ENDOCRINE SYSTEM - Parathyroid Glands
Plasma
(0.1%) 2.3-2.6mM 50% free ionised Ca 45% bound to protein (mainly albumin) 5% chelated to di-carboxylic acids eg citrate/lactate
Pre-proparathyroid hormone
115 residues which consist of 4 sections - signal sequence, pro sequence, biologically active sequence and C terminal fragment sequence Signalling sequence allows proteins to be secreted into ER Prohormone then gets cleaved and looses bothe the signal and pro sequece and then put into secretory vesicles as PTH. The N-terminal is the active part of PTH, C-terminal is inactive When released into circulation, some of hormone is cleaved at the N terminal which releases a more active form. If move C terminal off it gets broken down more quickly in blood. Keep 88aa hormone and has good level of activity which is prolonged. If cleave the activity jumps but it is cleaved again within minutes. Can vary activation of hormone
Optimal plasma Ca levels
8.5-10.6 mg/dl
Prosthetic group
A non-protein, but organic, molecule (such as vitamin) that is covalently bound to an enzyme as part of the active site.
PTH and Vit D
Act on osteoblsats to cause them to release a number of factors M-CSF, growth factors, IL-6, RANK ligand
Intestinal absorption of phosphate
Although due to intralumenal H+ and Pi concentrations, Pi probably can be absorbed as H2PO4- by passive means in the duodenum, transepithelial transport of HPO42- requires uptake from the lumen by an active transport system. The latter has been identified in many species as a Na(+)-Pi cotransport system at the brush-border membrane of the enterocyte. There is agreement that the transporter accepts H2PO4- and HPO42- alike. Movement of Pi from the cytosol across the basolateral membrane into the interstitial space has only been preliminarily characterized as transfer by facilitated diffusion. Na(+)-Pi cotransport across the brush-border membrane is under control by the active vitamin D metabolite, 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3). The sterol increases the intrinsic activity, i.e. the number and/or mobility of Na(+)-Pi carriers.
Osteoclast
Break down bone/resorb cells They are large, multinucleate cells that form through the fusion of precursor cells. Unlike osteoblasts, which are related to fibroblasts and other connective tissue cells, osteoclasts are descended from stem cells in the bone marrow that also give rise to monocytes (lymphocytes).
Calcium homeostasis components
Calcium homeostasis is a complex process involving the following 4 key components: serum calcium, serum phosphate, 1,25-dihydroxyvitamin D-3, and parathyroid hormone (PTH).
Regulation of PTH secretion
Chief cells have receptors on surface which are sensitive to ECF [Ca]. When Ca there, it binds to external site of receptor (Ca sensing receptor) which activates G protein and produces secondary messengers DAG (which produces PKC) and IP3. IP3 releases Ca from ER PKC phosphorylates a number of proteins In Chief cells, when there is a raise in intracellular Ca or activation of PKC, it inhibits PTH synthesis and release. This is opposite to other cells. If raise Ca concentration, inhibit exocytosis and suppresses PTH release If decrease Ca, signalling system slows down so get release of PTH
Negative feedback
Feedback inhibition of parathyroid hormone (PTH) release occurs primarily by direct effect of calcium at the level of the parathyroid gland. Although not well elucidated, 1,25-(OH)2 D3 appears to exert a mild inhibitory effect on the parathyroid gland as well.
Mature CGRP peptide
Following proteolytic processing in thyroid C cells get N-terminal peptide, CGRP and C terminal peptide
Mature calcitonin peptide
Following proteolytic processing in thyroid C cells get N-terminal peptide, Calcitonin and CGP
Osteoprogenitor
Osteoblast precursor Unspecialized bone stem cells derived from mesenchyme, the tissue from which almost all connective tissue are formed. They are the only bone cells to undergo cell division; the resulting cells develop into osteoblasts. Found along the inner portion of the periosteum, in the endosteum, and in the canals within bone that contain blood vessels. Cover the osteons.
M-CSF
Osteoblasts can release this factor to stimulate the differentiation of monocyte stem cells into osteoclast progenitor cells
Body phosphate homeostasis
Regulated by gut, bone and kidneys like Ca Less phosphate bound to proteins in plasma so bigger proportion will be filtered out through the day compared to Ca. Most is reabsorbed, but some passes out through urine.
