Calcium Regulatory Hormones

Calcium

There is a need for a constant Ca source because it is of major importance but cannot be stored, due to the impact it will have on the heart etc. 99% of the calcium in our body is in the bones and teeth. This means that if we don’t have dietary Ca, we take it from the skeletal system. We have a huge reservoir in the bones and teeth.

Provitamin Skin (7-dehydrocholesterol) Diet

Vitamin Vitamin D3 Vitamin D2

liver

Prohormone 25-hydroxyvitanin D3

Kidney Kidney and other tissue

Active hormone 1,25-dihydroxyvitamin D3 24,25-dihydroxyvitamin D3

Very little Ca is absorbed through the gut unless there is a strong signal from hormones. Rickets – children working in mines developed soft bones and became crippled. It was discovered that it was curable with codfish liver. Research started in earnest when monkeys imported form Africa and placed under canopy developed rickets as well.

Realization: there is something in the sunshine that helps form strong mineralized bones.

Osteoporosis – lose of bone, not losing calcium (which would be rickets).

Vitamin D is formed in skin as a result of UV radiation in a photochemical reaction. Also get vitamin D from diet, primarily milk. Major source of vitamin D3 is from exposure to light. Bedouin charred women, and old age home residents in Israel frequently suffer form vitamin D deficiency because of lifestyle and clothing style. In North America there is a vitamin D deficiency epidemic in the winters especially.

Vitamin D cannot be considered a hormone because hormones are characterized by complex feedback mechanism, more when necessary less when unnecessary. Vitamin D is not a hormone, it is converted to a prohormone, 25-hydroxyvitamin D3. this prohormone is carried by carrier molecules in blood stream. This step is not regulated, there is an enzyme responsible for conversion of Vit D to prohormone. It acts whenever Vit D is present. Regulatory step comes in to action in the conversion of the prohormone to the hormone.

Active hormone is Calcitriol (calcium and 3 hydroxyl on molecule), 1,25-dihydroxyvitamin D3 prohormone is sometimes called calcidiol. 24,25-dihydroxyvitamin D3 hormone is not active, it is more water soluble so it goes to catabolic pathway and leaves the system.

1. competition occurs between activating and deactivating enzymes working on prohormone.

2. but deactivating enzyme can also work in a degradative pathway by adding the other hydroxyl groups on position 24 of calcitriol.

Vitamin D

• Skin or Diet

• 25(OH0 vitamin D (liver, vitamin D 25-hyrozylase)

• 1,25 (OH)2 D – kidney, 35-hyrozyvitamin D 1 alpha hydroxylase

• 24,25 (OH)2 D and 1,24,25 (OH)3 D – kidney 25-hydroxyvitamin D, 24-hydroxylase.

Calcitriol

• Increases (directly); intestinal calcium and phosphate absorption, as well as renal calcium reabsorption (prevent loss).

  • Calcium and phosphate together because of hydroxyl-apatite gets Ca into bone and combine work on phosphate and Ca

Calcitriol may also:

• Encourage renal phosphate reabsorption

• Act directly on bones

• At high concentrations it may act on bone resumption and at low concentration it may act on bone formation. It has receptors but it is not clear exactly what is going on.

Absorption through intestine of calcium into blood stream, transporter (ECaC2) brings calcium in and then calcium is grabbed by Ca binding protein that hands calcium off to another transporter (PmCAlb). This system is affected by vitamin D.

There is calcium traffic that is not calcitriol mediated but most traffic is vitamin D mediated.

Hormone Receptors:

1. membrane receptors – responsive to peptide hormones

2. IC receptors – change transcription of target gene

Vitamin D enters into nucleus, and then calcitriol binds to VDR (vitamin D receptor). VDR and RXR, a receptor for 9-cis-retinoic acid, partner. RXR is a partner for many nuclear receptors, including thyroid receptor. RXR and VDR together bind to promoter of target gene of vitamin D. VDRE, vitamin D response element, VDR and another nuclear receptor form a heterodimmer. The heterodimmer starts transcription of target gene, calcitriol works on transcriptional level. It takes time for protein to begin to be synthesized and a while to disappear. It is a slow and heavy process going on and off.

