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Flashcards in Control of Mineral Metabolism Deck (31):
1

Calcium plays two important physiological roles.

1. The first is a structural role, since it is a major constituent of the mineral matrix of bone.
i. Bone calcium also serves as a reservoir for maintenance of plasma calcium.

2. The second is a biochemical role.
i. Calcium is an essential regulator of excitation-contraction coupling, stimulus-secretion coupling, blood clotting, membrane excitability, cellular permeability and other metabolic functions.

2

Balance of Calcium

a. The metabolic functions require that the plasma calcium be maintained within the narrow limits of 8-10 mg/dl (~2.5 mM).

b. Plasma phosphate is maintained between 3 and 4 mg/dl (~1.1 mM).
i. The structural functions require that overall calcium and phosphate balance be maintained within the body.

c. IN = OUT, and BALANCE is achieved, as long as the distribution of calcium in three compartments is maintained.
i. There is in addition, a fast exchange of up to 20 g/day between the ECF and labile bone, mediated by osteocytes.

3

Calcium Compartmentalization of the Body

a. Calcium is found within three major compartments:

1. Bone-99% of body calcium in the form of hydroxyapatite.

2. Intracellular compartment-contains a total of 10 g of calcium. Free cytosolic calcium is ~50-100 nM in resting cells. This level of cytosolic calcium is maintained by intracellular mobile calcium buffers, compartmentalization into ER calcium stores, by an ATP linked calcium pump and a Na/Ca antiporter.

3. Extracellular fluid-includes blood and interstitial spaces that are in equilibrium. Total concentration, 2.5 mM, ~1/2 is free, and filterable by the kidney, 10% as salts (bicarbonate and phosphate) which can also be filtered by the kidney and remainder bound to albumin.

b. The kidney filters about 10g of calcium/day (60mg/l x 170L/day).
i. About 98% of this is reabsorbed.

c. Homeostasis refers to the minute-to-minute maintenance of free calcium levels.
i. Free calcium levels are a regulated variable.

4

Phosphate

a. Phosphate also plays a structural role, since it is part of the mineral matrix of bone.
i. In addition, it is a common intracellular buffer.

b. It is required for phosphorylation reactions, which transfer energy from one compound to another as well as regulate cellular functions.

c. About 85% of serum phosphate is free in the ionized active form (as HPO4-2 and H2 PO4-2). Normal serum phosphate levels are about 3-4 mg/dl.

d. Like calcium, there is a net phosphate balance in the body between the various compartments.

5

Parathyroid Hormone

Large Summary

a. PTH is produced by the parathyroid gland.
i. It contains 84 amino acids but only the first 34 are required for activity.
ii. It is synthesized as a larger pre-prohormone of 115 amino acids.
iii. The first 25 are cleaved in the ER and another 6 are cleaved in the Golgi.


b. PTH leads to increased plasma calcium by several means:
1) BONE:
i. Rapid effect-increased efflux of labile bone calcium, not accompanied by phosphate
ii. Slow effect-increased bone remodeling, releases both calcium and phosphate (seen mainly in pathological conditions)

2) KIDNEY:
i. Increased calcium reabsorption in distal tubule
Decreased phosphate reabsorption
ii. Increased synthesis of 1,25 (OH)2 Vitamin D

3) GI TRACT:
i. Indirect via Vitamin D, which enhances calcium absorption-requires 1 day.

c. N.B. PTH thus has complex effects on calcium and phosphate levels in the blood.
i. The net consequence of PTH action is to increase serum calcium and decrease serum phosphate levels.

d. Regulation of PTH secretion:
i. PTH secretion is stimulated by a fall in the free ionized calcium in the plasma and inhibited by a rise. Since it acts to increase plasma calcium, this constitutes a classical negative feedback loop with plasma calcium as the regulated variable and the parathyroid gland as the sensor.

6

PTH leads to increased plasma calcium by several means:

1) BONE:
i. Rapid effect-increased efflux of labile bone calcium, not accompanied by phosphate
ii. Slow effect-increased bone remodeling, releases both calcium and phosphate (seen mainly in pathological conditions)

2) KIDNEY:
i. Increased calcium reabsorption in distal tubule
Decreased phosphate reabsorption
ii. Increased synthesis of 1,25 (OH)2 Vitamin D

3) GI TRACT:
i. Indirect via Vitamin D, which enhances calcium absorption-requires 1 day.

