Study Guide for Calcium and
Phosphate Metabolism
• The most important first messengers for Dental Biochemistry include parathyroid
hormone, calcitonin, insulin, and glucagon
• Name common second messengers and how they operate
• Define calcium balance
• Describe the properties and mechanism of action of parathyroid hormone (PTH)
including G-protein function
• Review the composition of bone, and know what bone mineral is
• What are the two main functions of parathyroid hormone in kidney?
• Describe the metabolic activation of vitamin D
• What are the actions of vitamin D?
• What is osteomalacia? Rickets? Hyperparathyroidism? Pseudohypoparathyroidism?
• What is osteoporosis?
• Describe physiological ways to establish and maintain maximum bone density
• What foods are good sources of dietary calcium?
Elements of the Body (Table 2-1)
Overview
Calcium (Ca2+)
Characteristic
Phosphate (HPO42-)
99%
Body total in bone/teeth
85%
1.3 mM (ionized)
Plasma concentration of
ionic form
1.3 mM
Carefully controlled
Normal variation of ionic
form
Within 50%
104-105
[extracellular]/[intracellular]
10
Cell signaling and 2nd
messenger
Neurotransmitter and hormone
release
Exocytosis of proteins
Muscle contraction
Blood clotting
Biomineralization
Regulatory roles
Nil
Important conclusion,
calcium is an important
regulator
Regulation of Calcium Metabolism
• Minerals; serum concentration
– Calcium (Ca2+); 2.2-2.6 mM (total)
– Phosphate (HPO42-); 0.7-1.4 mM
– Magnesium (Mg2+); 0.8-1.2 mM
• Organ systems that play an import role in Ca2+ metabolism
– Skeleton
– GI tract
– Kidney
• Calcitropic Hormones
–
–
–
–
Parathyroid hormone (PTH)
Calcitonin (CT)
Vitamin D (1,25 dihydroxycholecalciferol)
Parathyroid hormone related protein (PTHrP)
Sutherland Second Messenger Hypothesis
Understand this key concept
Second Messengers
Fig. 19-4
More Second Messenger
Three Forms of Circulating
2+
Ca
• Intake = output
• Negative calcium
balance: Output >
intake
– Neg Ca2+ balance
leads to osteoporosis
• Positive calcium
balance: Intake >
output
– Occurs during
growth
• Calcium is essential,
we can’t synthesize
it
Calcium Balance
Calcium and the Cell
• Translocation across the plasma membrane
• Translocation across the ER and mitochondrion;
Ca2+ ATPase in ER and plasma membrane
Anatomy and Feedback Inhibition
Parathyroid Hormone Structure
• Synthesized in
the 4 parathyroid glands
• PreProPTH
• 115 aa precursor
giving a 90 aa
prohormone
• Cleaved at -6/-7
84 residues in
the mature
peptide
• Regulator of
Ca2+
homeostasis
Parathyroid Hormone Biosynthesis
Regulation of PTH Secretion and
Biosynthesis
• Extracellular Ca 2+ regulates secretion of
PTH
– Low Ca 2+ increases
– High Ca 2+ decreases
• Ca2+ also regulates transcription
• High levels of 1,25 dihydroxyvitamin D3
inhibit transcription
Calcium Sensing
Receptor (CaSR)
• Parathyroid chief cells contain a Ca2+ sensing receptor (CaSR)
– 7 transmembrane segments (We will see a lot of 7 TM receptors)
– mM affinity for Ca2+
– GPCR of the GPLC and GI varieties
• Generates inositol 1,4, 5-trisphosphate which  increases
intracellular Ca2+
• There are two paradoxes
– The receptor responds to decreasing concentrations of
agonist
– Low extracellular Ca2+ increases intracellular Ca2+
– Also found in thyroid C cells (calcitonin), kidney, and brain
Circulating Forms of PTH
• Intact, active PTH of 84 aa
• Inactive carboxyterminal fragments lack the 1-34 active
domain
• PTH t1/2 (half life) is 2-3 min
• Liver (2/3rds) and kidney (1/3rd) are major sites of
fragmentation
Actions of Parathyroid Hormone
• Fine tunes Ca2+ levels in blood
– It increases Ca2+
– It decreases Pi
• Parathyroid hormone acts directly on bone to stimulate resorption and
release of Ca2+ into the extracellular space (slow)
– Gs protein-coupled receptors in osteoblasts increase cAMP and activate PKA
– Inhibits osteoblast function
– This occurs when PTH is secreted continuously; the opposite occurs when it is
given once daily by injection
• Two effects in kidney
– Parathyroid hormone acts directly on kidney to increase calcium reabsorption
and phosphate excretion (rapid)
• Gs protein-coupled receptors
• Parathyroid hormone acts on distal tubule
• Calcitonin inhibits
– Stimulates transcription of 1-alpha hydroxylase for Vitamin D activation in
kidney
• Vitamin D increases calcium and phosphate absorption
Parathyroid Hormone Receptor