VDRE: consensus (statistical creature of what base is most frequent found in each position) G/AGGTCA 3’ G/ACTTCA

GGGTGA 3’ GGGGCA Human osteocacin

GGTTCA 3’ GGTTCA Mouse osterocalcin

AGGTCA 3’ GGTGTG Human 24-hydroxylase

AGGGAG 3’ GGTTCA Human p21

Part of the VDR that binds to VDRE, the DNA binding domain is very similar to steroid receptors of other hormones, all have zinc fingers receptors. This patter is common to many hormones. Zinc finger receptor is the hallmark of all nuclear receptors. There are a few sensitive locations, where damage/mutation causes dysfunction of VDRE.

Slow action of calcitriol – need additional measures for quick response as well as a regulatory mechanism for calcitriol. Parathyroid hormone formed in 4 small glands around thyroid gland. These glands for polypeptide hormone which is highly conserved through evolution, most action is exerted by a specific 34 AA sequence in the hormone. Parathyroid hormone is transcribed and translated in the nucleus, then processed in the ER and GA. Prohormone is converted to the hormone and secreted in secretory granules. Concentration increase in circulation very quickly upon signal, polypeptide hormone binds to cell membrane receptors, this fast response takes only minutes.

Parathyroid gland functions as the “eyes” to see [Ca] in blood stream, PT hormone is secreted into circulation at all times. When [Ca] decreases, PT increases, and vice-versa. The PT gland has to sense PTH (parathyroid hormone) in circulation PTG must have receptors to sense PTH.

Calcium receptor has seven trasmembrane loops coupled to G-protein, thus work through adenyl cyclase.

High EC [Ca]: increases phosholypase C (PLC), decreases AC, thus PTH is low.

Low EC [Ca]: increases cAMP, decrease PLC, Ca so increases PTH.

EC Ca conditions impact IC Ca concentrations. Many EC Ca receptors occupied --- low PTH. Few EC Ca receptors occupied --- high PTH.

PTH receptor is mediated by increase of cAMP. PTHR is also coupled to PLC. PTHR can activate both cAMP and PTH. cAMP is very important for PTHR action.

PTH

Increases: to make sure all Ca in diet gets absorbed

• Kidney, 25-hydoroxyviatmin D dihydroxylase (necessary for calcitriol production), takes Ca from bone

• Bone resorption (increase concentration) Ca reservoir

• Bone formation? (intermittent)

Inhibits:

• Renal phosphate re-absorption, so no Ca gets into bone

How can PTH both do bone resorption and formation?

Osteoblast = bone formation

Osteoclast = bone resorber

PTH receptors should be found on osteoclasts since PTH is resorber but it is not there. PTHR is found on osteoblasts only! So PTH impacts cross-talk between osteoclasts and osteoblasts. Osteoclast receptors for RANK turns preosteoclast to osteoclast. RNAK ligand activates RANK. RANK is sent from osteoblasts to osteoclast. Osteoblast also secretes OPG, osteoprotegrin (protecting bone), which is a decoy receptor. OPG can bind RANK L so RANK L cannot bind RANK this increase concentration of PTH: [OPG] decreases while RANK L and RANK concentration is not impacted this causes indirect osteoclast activation. Decreased [PTH]: direct rearmament of preosteoblast to osteoblast, which is bone building.