Oxyphilic cells
Release acids Function is unknown
Chief cells
Secreted PTH in parathyroid glands Contains multiple vesicles containing hormones
Trabecular bone
Spongy or cancellous bone composed of thin plates that form a honeycomb pattern; predominantly found in the ends of long bones and the vertebral bodies Softer, less heavily calcified
PTH release
Sustained period of release get resorption of bone Pulsatile release - bone formation, stimulates osteoblast cells to increase in number and release osteoid Give osteoporotic pts short bursts of PTH
1,25(OH)2D
The final important function of parathyroid hormone (PTH) is conversion of 25-hydroxyvitamin D to its most active metabolite, 1,25-dihydroxyvitamin D-3 [1,25-(OH)2 D3], by activation of the enzyme 1-hydroxylase in the proximal tubules of the kidney.
Forms of Vitamin D
Vitamin D-3 (cholecalciferol) is formed in the skin when a cholesterol precursor, 7-dehydroxycholesterol, is exposed to ultraviolet light. Activation occurs when the substance undergoes 25-hydroxylation in the liver and 1-hydroxylation in the kidney to form 1,25 - (OH)2D3
Osteoblast
Bone forming cells They are connective tissue cells found at the surface of bone. They can be stimulated to proliferate and differentiate as osteocytes. Bone formation due to osteoblasts releasing osteoid and different types of collagen and proteins that can bind Ca. Proteins released into osteoid, bind Ca and phosphate, promote nucleation of hydroxyapatite Forms precipitate which promotes bone formation.
Osteoid
Intracellular matrix Interlacing trabeculae (connecting osteocytes) of woven bone surrounded by osteoblasts Unmineralized bone matrix composed of proteoglycans, glycoproteins, and collagen Secretes proteins - largely type 1 collagen - which is then calcified
IL-6 and RANK ligand
Involved in promoting osteoblast precursors to mature into osteoclasts and stimulates function of osteoclasts - release of acid for bone resoprtion and release of Ca from bone etc
Fast and slow effect of PTH on bone
Parathyroid hormone (PTH) promotes absorption of calcium from the bone in 2 ways. The rapid phase brings about a rise in serum calcium within minutes and appears to occur at the level of the osteoblasts and osteocytes. Although it may seem counterintuitive that the cells that promote deposition of bone are involved in resorption, these cells form an interconnected network known as the osteocytic membrane overlying the bone matrix, but with a small layer of interposed fluid termed bone fluid. When parathyroid hormone (PTH) binds to receptors on these cells, the osteocytic membrane pumps calcium ions from the bone fluid into the extracellular fluid. The slow phase of bone resorption occurs over several days and has 2 components. First, osteoclasts are activated to digest formed bone, and second, proliferation of osteoclasts occurs. Interestingly, mature osteoclasts lack parathyroid hormone (PTH) membrane receptors; activation and proliferation appear to be stimulated by cytokines released by activated osteoblasts and osteocytes or by differentiation of immature osteoclast precursors that possess parathyroid hormone (PTH) and vitamin D receptors.
Fluxes in Ca and phosphate
1. The small intestine is the site where dietary calcium is absorbed. Importantly, efficient absorption of calcium in the small intestine is dependent on expression of a calcium-binding protein in epithelial cells. 2. Bone serves as a vast reservoir of calcium. Stimulating net resorption of bone mineral releases calcium and phosphate into blood, and suppressing this effect allows calcium to be deposited in bone. 3. The kidney is critically important in calcium homeostasis. Under normal blood calcium concentrations, almost all of the calcium that enters glomerular filtrate is reabsorbed from the tubular system back into blood, which preserves blood calcium levels. If tubular reabsorption of calcium decreases, calcium is lost by excretion into urine.
Body Ca homeostasis
3 ways in which PTH and Vit D regulate Ca levels: 1. PTH stimulates the release of Ca++ from bone, in part by stimulating bone resorption. Or can stimulate bone formation to lower Ca levels. -main Ca sink/buffer for plasma Ca is in bone 2. PTH decreases urinary loss of Ca++ by stimulating Ca++ reabsorption. Alternatively can increase amount filtered out into urine. 3. PTH indirectly stimulates Ca++ absorption in the small intestine by stimulating synthesis of 1,25(OH)2D in the kidney. Or can increase amount of Ca secreted into gut and lost via faeces.
