Asem: Calcium Regulation

minute-to-minute (makes sense because of short half-life)

PTH regulates ____ concentrations of free calcium.

PTH suppresses the expression of Na+/Pi symporter (NPT) in apical membrane. PTH also causes the removal of existing Na+/Pi symporter from apical membrane.

PTH *decreases phosphate reabsorption* by renal proximal tubule. How does it do this?

• PTH stimulates the expression and activation of 1-hydroxylase enzyme. • So, there is a direct effect of PTH on calcitriol (Vit. D) production!

PTH *stimulates calcitriol production* in renal proximal tubules. How does it do this?

1. *epithelial calcium channels* (TRPV5, TRPV6, etc) in the apical membrane. 2. *calcium binding protein* (calbindin) in cytoplasm. 3. plasma membrane *Calcium ATPase* (PMCA) 4. *sodium/calcium exchanger* (NCX) basolateral membrane.

PTH increases calcium reabsorption from distal tubules of nephrons and thick ascending limb (to a lesser extent). It does so by increasing expression of (4):

Vitamin D absorbed from the intestinal tract may be (1) transported in association with chylomicrons through lymphatic to venous system to the heart, and (2) bound to vitamin D-binding protein (DBP) through the hepatic portal vein to the kidney. Vitamin D is transformed in the liver to 25-hydroxyvitamin D which is transformed to in the kidney to active 1,25-dihydroxyvitamin (calcitriol). Alternately, both 25-hydroxyvitamin D and 1,25-dihydroxyvitamin (calcitriol) may transformed to inactive forms in the kidney.

Summarize vitamin D metabolism

Calcitonin reduces plasma Ca2+ levels by: 1. inducing bone formation and enhancing calcium deposition in bone. 2. inhibiting the functions of osteoclasts. 3. inhibiting the formation/differentiation of new osteoclasts. 4. attenuating the functions and numbers of osteoblasts.

The *major* functions of calcitonin are on bone. What does calcitonin do to bone?

Calcitonin decreases calcium reabsorption by renal thick ascending limb and distal tubule (opposes PTH effect on calcium handling). (renal actions of calcitonin are minuscule compared to those of PTH)

The *minor* functions of calcitonin are on kidneys. What does calcitonin do to kidneys?

(1) modulating *bone resorption* (2) influencing calcium reabsorption by *renal distal tubule* (3) regulating *calcitriol production* in kidney; (calcitriol regulates calcium and phosphate absorption by intestines)

What actions does PTH make to regulate plasma free Ca++? (3)

regulating the transport maximum (Tm) for phosphate reabsorption in renal tubules

What actions does PTH make to regulate plasma phosphate?

*acidemia* = excess H+ displaces calcium from albumin and inorganic ions plasma free ionized calcium concentration increases (hypercalcemia).

What are the effects of acidemia on free calcium?

*alkalosis* = calcium displaces H+ from albumin and inorganic ions plasma free ionized calcium concentration decreases (hypocalcemia)

What are the effects of alkalosis on free calcium?

• Low plasma albumin = increased ionized calcium • High plasma albumin = decreased ionized calcium.

What are the effects of low and high plasma albumin on free calcium?

Calcitriol acts to maintain adequate calcium levels for body functions. The main targets of calcitriol action are small intestines, bone, kidney and parathyroid gland.

What are the functions of calcitriol in the body?

regulate calcium availability (free/ionized)

What are the main functions of PTH/Calcitriol?

High affinity vitamin D receptors are expressed in plasma membrane and are coupled to G-proteins. Calcitriol exerts rapid non-genomic actions via the plasma membrane receptors in its target cells, for example, induction of rapid calcium absorption in the duodenum (a process known as transcaltachia).

What are the non-genomic actions of calcitriol? (receptor type)

50% ionized 40% albumin-bound 10% inorganic ion-bound (PO4, SO4, HCO3, citrate)

What are the proportions of calcium forms?

1) kidney 2) digestive system 3) bones

What body systems/organs are responsible for Ca:P balance?

Calcitonin acts primarily on the bone and kidney to reduce the serum concentrations of calcium and phosphate. The effects of calcitonin are best seen in young and growing animals.

What does calcitonin do?

(a) total phosphate in the body (b) the processes that regulate distribution of phosphate between bone and ICF and ECF compartments.

What does the maintenance of normal *phosphate* levels dependent upon?

osteoprotegerin (secreted by osteoblasts) The action of osteoclasts on bone resorption is terminated by a different protein produced by osteoblast. Not long after the activation of osteoclasts the osteoblasts secrete OPG (osteoprotegerin). OPG acts as a soluble decoy receptor of RANKL and reduces the stimulatory effect of RANKL on osteoclasts.