7

Regulation of PTH secretion:

a. i. PTH secretion is stimulated by a fall in the free ionized calcium in the plasma and inhibited by a rise.

b. Since it acts to increase plasma calcium, this constitutes a classical negative feedback loop with plasma calcium as the regulated variable and the parathyroid gland as the sensor.

8

Calcitonin

a. Calcitonin is produced by parafollicular or C cells of the thyroid. It is a 32 amino acid peptide.
i. It is secreted in response to elevated calcium as well as certain GI hormones such as gastrin, cholecystokinin, secretin and glucagon.

b. Calcitonin acts on bone to decrease efflux of labile bone calcium.
i. The level necessary to produce this effect is rather high and many have questioned its role in maintaining normal calcium balance.
ii. However, it is useful in therapeutically in slowing down high turnover bone disorders.

9

Synthesis and Secretion of Vitamin D

a. 7-dehydrocholesterol in skin is acted on by sunlight to produce Vitamin D (biologically inert).

b. In the liver, one hydroxyl group is added to form 25-OH Vitamin D.

c. In the kidney, a second hydroxyl is added in a reaction catalyzed by 1-hydroxylase yielding 1,25 (OH)2 Vitamin D, the most active form. The kidney also has 24-hydroxylase activity which leads to the production of 24,25 (OH)2 Vitamin D which is inactive.

d. 1,25 (OH)2 Vitamin D is mostly transported in the blood bound to transcalciferin.

10

Actions of Vitamin D

a. The major action of 1,25 (OH)2 Vitamin D is on the GI tract where it eventually interacts with a nuclear receptor increasing the synthesis of specific mRNAs and then proteins.
i. One of these proteins is a calcium binding protein that appears in the lumen of the intestine.
ii. It is not clear exactly how both calcium and phosphate transport are promoted, but it is possible that pinocytosis of this binding protein is involved.

b. 1,25 (OH)2 Vitamin D mobilizes bone in a way similar to PTH, possibly simply by sensitizing the bone to PTH action.
i. In recovery from chronic Vitamin D deficient states (e.g., rickets) however, the improvement in plasma calcium brought about by increased calcium and phosphate absorption leads to increased calcium deposition in bone.

c. The physiologic significance of the mobilization of bone calcium by 1,25 (OH)2 Vitamin D is not at all clear.

11

Regulation of 1,25 (OH)2 Vitamin D synthesis:

Several factors converge on the renal hydroxylase reactions to influence the synthesis of 1,25 (OH)2.

a. 1,25 (OH)2 Vitamin D negatively affects the 1-hydroxylase; in this was, 1,25 (OH)2 Vitamin D acts in a negative feedback loop and regulates its own synthesis.

b. Often, the two hydroxylase enzymes of the kidney, 1-hydroxylase and 24-hydroxylase, are reciprocally regulated by influencing factors.
i. For example, increased levels of PTH positively and negatively affect the activities of 1-hydroxylase and 24-hydroxylase, respectively.

c. High PTH will lead to increased levels of 1,25 (OH)2 Vitamin D, which then acts on the GI tract to increase calcium absorption. In this way, PTH and 1,25 (OH)2 Vitamin D act synergistically. In contrast, decreased levels of phosphate positively and negatively affect the activities of 1-hydroxylase and 24-hydroxylase, respectively.

d. Thus, if plasma phosphate falls, 1,25 (OH)2 Vitamin D synthesis is increased; in turn, the increased levels of 1,25 (OH)2 Vitamin D will act on the GI tract and promote increased phosphate absorption and return of plasma phosphate levels to normal.

e. Cyclic AMP is thought to be involved in mediating influences of PTH and phosphate on 1-hydroxylase and 24-hydroxylase activities.

12

Short term regulation of blood calcium

a. The short term (minute-to-minute) regulation of blood calcium (homeostasis) is carried out primarily by PTH acting to mobilize calcium into the plasma when levels begin to become low.

b. Calcitonin may be useful in increasing the rate of storage of an acute calcium load. PTH will mobilize calcium movement from the bone compartment into the blood.

c. If this is continued for a prolonged time, calcium balance is affected negatively.