Parathyroid glands
4 glands at poles of thyroid Chief cells - parathyroid hormones (parathormone) Oxyphilic cells - unknown function PTH - 84 aa 9500 mol wt Primary action - maintenance plasma [Ca] PTH + Vit D3 - increase plasma Calcitonin - decrease plasma Body Ca - 99% bone/teeth (remaining 1% intracellular)
Calbindin
A calcium-binding protein, located all along the small intestine, but it is most abundant in the duodenum and proximal jejunum Requires Vit D for synthesis Binds to Ca in cells, to make sure Ca conc is low in cells and thus concentration gradient is high.
Bone remodelling
Bone may seem to be stable and unchanging, but in fact, bone is constantly being remodeled. Bone remodeling is triggered by a need for calcium in the extracellular fluid, but it also occurs in response to mechanical stresses on the bone tissue.
Bone resorption
Bone resorption can be triggered by parathyroid hormone (PTH) in response to hypocalcemia. PTH stimulates the generation of new osteoclasts (osteoclastogenesis). After osteoclasts form, the first step necessary for bone resorption to occur is that the mature osteoclast needs to tightly adhere to the bone, creating a specialized isolated compartment. Once the osteoclast adheres to the bone surface, the membrane adjacent to the bone differentiates as the ruffled membrane. The ruffled membrane contains proteins that acidify the compartment adjacent to the bone. The acid dissolves the minerals in the bone; subsequently, digestive enzymes break down type I collagen and other proteins. Bone resorption ends when the osteoclast dies by apoptosis.
Hydroxyapatite
Ca10(PO4)6(OH)2 A crystallized mineral salt in bone matrix that forms when calcium phosphate combines with calcium hydroxide More than 99% of the total body calcium is stored in bone in the form of phosphate and hydroxide salts, predominantly as hydroxyapatite. Normally, a very small portion of this calcium is available for exchange in the serum.
Processing of Pro-calcitonin in the parafollicular C cells within the thyroid or brain neurons
Calcitonin is originally synthesised in bigger precursor Procalcitonin gene can be processed into calcitonin or into a different peptide CGPR Calcitonin - antagonises PTH levels, though less in humans CGPR - involved in pain transmission and causes vasoconstriction Precursor contains different regions, Calcitonin, CCP and CGPR regions If processed in thyroid C cells then get rid of CGPR peptide and processed to get common region with calcitonin and CGP peptides In neurons in brain get rid of calcitonin peptide and get CGRP combined with common region. Break down common region and get CGPR which are NT in brain
Intestinal Absorption of Ca
Calcium is absorbed in the mammalian small intestine by two general mechanisms: a transcellular active transport process, located largely in the duodenum and upper jejunum; and a paracellular, passive process that functions throughout the length of the intestine. The transcellular process involves three major steps: entry across the brush border, mediated by a molecular structure termed CaT1 on apical surface, intracellular diffusion, mediated largely by the cytosolic calcium-binding protein (calbindin); and extrusion, mediated largely by the CaATPase. When calcium intake is low, transcellular calcium transport accounts for a substantial fraction of the absorbed calcium. Biosynthesis of Calbindin is fully and CaT1 function is approximately 90% vitamin D-dependent. At high calcium intakes CaT1 and Calbindin are down regulated because 1,25(OH)(2)D(3), the active vitamin D metabolite, is downregulated. Vit D binds to receptors, acts as transcription facotrs, increasing synthesis of proteins such as calbindin.