What is OPG?

Calcitonin is a 32-amino acid polypeptide secreted by "C-cells" (called parafollicular cells) in the thyroid gland. The C-cells are located in the interstitial space of the thyroid gland.

What is calcitonin and where is it secreted?

Intestinal calcium absorption proceeds independently of calcitriol (vitamin D). Excess calcium is excreted in urine of hind-gut fermenters (cloudy/chalky appearance) As such hypercalcemia is a key indicator of renal failure in hind-gut fermenters. (renal and bone calcium regulation remains the same as in other animals)

What is different about calcium handling in hindgut fermenters (horses, rabbits, etc.) What is hypercalcemia an indicator of?

Hypoparathyroidism may be result from malfunction of parathyroid gland (idiopathic causes) - low PTH levels - severe hypocalcemia - hyperphosphatemia

What is hypoparathyroidism and what are symptoms?

occurs in nursing animals that are calcium deficient and are unable to maintain normal calcium levels even with bone resorption; cycle of muscle contraction becomes more frequent

What is lactation tetany of animals?

the excessive and rapid removal of calcium from bones during early lactation

What is lactational osteoporosis?

hypocalcemia (parturient paresis)

What is milk fever?

PTH

What is more important, PTH or calcitonin?

calcitriol deficiency the osteoid in bones of *adult* animals are inadequately mineralized lameness and pelvic fracture may occur

What is osteomalacia?

calcitriol deficiency the osteoid in bones of *young* animal are inadequately mineralized

What is rickets?

Calcitriol inhibits its own production in the kidney

What is the action of calcitriol itself on the kidneys?

Calcitriol stimulates intestinal phosphate absorption. Calcitriol increases the expression of sodium/phosphate cotransporter (NPT2) in apical membrane of the enterocyte.

What is the action of calcitriol on phosphate?

calcitriol

What is the active form of Vit D3?

Caused by a disease of the parathyroid gland itself (e.g., a neoplasia of parathyroid gland secretes excessive PTH) - increase renal calcium reabsorption - decrease phosphate reabsorption manifests as = hypercalcemia and hypophosphatemia

What is the cause of *primary hyperparathyroidism*? What are symptoms? How does it manifest?

Caused by a disease of a different organ that has secondary effect on the parathyroid gland Ex 1 = a diseased kidney --> reduced GFR --> poor filtration leads to hyperphosphatemia --> excessive PTH secretion Ex 2 = a diseased kidney w/ reduced calcitriol production --> no negative feedback from calcitriol on PTH gland --> excessive PTH secretion

What is the cause of *secondary hyperparathyroidism*? Give two examples. What are symptoms? How does it manifest?

99% bones 1% ICF 0.1% ECF

What is the distribution of calcium in the body?

1. *increases calcium reabsorption* from distal tubules of nephrons and thick ascending limb (to a lesser extent). 2. *decreases phosphate reabsorption* by renal proximal tubule 3. *stimulates calcitriol production* in renal proximal tubules

What is the effect of PTH on kidneys?

Calcitriol enhances the expression of RANKL in osteoblasts; therefore calcitriol *amplifies the action of PTH* on bone resorption. Adequate levels of calcitriol are required for normal effect of PTH on bone resorption.

What is the effect of calcitriol in the bone?

Via expression of: (1) the apical calcium channel (*TRPV6 or TRPV5*) (2) the cytosolic calcium binding protein (*calbindin*) (3) calcium ATPase (*PMCA*) (4) Na+/Ca2+ exchanger (*NCX*) in basolateral plasma membrane

What is the effect of calcitriol in the kidney?

Calcitriol enhances the expression of proteins involved in transcellular calcium absorption in small intestine. The proteins include: (1) epithelial calcium channels (*TRPV5, TRPV6, etc*) in the apical membrane of enterocyte; (2) calcium binding protein (*calbindin*) in cytoplasm of enterocyte; (3) plasma membrane calcium ATPase (*PMCA*) and sodium/calcium exchanger (*NCX*) located in the basolateral membrane of enterocyte.

What is the effect of calcitriol in the small intestine?

PTH stimulates calcitriol production, calcitriol suppresses PTH synthesis.

What is the feedback relationship between PTH and calcitriol?