13

Long Term Regulation of Calcium Balance

a. Vitamin D is the important hormone in the long-term regulation of body calcium and phosphate stores by regulating the intestinal absorption of these minerals.

b. A simple way to consider this is if IN < OUT, one way to achieve balance is to increase "IN" by increasing the amount of calcium that can be absorbed from the GI tract.

14

Hyperparathyroidism:

a. In primary hyperparathyroidism, increased PTH increases calcium levels in plasma and urine (leading to renal stones).

b. The effects of hypercalcemia include vague or overt symptoms of muscle weakness, depression and GI disorders.

c. In severe cases, bone demineralization leads to bone pain and fractures.

d. Secondary hyperparathyroidism can result from any disorder where plasma calcium is low such as rickets and renal failure.

15

Hypoparathyroidism:

a. The major symptom of the decreased plasma calcium caused by a lack of PTH is increased neuromuscular excitability that can cause muscle cramps, seizures as well as mental changes.

b. One test for hypoparathyroidism is to tap the facial nerve, which evokes facial muscle spasms, called Chvostek’s sign).

c. There is a PTH dependent decrease in calcitriol levels causing decreased serum calcium levels due to less absorption from intestines and less reabsorption through the kidneys.

d. Bone demineralization is usually not a problem because of increased serum phosphate (due to increased reabsorption of phosphate from kidneys).
i. It is usually treated with Vitamin D and calcium supplements.

16

Vitamin D deficiencies:

a. This condition is called rickets in children and osteomalacia in adults.

b. Rickets can lead to severe skeletal deformities whereas symptoms of osteomalacia can include bone pain and pathological fractures.

c. Dietary lack of Vitamin D is rare in the USA because of supplementation of milk. It can be seen in certain liver diseases and renal dysfunction, both of which interfere with synthesis of the active hormone.

17

Calcium Important Function

Calcium is important for:

a. Bone structure

b. Signalling as a 2nd messenger in cellular pathways
-kinases and other proteins

c. Synaptic signalling- need Ca2+
i. low calcium leads to Hyperexcitable state---> tetany and seizures
-low calcium makes the outer surface of the membrane more negative, easier to depolarize (
ii. High calcium, will see a decrease excitability

18

Important Function of Phosphate (PO4 3-)

a. Important for bone structure (phosphate and calcium are both important)

b. High energy compounds rely on phosphate

c. Membrane phospholipids

d. Regulation of enzymes (can turn enzymes on and off)

e. Phosphates are needed as backbone for DNA and RNA

19

Regulation of Calcium

a. Will obtain calcium from diet, will enter the gut and go into the serum
i. We get about one gram of calcium from our diet
ii. We absorb calcium somewhat poorly, a good portion is lost to feces

b. In the blood, about 50% of calcium is free, 40% is bound to Albumin and other proteins, the remaining 10% of calcium exists as a salt--> Ca bound to phosphate and bicarbonate


c. The Ca2+ will enter different compartments (serum, intracellular, bone, and kidney)
i. There is exchange of calcium between the serum and the bone (these two compartments are exchanging)
ii. Exchange of Calcium between bone and serum. Several different cells regulate this process
1. Surface Osteoblasts
2. Osteocytes
3. Osteoclasts

d. Serum calcium is roughly 0.6, will get taken up from the blood into bone by Osteocytes (calcium will enter the Canaliculi fluid of the bone before getting taken up by Osteocytes)
i. Calcium will get pumped out into blood by the Osteoblasts
ii. Osteocytic Osteolysis--> Transfer of 10g/day of calcum between bone and blood

e. Osteoclasts will break down bone, chew up matrix and create Calcium and Phosphate
i. Will release both calcium and phosphate

f. Kidney and Calcium regulation
i. roughly 10 grams of Calcium go through the Kidney, nearly 98% is reabsorbed
ii. only small amount (100-200 mg) is excreted

20

The composition of calcium in blood

In the blood, about 50% of calcium is free, 40% is bound to Albumin and other proteins, the remaining 10% of calcium exists as a salt--> Ca bound to phosphate and bicarbonate