• 7 TM
• GPCR
G-Protein Cycle (Fig. 19-10)
Regulation of Adenylyl Cyclase
(Fig. 19-11)
7 and 12 TM segments
Cyclic AMP Metabolism (Fig. 19-12)
Know each step involved in the generation of
cAMP by PTH (words, not structures)
• Inorganic (67%)
Bone
– Hydroxyapatite 3 Ca10(PO4)6(OH)2
– There is some amorphous calcium phosphate
• Organic (33%) component is called osteoid
– Type I collagen (28%)
– Non-collagen structural proteins (5%)
•
•
•
•
•
Proteoglycans
Sialoproteins
Gla-containing proteins (gamma carboxyglutamate)
Phosphoproteins
Bone specific proteins: osteocalcin, osteonectin
– Growth factors and cytokines (Trace)
• Bone undergoes continuous turnover or remodeling
throughout life
– About 20% of bone is undergoing remodeling at any one time
Bone Composition
Calcium and the Skeleton
• A, absorption is stimulated by Vit D; S, secretion
• GF, glomerular filtration; TR, tubular reabsorption
of Ca2+ is stimulated by PTH
Osteoblast and Osteoclast
Function
• Osteoblasts
• Bone formation
• Synthesis of matrix
proteins
– Type I collagen
– Osteocalcin
– Others
• Mineralization
• Activation of osteoclasts
via RANKL production
• Osteoclasts
• Bone resorption
– Degradation of
proteins by enzymes
– Acidification
• RANK is activated by
RANKL, and this
leads to cells
differentiation to
osteoclasts
Bone Remodeling
• Osteoclasts dissolve
bone
– Large multinucleated
giant cells
• Osteoblasts produce
bone
– Have receptors for PTH,
CT, Vitamin D,
cytokines, and growth
factors
– Main product is collagen
• When osteoblasts
become encased in
bone, they become
osteocytes
PTH and Osteoblastogenesis
Osteoclast Mediated Bone Resorption
Osteoclastogenesis: RANKL, RANK,
and OPG
• Osteoblasts activate osteoclasts, formation of a multinuclear cell
• The molecular participants in this pathway are the membraneassociated protein named RANKL (receptor activator of nuclear factor
kappa B ligand,) a member of the tumor necrosis factor family of
cytokines
• Its cognate receptor is RANK; TRAF, TNF receptor associated factors
– Mediates activation of NF-kappa-B by unknown mechanism
• OPG (osteoprotegerin) is a soluble "decoy" receptor for RANKL
• RANKL is expressed on the surface of osteoblastic stromal cells
• By binding to RANK, its receptor, on osteoclast precursors, RANKL
enhances their recruitment into the osteoclastogenesis pathway in the
physiology of bone metabolism
• RANKL also activates mature osteoclasts to resorb bone
• RANKL is a factor through which osteoblasts regulate osteoclasts, and
bone formation is coupled to bone resorption
RANK and RANKL
Osteoclastogenesis
PTH and
Kidney
PTH acts
on the
distal
tubule
Calcitonin
• Product of
parafollicular C cells
of the thyroid
• 32 aa
• Inhibits osteoclast
mediated bone
resorption
– This decreases serum
Ca2+
• Promotes renal
excretion of Ca2+
Calcitonin
•
•
•
•
Probably not essential for human survival
Potential treatment for hypercalcemia
7 transmembrane segment receptor
Stimulates cAMP production in bone and
kidney
Vitamin D Metabolism
Transport and Metabolic Sequence of
Activation of Vitamin D
Proposed Mechanism of Action of
1,25-DihydroxyD3 in Intestine
Vitamin D-dependent Ca2+ Absorption
• Duodenum>jejunun>ileum
• Absorption is greater at low pH
– The pH of the stomach is about 2
– Peak absorption at the beginning of the
duodenum
Vitamin D Deficiency: Rickets
• Inadequate intake and absence of sunlight
• The most prominent clinical effect of Vitamin D
deficiency is osteomalacia, or the defective
mineralization of the bone matrix
• Osteoblasts contain the vitamin D receptor
• Vitamin D deficiency in children produces rickets
• A deficiency of renal 1α-hydroxylase produces
vitamin D-resistant rickets
– Sex linked gene on the X chromosome
– Renal tubular defect of phosphate resorption
– Teeth may be hypoplastic and eruption may be retarded
Rickets
Vitamin D-Resistant
Rickets
• Above: Hypoplastic teeth
• Below: Minimal caries can
produce pulpitis; periapical
abscesses are thus common
• Lack 1-hydroxylase in kidney
• Rx: Respond well to 1, 25dihydroxyD3
PTHrP; Parathyroid Hormone related
Protein
• It is synthesized as 3 isoforms as a result of alternative
splicing (139, 141, 173 aa)
• Can activate the PTH receptor
• Plays a physiological role in lactation, possibly as a
hormone for the mobilization and/or transfer of calcium to
the milk