Interplay – perturbation – inject calcium binder, therefore the free calcium decrease. PTH increases in close conjunction with decrease in free calcium, urinary cAMP increases also. PTH acts on kidney to produce 1 alpha hydroxylase and to inhibit phosphate re-absorption. cAMP increase in target cells as a result some cAMP leacks into the urine. cAMP in urine is an indicator that PTH has acted, if it is not present it is an indicator that PTH has not been able to do its job. Increase in serum 1,25 (OH)2 D3 occurs too, but this increase continues after serum Ca level had dropped. This is because of the slow action of 1 alpha hydroxylase. Serum phosphate decreases and comes back to normal along with calcium so we are now ready to put calcium phosphate into the bone.

PTH secretion and synthesis:

Stimulated by: low serum calcium

Inhibited by: high serum calcium and calcitriol

Calcitriol inhibits PTH secretion and synthesis. But PTH with not decrease if there is no calcium entry. Synergistic interaction: when ca increase to normal levels, Ca acts synergistically with calcitriol to decrease PTH. Calcitriol cannot lower PTH levels alone.

Liver, Vitamin D 25-hydrolyase – is not regulation, rate is determined by substrate availability.

Kidney, 25-hydroxyvitamin D 1 alpha hydroxylases – is stimulated by PTH and low serum phosphate, and inhibited by calcitriol.

Kidney 25 hydroxyvitamin D 24 hydroxylase – is stimulated by calcitriol and inhibited by PTH and low phosphate serum levels.

Calcitonin: is secreted from the thyroid gland (secretion increases in response to high serum calcium), it acts through G proteins coupled receptors coupled to AC and PLC. The action of calcitonin is related to increased cAMP levels, and it inhibits osteoclast function. Someone without a thyroid gland has no dramatic bone problems even-though no calcitonin. So calcitonin can act as a therapeutic measure. Until recently, salmon calcitonin was injected as therapy.

Normal Response:

Plasma Ca increase ---- PTH secretion increase ---- plasma phosphate decrease --- bone resorption increase ---- calcitriol increase --- intestinal calcium absorption increase --- plasma calcium increase --- PTH secretion decrease --- calcitriol decrease and 24,25 (OH)2 D increase.

• in bold used to increase PTH section

parathyroid PTH

Levels between 9 and 10

Bone resorption does not equal reabsorption

But both bring Ca into serum.

Disorders:

• Primary hyperparathyroidism – adenoma in PTG secretes PTH without feedback

• Secondary Hyperparathyroidism

• Vitamin D deficiency

• Renal failure

• 1 alpha hydroxylase deficiency

• End organ resistance to calcitriol

Primary hyperparathyroidism –

In serum: increased ca, decreased phosphate, increased calcitriol, 25 (OH) D is normal, cAMP increased in urine and PTH increased.

Kidney stones because there is too much CA in system. Ca is 10% above normal, PTH levels are normal. This means that PTH levels should have been very low since Ca is high. Normal PTH levels when calcium is high is not normal; in this case, PTH should be low. Remember specific situation when looking at lab results!

Disorders I

• Primary hyperparathyroidism

• Secondary hyperparathyroidism

• Vitamin D deficiency

• Renal failure

• 1 alpha hydroxylase deficiency

• end organ resistance to calcitriol

From outside we can access

Serum Urine

Calcium cAMP

PTH(parathyroid Hormone)

Calcitriol

25 (OH) D (vitamin D)

Phosphate

Defects are known in every part of this system.

Primary hyperparathyroidism is too much parathyroid hormone when there is no need. It is usually due to adenoma, PTH gland so it loses sensitivity to PTH. Extreme cases exist. Moderate cases are harder to diagnosis. Many years of primary hyperthyroidism will create bone problems in patient. Not lethal but it will decrease patient’s quality of life. Expect low PTH in serum test, see normal, it’s a warning.

Secondary hyperparathyroidism (calcium deficiency) – occurs when there is high PTH in the blood. Sensors in PTH gland feel that there is not enough calcium in the system. High PTH in bloodstream means system is taking calcium out of bones. Long term, this is bad for you because of skeletal deterioration. How can we be sure PTH levels is high? Not easy to tell, PTH has a big range so it has to say if it is normal level for that individual. Elderly especially are at a bigger risk of malnourishment, lack of calcium. Check PTH and then treat if it is not perfectly normal. If PTH levels then decrease, we can say he probably had secondary hyperparathyroidism. If it doesn’t decrease, they probably didn’t have secondary hyperparathyroidism.