21

Bone and Calcium exchange

a. There is exchange of calcium between the serum and the bone (these two compartments are exchanging)

b.Exchange of Calcium between bone and serum. Several different cells regulate this process
1. Surface Osteoblasts
2. Osteocytes
3. Osteoclasts

c. Serum calcium is roughly 0.6, will get taken up from the blood into bone by Osteocytes (calcium will enter the Canaliculi fluid of the bone before getting taken up by Osteocytes)
i. Calcium will get pumped out into blood by the Osteoblasts
ii. Osteocytic Osteolysis--> Transfer of 10g/day of calcum between bone and blood

d. Osteoclasts will break down bone, chew up matrix and create Calcium and Phosphate
i. Will release both calcium and phosphate

22

Regulation of Calcium Metabolism

Wiki

a. The plasma ionized calcium concentration is regulated to within very narrow limits (1.3–1.5 mmol/L), despite being the central hub through which calcium is moved from one body compartment to the other

b. This is achieved by both the parafollicular cells of the thyroid gland, and the parathyroid glands constantly sensing (i.e. measuring) the concentration of calcium ions in the blood flowing through them.

c. When the Ca concentration rises the parafollicular cells of the thyroid gland increase their secretion of calcitonin (a proteinaceous hormone) into the blood.
i. At the same time the parathyroid glands reduce their rate of parathyroid hormone (or PTH, also a proteinaceous hormone) secretion into the blood.

d. The resulting high levels of calcitonin in the blood stimulate the skeleton to remove calcium from the blood plasma, and deposit it as bone.

e. The reduced levels of PTH inhibit removal of calcium from the skeleton.
i. . The low levels of PTH have several other effects: they increase the loss of calcium in the urine, but more importantly inhibit the loss of phosphate ions via that route.

f. Phosphate ions will therefore be retained in the plasma where they form insoluble salts with calcium ions, thereby removing them from the ionized calcium pool in the blood.

g. The low levels of PTH also inhibit the formation of calcitriol (1,25 dihydroxyvitamin D3) from cholecalciferol (vitamin D3) by the kidneys.

h. The reduction in the blood calcitriol concentration acts (comparatively slowly) on the epithelial cells (enterocytes) of the duodenum inhibiting their ability to absorb calcium from the intestinal contents
i. The low calcitriol levels also act on bone causing the osteoclasts to release less calcium ions into the blood plasma

23

When Calcium levels are low (Wiki explaining regulation)

a. When the plasma ionized calcium level is low or falls the opposite happens. Calcitonin secretion is inhibited and PTH secretion is stimulated, resulting in calcium being removed from bone to rapidly correct the plasma calcium level.

b. The high plasma PTH levels inhibit calcium loss via the urine while stimulating the excretion of phosphate ions via that route.
i. They also stimulate the kidneys to manufacture calcitrol (a steroid hormone), which enhances the ability of the cells lining the gut to absorb calcium from the intestinal contents into the blood, by stimulating the production of calbindin in these cells.

c. The PTH stimulated production of calcitriol also causes calcium to be released from bone into the blood, by the release of RANKL (a cytokine, or local hormone) from the osteoblasts which increases the bone resorptive activity by the osteoclasts.
i. These are, however, a relatively slow processes

24

Phosphate Metabolism

a. Body is more efficient at regulating phosphate than calcium

b. In the bone, phosphate is released by osteoclast activity

c. In the Kidney, it will retake the majority of the phosphate

25

Parathyroid Hormone (PTH)

a. PTH-A peptide hormone that is created by chief cells in the parathyroid gland

b. Low serum Calcium will trigger the release of PTH
i. Chief cells have a Calcium Sensor that is a transmembrane protein receptor; Calcium is normally bound to these receptors
ii. low calcium means less Ca bound to the Transmembrane Receptor----> will act through a Gq protein to increase IP3
iii. High IP3 will cause release of vessicles filled with PTH

c. PTH will increase serum calcium back to its homeostatic levels
1) PTH will increase Osteoclast activity
i. happens through osteoblasts
2) PTH will increase the Osteocytic Osteolysis activity
3) PTH will increase Ca absorption at the kidney
4) PTH will have indirect effect of increasing Vitamin D3 (and calcium absorption at gut)

d. However, Ca and Phosphate both increase, will have just a very insoluble salt together
i. PTH will also increase the resorption of phosphate from the bone, creating the salt product with calcium
ii. PTH at the Kidney, phosphate will be Secreted at a higher rate
*Critical to understand that this allows more Ca to be absorbed, without salt creation from the kidney

26

Low serum Calcium will trigger the release of PTH

What is the cellular mechanism?