• May be important in fetal development
• May play a role in the development of hypercalcemia of
malignancy
– Some lung cancers are associated with hypercalcemia
– Other cancers can be associated with hypercalcemia
PTHrP; Parathyroid Related Protein
Causes of Hypocalcemia
Hypoparathyroid
Nonparathyroid
PTH Resistance
Postoperative
Vitamin D
deficiency
Pseudohypoparathyroidism
Idiopathic
Malabsorption
Post radiation
Liver disease
Kidney disease
Vitamin D
resistance
Sequence of Adjustments to
Hypocalcemia
Pseudohypoparathyroidism
• Symptoms and signs
– Hypocalcemia
– Hyperphosphatemia
– Characteristic physical appearance: short stature, round face, short
thick neck, obesity, shortening of the metacarpals
– Autosomal dominant
• Resistance to parathyroid hormone
• The patients have normal parathyroid glands, but they fail to respond
to parathyroid hormone or PTH injections
• The rise in urinary cAMP after parathyroid hormone fails to occur
• The cause of the disease is a 50% deficiency of Gs in all cells
• Symptoms begin in children of about 8 years
– Tetany and seizures
– Hypoplasia of dentin or enamel and delay or absence of eruption
occurs in 50% of people with the disorder
• Rx: vitamin D and calcium
Pseudohypoparathyroidism
Elfin facies, short stature,
enamel hypoplasia
Signs and Symptoms of
Hypercalcemia
• Neurologic
– Lethargy, drowsiness, depression, confusion
– Can lead to coma and death
• Neuromuscular
– Muscle weakness, hyptonia, decreased reflexes
• Cardiac
– Arrhythmias
• Bone
– Ache, pain, fracture
Causes of Hypercalcemia
Common
Uncommon
Malignant disease, e.g.
some lung cancers
Renal failure
Hyperparathyroidism
Sarcoidosis
Vitamin D toxicity
(excessive intake)
Multiple myeloma
Causes of Hypercalcemia
• Primary hyperparathyroidism
– Most people are asymptomatic
– Classically affects skeleton, kidneys, and GI tract
• Triad of complaints: bones, stones, and abdominal groans
– Renal stones are most common single presenting complaint
– Usually due to an adenoma (tumor)
Hyperparathyroidism
• The disorder is characterized by hypercalcemia,
hypercalcuria, hypophosphatemia, and
hyperphosphaturia
• Parathyroid hormone causes phosphaturia and a decrease
in serum phosphate
• Calcium rises and it is also secreted in the urine
• Most common complication are renal stones made of
calcium phosphate
– Stone chemistries: calcium, phosphate, urate
• Most serious complication is the deposition of calcium in
the kidney tubules resulting in impaired renal function
Primary Hyperparathyroidism
• Calcium excretion > calcium intake
• Large regions of bone are replaced by connective
tissue
• Two lesions: maxilla and forehead
Hyperparathyroidism
• Left: Giant Cell Granuloma
• Right: Loss of lamina dura, pathognomonic oral
change in hyperparathyroidism
Lamina Dura
• Tooth sockets are bounded by a thin radiopaque
layer of dense bone
• Lamina dura: “hard layer”
Congenital Hypoparathyroidism
• Hypoplasia of the teeth, shortened roots,
and retarded eruption
Hypercalcemia of Malignancy
• Treatment improves quality of life when
Ca2+ is elevated but not yet life threatening
• Treat with bisphosphonates
– Inhibits osteoclastic activity
• When serum Ca2+ > 3.00 mM treat with
NaCl IV
Bisphosphonates
Osteoporosis
• Osteoporosis is characterized by a significant reduction in bone
mineral density compared with age- and sex-matched norms
• There is a decrease in both bone mineral and bone matrix
• Osteoporosis is the most common metabolic bone disease
• Affects 20 million Americans and leads to 1.3 million fractures in the
US per year
• Women lose 50% of their trabecular bone and 30 % of their cortical
bone
• 30% of all postmenapausal women will sustain an osteoporotic
fracture as will 1/6th of all men
• The cost of health care and lost productivity is $14 billion in the US
annually
Normal and Osteoporotic Bone
Factors that Affect Peak Bone Mass
• Gender (M>F), males have greater PBM than females
• Race (Blacks >Whites)
• Genetics (osteoporosis runs in families and this may be the
predominant factor)
– Estrogen receptor gene
– Type I collagen gene
– Vitamin D receptor gene
• Gonadal steroids (estrogen and testosterone increase bone
mass)
• Growth hormone (increases bone mass)
• Calcium intake (supplements work)
• Exercise (increases bone mass)
Sequelae of
Osteoporosis
Osteoporosis
Bone Density as a Function of Age
