Control of Mineral Metabolism Flashcards
(31 cards)
Calcium plays two important physiological roles.
- 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. - 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.
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.
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.
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.
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.
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.
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.
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 oflabile bone calcium.
i. The level necessary toproduce 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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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
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
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
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
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
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
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
