cells called basic multicellular units Remodeling involves the resorption (breakdown) of old bone by osteoclasts and formation of new bone at the resorption sites. ->These events are closely coupled and involve interactions among different cell types. >Both cortical bone and trabecular bone undergo remodeling. >The annual turnover rate for cortical bone is about 4% per year, whereas trabecular bone turns over at a rate of about 20% per year. >Remodeling enables bone to: (1) adapt to mechanical strain (2) replace worn and damaged bone (3) mobilize bone mineral in response to hormones that maintain calcium homeostasis. -Effects of loading on bone >A low amount of loading leads to bone LOSS >A high loading results in an INCREASE in bone mineral density

Bone is Continuously Being Remodeled

Osteoblasts promote the synthesis and mineralization of bone during replacement phase of the remodeling process. -Osteoblasts create new bone by secreting organic matrix - *osteoid* - and then mineralizing it with *calcium phosphate* crystals -Bone matrix proteins secreted by osteoblasts include type I collagen (the most abundant organic component of the matrix) and a variety of non-collagen proteins that play essential roles in controlling matrix formation and mineralization. -Alkaline phosphatase: Alkaline phosphatase diffuses into the blood. >Clinically, the circulating level of alkaline phosphatase is commonly used as an *indicator of bone formation.* >Alkaline phosphatase levels are ELEVATED during childhood growth, following a major fracture and in certain bone destroying diseases which require bone repair, such as rickets and osteomalacia. -Osteoblast growth and differentiation are controlled by locally produced factors such as bone morphogenic proteins, growth factors (e.g. IGF-1), cytokines (IL-1 and IL-6), and mechanical forces. -Osteoblasts also express receptors for a range of hormones including PTH, 1,25(OH)2D3, estrogens, and glucocorticoids. -Osteoclasts dissolve bone (bone resorption) by solubilizing the crystals >Osteoblast regulate osteoclast differentiation and resorptive activities

Osteoporotic Bone

Osteoporosis = "brittle bones" Occurs in both men and women -Estrogen INHIBITS bone resorption by *decreasing osteoclast activity and increasing osteoblast activity* >Increased incidence of osteoporosis as estrogen levels fall in *post-menopausal women* -Condition in which bone resorption is greater then bone formation: -Reduced osteoblast activity -Increased osteoclast activity -Plasma calcium, phosphate, PTH, and Vitamin D levels are usually normal -Overall: results in REDUCTION of bone mass

Regulation of PTH Secretion and PTH Actions on Bone, Kidney, and Intestine

PTH mobilizes calcium from bone: Calcium is released from bone in response to PTH stimulation. -PTH acts primarily through a mechanism involving the *resorption of mineralized bone by osteoclasts*. -PTH promotes the differentiation and activation of osteoclasts indirectly by binding to and activating *PTH/PTHrP* receptors in the plasma membrane of osteoblasts and other cells of osteoblastic lineage.

PTH/PTHrP Receptor

The PTH/PTHrP receptor is a cell surface receptor that elicits a response in the target cell by INCREASING intracellular cAMP levels via a Gs protein-dependent mechanism. -Since many PTH actions can be mimicked by cAMP analogs, cAMP is considered to be an important mediator of PTH action. -The PTH/PTHrP receptor also turns on the IP3/diacylglycerol second messenger pathway by means of a Gq protein which activates phospholipase C.

Ca2+ Homeostasis

The primary function of PTH is to ensure that the *calcium concentration of extracellular fluid stays within normal limits.* (8.4 and 10.2 mg/dl) -The principal targets of PTH action are the *kidneys and bone.* -PTH regulates specific functions in these tissues by binding to and activating the *type 1 PTH receptor.* >This receptor is also called the PTH/PTHrP receptor because it binds both PTH and a protein called *PTH related protein (PTHrP).* Calcium Balance: -In a state of calcium balance, the amount of calcium excreted by the kidney is EQUAL to the amount of calcium absorbed by the small intestine. -Negative calcium balance (net LOSS of calcium) is associated with: 1.) the immobilization of a limb 2.) prolonged bed rest 3.) certain forms of cancer 4.) weightlessness during space flight. -Positive calcium balance (net GAIN of calcium) is associated with skeletal GROWTH.