FDA Approved Rx’s for Osteoporosis
• Bisphosphonates (alendronate and risedronate),
calcitonin, estrogens, parathyroid hormone and
raloxifene are approved by the US Food and Drug
Administration (FDA) for the prevention and/or
treatment of osteoporosis
• The bisphosphonates (alendronate and
risedronate), calcitonin, estrogens and raloxifene
affect the bone remodeling cycle and are classified
as anti-resorptive medications
• Teriparatide, a form of parathyroid hormone, is a
newly approved osteoporosis medication. It is the
first osteoporosis medication to increase the rate
of bone formation in the bone remodeling cycle
Treatments (Continued)
• Exercise, activity
• Calcium intake should be 1000-1500 mg/day
– Postmenapausal women take in less than 500 mg/day
– Males and females should take in 1000-1500 mg/day
– All adults greater than 65 years should take 1500 mg/day
– Three glasses of milk or three cups of yogurt per day provide 1000-1500 mg/day
• Estrogen treatment
– Estrogen inhibits osteoclastic activity
– Bone density increases 3-5% per year for the first three years after menopause
– This therapy needs to be individualized
• Estrogen may increase the incidence of breast cancer, heart attacks, stroke, blood
clots
• That it may exacerbate cardiovascular disease is controversial
– All the data are not in yet, and estrogen treatment is under review; for more
information go to http://www.fda.gov/bbs/topics/NEWS/2003/NEW00863.html
Treatments (Continued)
• Raloxifene (Brand name Evista) is a selective estrogen receptor
modulator
• Decreases in estrogen levels after menopause lead to increases in bone
resorption and bone loss. Bone is initially lost rapidly because the
compensatory increase in bone formation is inadequate to offset
resorptive losses. This imbalance between resorption and formation is
related to loss of estrogen, and may also involve age-related
impairment of osteoblasts or their precursors
• Raloxifene reduces resorption of bone and decreases overall bone
turnover. These effects on bone are manifested as increases in bone
mineral density (BMD)
• Raloxifene’s biological actions, like those of estrogen, are mediated
through binding to estrogen receptors. This binding results in the
modulation of expression of multiple estrogen-regulated genes in
different tissues
Treatments (Continued)
• Bisphosphonates inhibit osteroclasts
– Alendronate (Brand name Fosamax)
– Risedronate (Brand name Actonel)
• Calcitonin (Brand name Miacalcin )
– From salmon
– Given intranasaly
– Probably least effective Rx
• Vitamin D
– Most Americans consume less than recommended amount
– 800 IU per day seems safe and not enough to cause vitamin D
toxicity
Treatments (Continued)
• Parathyroid hormone (Brand name Forteo)
– Teriparatide, a form of parathyroid hormone, is approved for the
treatment of osteoporosis in postmenopausal women and men who
are at high risk for a fracture
– Chronically elevated PTH leads to bone loss; however, intermittent
PTH (once daily bolus injection) leads to new bone synthesis
– Must be injected daily, a major disadvantage
– Cost about $7000 per year
• Future Rx’s
• Sodium fluoride
– Considered a possibility for years
– Adoption seems unlikely
• Strontium ranelate
– NEJM 350 (2004) 459-468.
Calcium Content of Foods
• http://www.nal.usda.gov/fnic/foodcomp/Data
/SR16/wtrank/wt_rank.html
Selected Web Sites
Properties of Parathyroid Hormone
• It is a small protein hormone
• It acts on its 7 transmembrane segment receptor
– Gs protein-coupled receptors in osteoblasts increase [cAMP] and
activates protein kinase A
– Parathyroid hormone acts directly on bone to stimulate resorption
and release of Ca2+ into the extracellular space (slow)
– Parathyroid hormone acts directly on kidney to increase calcium
reabsorption and phosphate excretion (rapid)
– Parathyroid hormone acts on distal tubule and stimulates calcium
resorption
– Parathyroid hormone stimulates transcription of 1-alpha
hydroxylase for vitamin D activation in kidney
• Know how 7 transmembrane segment receptors activate or
inhibit adenylyl cyclase via the G-protein cycle
• Know the pathway for the formation of active vitamin D
Transport and Metabolic Sequence of
Activation of Vitamin D
PTH
Biosynthesis
• PTH is co-secreted
with chromogranin
A, a protein;
significance
unknown
Sequence of Adjustments to
Hypocalcemia