Calcitriol acts through its nuclear receptor (nuclear vitamin D receptor, VDR) that belongs to the superfamily of nuclear receptors (which include receptors for steroid-hormones, thyroid hormone, retinoic acid receptors). Calcitriol binds to specific DNA sequences (vitamin D response elements) in promoters of genes in target tissues to regulate their transcription. The nuclear vitamin D receptor forms a heterodimer with retinoid X receptor (RXR) as it regulates gene transcription.

What is the genomic action of calcitriol? (receptor type)

<5 min

What is the half-life of PTH in circulation?

1) Bones, cartilage, teeth, etc 2) second messenger in cells 3) blood clot formation 4) exocytosis/secretory processes 5) neural impulse transmission 6) muscle contraction 7) cofactor in many enzyme reactions

What is the importance of calcium?

1) bones 2) metabolic processes (intermediary metabolism and ATP formation) 3) muscle contraction 4) protein phosphorylation (second messenger) 5) plasma membrane phospholipid 6) nucleic acids and nucleotides 7) buffer cells, plasma, urine

What is the importance of inorganic phosphate?

lower calcium in the blood

What is the main function of calcitonin?

1) total amount of calcium in the body 2) distribution process between bone and ECF (PTH, calcitonin, calcitriol)

What is the maintenance of normal *calcium* levels dependent upon?

PTH stimulates osteoclast activity indirectly via activation of osteoblasts. Osteoclasts cause bone resorption. PTH stimulates osteoblasts to produce RANKL and M-CSF (monocyte colonystimulating factor). Osteoblast-derived RANKL stimulates osteoclast precursors to differentiate and fuse into multinucleated osteoclasts (osteoclastogenesis). M-CSF also cause osteoclastogenesis. RANKL also acutely activates differentiated osteoclasts to resorb bone

What is the overall effect of PTH on bone?

accretion

What is the process of bone formation?

osteolysis

What is the process of bone resorption?

chief/principal cells of the parathyroid gland

What is the source of PTH?

High plasma concentrations of free calcium (hypercalcemia) stimulate calcitonin secretion. By comparison, low calcium stimulates parathyroid hormone secretion.

When is calcitonin secreted in comparison to PTH?

when PTH activates the 1a-hydroxylase enzyme

When is the active form of calcitril produced?

when calcium is high in the body and activates the 24-hydroxylase enzyme

When is the inactive form of calcitriol produced?

1. renal tubular reabsorption increase 2. gastrointestinal absorption increase 3. bone calcium and phosphate resorption increase

When plasma calcium or phosphate concentrations decrease, (3)

Calcitonin has specific receptors (CTR) in plasma membranes of osteoclasts. The calcitonin receptor is coupled to G-protein and adenylate cyclase and phospholipase C.

Where are calcitonin receptors located and what type of receptor are they?

In plasma membranes of osteoblasts in bone and cells in renal proximal and distal kidney tubules. The PTH-receptor is coupled to G-protein and adenylate cyclase and phospholipase C.

Where is the PTH receptor found? What kind of receptor/signaling pathway is it?

HYPO

____________(hypo/hyper)phosphatemia stimulates calcitriol production in renal proximal tubules.

PTH and calcitriol calciotropic

____________________ are considered the most important hormones in the maintenance of blood calcium levels; they are called _____________ hormones

yes

can hypoparathyroidism occur with normal renal function?

In summary, estradiol and testosterone regulate bone metabolism. Both estradiol and testosterone act directly on the growth plate to cause its fusion. However, whereas estradiol acts to inhibit bone elongation directly, testosterone acts to increase bone elongation directly.

summarize the actions of estrogen and testosterone on bone metabolism

*High levels of calcitriol inhibit PTH secretion by suppressing transcription of PTH gene. Calcitriol stimulates the expression of calcium sensing receptor in chief cells.* The sensitivity of chief cells (in parathyroid gland) to ionized calcium is decreased in calcitriol deficient subjects; therefore normal calcium concentrations cannot suppress PTH secretion. The end result will be increased PTH secretion and increased bone resorption.

what is the effect of calcitriol on the parathyroid gland?

- cells of thick ascending limb of loop of Henle and distal tubules. - intestine - brain

In what other tissues is CaSR expressed?

PTH receptors are expressed in plasma membrane of osteoblasts (but not on osteoclasts).

ALL ACTION OF PTH IN BONE IS *INDIRECT* why?

1. skin 2. liver 3. kidney

Calcitriol production requires transformations of pro-vitamin precursors in: (3)

day-to-day

Calcitriol regulates ____ concentrations of free calcium.

No, it *indirectly* stimulates calcitriol production by increasing PTH, which activates 1-alpha hydroxylase in renal tubules

Does low calcium directly stimulate calcitriol production?