1. Chief cells have a Calcium Sensor that is a transmembrane protein receptor; Calcium is normally bound to these receptors

2. low calcium means less Ca bound to the Transmembrane Receptor----> will act through a Gq protein to increase IP3

3. High IP3 will cause release of vessicles filled with PTH

27

Parathyroid Hormone at Bone and Kidney

a. PTH will increase serum calcium back to its homeostatic levels
1) PTH will increase Osteoclast activity
i. happens through osteoblasts
2) PTH will increase the Osteocytic Osteolysis activity
3) PTH will increase Ca absorption at the kidney
4) PTH will have indirect effect of increasing Vitamin D3 (and calcium absorption at gut)

b. However, Ca and Phosphate both increase, will have just a very insoluble salt together
i. PTH will also increase the resorption of phosphate from the bone, creating the salt product with calcium
ii. PTH at the Kidney, phosphate will be Secreted at a higher rate

c. Critical to understand that Kidney PTH activity allows more Ca to be absorbed, without salt creation from the phosphate

28

Vitamin D3

a. Vitamin D3 is by itself inactive, needs to get two hyroxylations to become active
i. At the Liver, will get its first Hydroxylation (25OH Vit D)
ii. Then goes to the Kidney for the 2nd hydroxylation, will become the active form 1,25 OH Vitamin D

b. All these hyroxylation enzymes at the liver and kidney are regulated by Parathyroid hormone

c. Main purpose of active Vit D is for Calcium absorption at the gut
i. Vit D increases synthesis of Calbindin, allows Ca absorption at gut

*Cortisol will antagonize the actions of Vitamin D

29

How Cortiosl hurts bone absorption

a. Main purpose of active Vit D is for Calcium absorption at the gut
i. Vit D increases synthesis of Calbindin, allows Ca absorption at gut

b. Cortisol will antagonize the actions of Vitamin D

30

Hyper and Hypo Parathyroid Hormone Conditions

a. Hyperparathyroid Function
1. Tumors that release PTH
i. will see high serum calcium, get hypercalcemia.
ii. will see patient being sluggish, low mm tone
iii. Major increased risk for phosphate or Ca stones

b. Hypoparathyroid Function
1. Most often happens from surgery errors
i. will have low PTH--> low serum calcium
ii. increased risk for siezurs and high cell excitability, due to lower Ca2+ meaning cell membrane surface is more negative
2. Vitamin D3 deficiency can lead to calcium deficiency
i. can get rickets

31

Three hormones that control Calcium levels:

Good Summary (long)

The 3 hormones are Parathyroid Hormone (PTH), 1,25 dihydroxyvitamin D (1,25 (OH)2 Vitamin D), and Calcitonin; the 3 organ systems are the skeleton, the kidneys and the intestines.

1. PTH is an 84 amino acid peptide produced by the parathyroid glands.
i. It raises serum calcium levels by direct actions on the skeleton and kidneys and indirect actions on the intestines.
ii. PTH directly stimulates osteoclastic bone resorption, which releases ionized calcium and phosphate from the skeleton into the extracellular fluid.
iii. PTH directly inhibits renal calcium excretion but promotes renal phosphate excretion.
iv. And PTH stimulates conversion of 25 OH Vitamin D into 1,25 (OH)2 Vitamin by renal 1 alpha hydroxylase and thereby indirectly stimulates intestinal calcium and phosphorus absorption.

2. 1,25 (OH)2 Vitamin D raises serum calcium levels by directly enhancing intestinal calcium and phosphorus absorption and by promoting osteoclastic bone resorption, an effect that occurs mainly at high 1,25 (OH)2 Vitamin D levels.

3. Calcitonin is a 32 amino acid peptide produced by the parafollicular c-cells in the thyroid gland.
i. It lowers serum calcium by inhibiting osteoclastic bone resorption.