Vitamin D deficiency

Severe situations present with:

• calcium decrease, no absorption through intestine

• increase in PTH

• decrease in calcitriol

• 25 (OH) D- this is the best sign, as long as there is substrate this should produce vitamin D, it goes down.

• Phosphate decrease because PTH needs to get calcium into serum not bone so it will want to get phosphate out of the system into urine. Calcitriol wants to get things into bone.

• cAMP (in urine)- signifies action of PTH so it will increase

How will you treat someone with vitamin D deficiency?

• Sit in sun

• Give them vitamin D in pills or drops; unless they have liver problems where they can’t convert vitamin D to 25 (OH)D, in which case give them 25 (OH)D.

Renal Failure

Kidney will no synthesize calcitriol

Signs

• Calcitriol decreases

• PTH increases

• Calcium decreases

• 25 (OH)D no change (substrate is present and conversion occurs in the liver)

• Phosphate not informative because the effect of PTH on phosphate occurs through the kidneys

• cAMP not measurable

We want to get PTH down so bones don’t breakdown.

Treatment

• Bypass problem by giving patient calcitriol. This is treatment of choice in the US.

• Elsewhere, 1 alpha D₃ is given . Calcitriol has a very short plasma half-life, so patient has to have a strict regiment to keep levels stable. 1 alpha D₃ has hydroxyl at 1 alpha position doesn’t have OH yet. It is put on by the liver. So 1 alpha D₃ is inactive, and liver slowly adds on OH on the 25 position creating slow release of active drug into blood stream.

1 alpha hydroxylase deficiency

this is a genetic disease, mutation in 1 hydroxylase enzyme. Everything else is fine. You will get this syndrome in infants. Baby will come with rickets, soft bones, heart and skeletal problems.

Expect:

Serum Urine

Calcium decrease cAMP increased

PTH increase

Calcitriol very low

25 (OH) D normal

Phosphate decrease

Classical pattern:

Treatment: give patient calcitriol (can’t give 1 alpha hydroxylase, can’t give enzymes)

End organ resistance to calcitriol

Defective receptor to calcitriol

Effects

• Calcium decreased because no calcitriol receptor to calcitriol

• PTH increased

• Calcitriol really increased system does what it can.

• 25 (OH) D normal or slight decrease

• phosphate decreased

• cAMP increased

very calcitriol levels but no calcium absorption

Treatment: give calcium directly bloodstream to circumvent everything on the way. Using a pump, each night put calcium into bloodstream to supply calcium for skeletal health and growth.

Disorders III (skip Disorder II)

Extra renal production of calcitriol (skip)

Circulating PTH related to peptide:

Hypercalcemia of malignancy happens in breast, prostate, intestine cancers. Happened even in absence of overt bone metassisis. Signs high calcium and bone reabsorption.

Serum

• high calcium

• very low PTH

• somewhat high calcitriol

• normal 25 (OH) D

• very low phosphate

• high cAMP

This picture presents as high PTH, i.e. low phosphate and high cAMP, but PTH is very high. Something is binding to PTH receptors and approximating PTH action.

Result of cultures of tumor supernatant, protein with a high PTH related peptide, a high homology with PTH. However, PTHrp was also found in cow milk, human milk, etc. So this is a normal program, just improper synthesis. PTHrp is important hormone, found many places, not just lactation, also in placenta, bone, skin. PTHrp is an example of a hormone discovered through malignancy. It is a paracrine hormone. Level in circulation is zero, normally. PTH is its “brother” hormone, an endocrine hormone. Sometimes PTHrp will be the first diagnosis of malignancy.

Category: Biochemistry Notes , Physiology Notes