Calcitriol is necessary for action of PTH on osteoclastogenesis. However, too much calcitriol can inhibit PTH because it inhibits transcription of PTH gene.

Explain the relationship between calcitriol and PTH?

• PTH secretion inhibited. • Bone resorption is decreased • Calcium excretion by kidney increased. • Calcitriol formation is inhibited. • Inactive vitamin D is formed. • Intestinal calcium absorption is attenuated.

Hypercalcemia:

• PTH and calcitriol act to restore blood calcium to normal levels. • PTH increases calcium resorption from bones and decreases calcium excretion by the kidney • Calcitriol increases intestinal calcium absorption.

Hypocalcemia:

In females, estrogens regulate bone formation. Estradiol enhances osteoblast survival while it promotes apoptosis of osteoclasts. Also, estradiol decreases bone resorption. Estradiol stimulates growth hormone secretion by the anterior pituitary gland. Growth hormone via insulin-like growth factor-1 (IGF1) causes elongation (linear growth) of bones. After a period of time estradiol causes fusion of growth plate. Estradiol has direct actions on the growth plate where it inhibits chondroblast/chondrocyte proliferation and matrix synthesis.

In *females*, what are the effects of gonadal steroid hormones on *bone metabolism*?

In males, androgens (especially testosterone) increase bone formation (elongation) directly, - Testosterone suppresses bone resorption. - Testosterone enhances osteoblast survival but it promotes apoptosis of osteoclasts. Testosterone stimulates growth hormone secretion --> elongation of bones. Testosterone causes fusion of growth plate (via an estrogen-dependent process) as the male ages.

In *males*, what are the effects of gonadal steroid hormones on *bone metabolism*?

24-hydroxylase is activated & inactive vitamin D3 or 24,25(OH)2-D3 is formed

How does high calcium suppress calcitriol production in kidney?

The plasma membrane of chief cells possess receptors for extracellular calcium (calciumsensing receptors, CaSR) that are coupled to Gproteins. The calcium-sensing receptors are able to detect very small changes in plasma free calcium concentrations and transmit this information by modifying signaling pathways in the chief cell. The PTH gene expression and PTH synthesis as well as PTH secretion are regulated by the CaSR signaling pathway. *High plasma concentrations of free calcium suppress CaSR-mediated signaling pathways and PTH production and secretion is inhibited.* By comparison, CaSR-mediated signaling pathways are activated whenever plasma free calcium levels decrease and PTH production and secretion is increased.

How does the parathyroid gland sense and respond to plasma calcium?

prepro-PTH > cleaved to pro-PTH (ER) > processed to PTH (golgi/secretory vesicle)

How is PTH synthesized?

In basal layers of skin, the provitamin, 7-dehydrocholesterol is converted to vitamin D3 (cholecalciferol) by ultraviolet radiation. Vitamin D3 (cholecalciferol) must be activated through two hydroxylation processes in liver and kidney. Cholecalciferol is hydroxylated first in liver (by cholecalciferol 25-hydroxylase) to form 25-hydroxyvitamin D3 (25-hydroxycholecalciferol or calcifediol or calcidiol); this is followed by a second hydroxylation in the proximal tubule of kidney to form 1,25-dihydroxycholecalciferol (1,25 dihydroxyvitamin D3, [1,25(OH)2D3] or calcitriol) the most active form of vitamin D3. The enzyme that catalyzes the hydroxylation in kidney is 25-hydroxy-D3-1αhydroxylase (25OHD3-1αhydroxylase) or simply 1α-hydroxylase.

How is vitamin D (calcitriol) produced?

yes! yes!

Is there a direct effect of calcitriol (Vit. D) on PTH secretion? Is there a direct effect of PTH on calcitriol production?

hind-gut fermenters

Intestinal calcium absorption proceeds independently of calcitriol (vitamin D) in __________________ (such as horses and rabbits) regardless of the dietary content of calcium or demands of the body.

False

T/F. Calcitonin levels may reach a 0 level in the body.

1. hypocalcemia 2. hyperphosphatemia

The chief cells of parathyroid glands secrete PTH in response to:

calcitriol (active form of vitamin D)

The expression of PTH gene is also suppressed by ___________. Further, this up-regulates the expression of CaSR gene.

55-60% of plasma calcium (ion-bound + ionized)

What % of calcium is filtered by the renal glomerulus?

PTH, calcitonin, calcitriol

What 3 things regulate movement of calcium AND phosphate between bones and ECF?