Bone reabsorption by osteoclast
Cytokine Receptor -RANK On surface of osteoclasts When cytokine such as IL-6 or RANK ligand bind it promotes bone reabsorption When calcitonin binds to receptor it activates PKA which inhibits bone reabsorption Integrins on osteoclast mate with vitronectins on the bone CO2 enters cells and combines with H20 to form H+ protons Acidic protons get pumped out of osteoclast by V-type protein pump into space between cell and bone (lacuna). Also secrete lysosomal enzymes and TRAP into lacuna
Vitamin D2
Ergocaciferol Form of Vitamin D found in diet Primarily found in plants and yeasts and is commonly used as a dietary supplement
Cortical Bone
Hard, dense, strong bone that forms the outer layer of bone; also called compact bone 80% of bone Heavily calcified
Feedback loops controlling plasma [Ca]
If decrease levels of Ca in blood you remove inhibition of PTH release by chief cells. PTH causes an increase in bone Ca reabsorption, an increase in tubule Ca reabsorption in kidney, inhibits tubule phosphate reabsorption and increases activation of vitamin D. Vitamin D then causes increase in intestine Ca absorption and bone Ca reabsorption, increase in tubule Ca and phosphate reabsorption in kidney. But also negatively feedbacks on itself to inhibit synthesis. Ca levels increase due to intestine reabsorption, kidney reabsorption and bone reabsorption
Osteoprotegrin
Osteoblasts also express a secreted factor called osteoprotegerin. Osteoprotegerin "protects bone" by preventing bone resorption. Osteoprotegerin works as a decoy receptor for RANKL: it binds RANKL and therefore prevents binding to RANK and stimulation of osteoclastogenesis. The ratio of osteoprotegerin:RANKL produced by osteoblasts will determine the extent of bone resorption.
Osteocytes
Osteocytes are bone cells. Osteocytes manufacture type I collagen and other substances that make up the bone extracellular matrix. Osteocytes will be found enclosed in bone.
PTH effects on bone
PTH has a rapid effect (occurring within minutes), whereby it stimulates osteoblasts to pump Ca++ ions out of the fluid surrounding the bone (which has a higher Ca++ concentration) and into the ECF. Over a longer time course, PTH stimulates bone resorption by stimulating osteoclastogenesis. Although PTH stimulates bone resorption, it is actually the osteoblasts that express PTH receptors. PTH stimulation of osteoblasts causes them to express a signaling molecule that activates osteoclasts
Osteoclastogenesis
PTH stimulates bone resorption by osteoclasts, but it does so indirectly. Receptors for PTH are located on osteoblasts, which then signal to bone marrow-derived osteoclast precursors to stimulate their fusion, differentiation and activation. Osteoclast precursors express a cell-surface receptor known as RANK. Osteoblasts express RANKL (RANK Ligand) on the extracellular surface of their plasma membrane. When they are stimulated by PTH, osteoblasts up-regulate expression of RANKL, which binds to RANK, activating signalling pathways that promote osteoclast differentiation and survival.
Physiological functions of Ca
Prosthetic group for many enzymes and structural proteins (+/- calmodulin) - binds and activates regulated protein so it can bind and regulate other hormones Structure of the plasma membrane (essential for structure of Na channel and permeability of plasma membrane to Na) Excitation-contraction coupling in muscle Excitation-secretion coupling at axonal terminals and in endocrine and exocrine glands - exocytosis Blood coagulation - activates platelets and clotting factors Major intracellular second messenger
PTH effects on kidney
The primary response to parathyroid hormone (PTH) by the kidney is to increase renal calcium resorption and phosphate excretion. In the kidney, parathyroid hormone (PTH) blocks reabsorption of phosphate in the proximal tubule while promoting calcium reabsorption in the ascending loop of Henle, distal tubule, and collecting tubule. The other key effect of PTH on the kidney is to stimulate production of 1,25(OH)2D, the active form of vitamin D. The role of PTH is to stimulate the regulated kidney enzyme, resulting in the production of 1,25(OH)2D. This is extremely important for bone health and Ca++ homeostasis because 1,25(OH)2D works in the small intestine to promote Ca++ absorption.
Osteons
The structural unit of compact bone that consists of a central canal (either a Haversian or Volkmann's canal) surrounded by a number of concentric rings of bony matrix called lamellae.
Vitamin D
Vitamin D-3 (cholecalciferol) is formed in the skin when a cholesterol precursor, 7-dehydroxycholesterol, is exposed to ultraviolet light. Activation occurs when the substance undergoes 25-hydroxylation in the liver and 1-hydroxylation in the kidney. The primary action of 1,25-(OH)2 D3 is to promote gut absorption of calcium by stimulating formation of calcium-binding protein within the intestinal epithelial cells. Vitamin D also promotes intestinal absorption of phosphate ion, although the exact mechanism is unclear. In bone, vitamin D may play a synergistic role with parathyroid hormone (PTH) in stimulating osteoclast proliferation and bone resorption. Compared to parathyroid hormone (PTH), vitamin D exerts a much slower regulatory effect on calcium balance.
