30. Calcium, PTH, Vitamin D

Rickets in the 21st Century

"Extinct bone disease, rickets makes shock comeback in UK, November 13, 2010. One in five children in Southhampton show signs of rickets. In 2001, Georgia Dept of Public Health and CDC reported 6 cases of nutritional rickets. Problematic in communities: Exclusively breast-fed infants Inadequate sun-exposure Low dietary calcium intake

*Know the biosynthesis of Vitamin D.*

*1,25-(OH)-D [calcitriol] active form* Calcitriol is product of 2 hydroxylations: Precursor: Calciferol created by UV light in the skin 1. Hydroxylated in liver (25-hydroxyvitamin D) 2. Hydroxylated again in the kidney by 1α hyroxylase [25(OH)D] --1α hyroxylase--> [1,25(OH)D] or the active form *calcitriol* Calcitriol is released into the circulation and acts as an endocrine hormone.

Inverse Relationship between PTH Secretion and *Acute Plasma Ca2+*

*Exquisitely sensitive mechanism* PTH maximal at low Ca2+ (for acute hypocalcemia) PTH secretion very sensitive to Ca2+ decrease Normal to High Ca2+ PTH finite

Roles of Ca2+ in Mediating Diverse Cell Functions

*highest regulated ion in the body -- regulated in cytosol and in the extracellular fluid* Contraction of skeletal, cardiac and smooth muscle Neurotransmitter release and nerve transmission Secretion of peptides from cells Mediates actions of many hormones Cell division and growth Regulation of enzyme activity Structural role in bone

PTH

1) PTH binds to the PTH receptor on osteoblasts and stimulates the osteoblast to secret M-CSF 2) M-CSF binds to its receptor on preosteoclasts and stimulate the expression of RANK on the osteoclast 3) PTH also stimulates osteoblasts to secrete RANKL 4) RANKL binds to RANK on preosteoclasts causing them to differentiate into multinucleated osteoclasts.

What is the rate limiting step of calcitriol synthesis?

1,25-(OH)2-D3 active form *** Rate-limiting*** ***PTH stimulates 1α hyroxylase Low affinity for vitamin D carrier protein Short half-life, 15 hours

Biosynthesis and Secretion of PTH Paradox**: Hi Ca2+ inhibits PTH release

Acute low Ca2+ in ECF PTH secretion stimulated Chronic low Ca2+ in ECF Stimulate transcription mRNA High Ca2+ in ECF Proteolysis (>90% of PTH)

*Know the biochemical basis for the action of vitamin D on calcium and phosphate metabolism in the bone, kidney and gut.*

Binds to vitamin D receptor bone -- Promotes mobilization of Ca++ from bone by increasing the number of osteoclasts *kidney* -- Increase calcium *gut* -- Increase Ca++ absorption; Increase PO4 absorption calcitriol binds to vit. D receptor --> up-regulates calcium channels, calbindin, and Na+/PO4 transporters

*Define calcium homeostasis, how it is maintained and the organs involved.*

Bone -- large reservoir of Ca++ & PO4 Kidney -- controls Ca++, PO4, HCO3 resorption, can activate vitamin D Intestine -- Ca++, PO4 Absorption It is maintained by hormones PTH, Calcitonin, and Vitamin D.

Bone Remodeling

Bone formation is stimulated by weight bearing exercise Bone is resorbed with immobility and weightlessness

Daily Calcium Balance in Adult

Bone i largest reservoir of Ca in body

*Describe the process of bone remodeling and its endocrine regulation.*

Bone resorption and bone formation are tightly coupled Osteoclasts express receptors for M-CSF and RANKL Osteoblasts secrete the ligands M-CSF and RANKL PTH & Calcitonin Regulate the process

Calcium Transport Processes

Ca is an important signaling messenger inside the cytosol

Effects of Calcitonin on Bone Calcitonin receptors on Osteoclasts.

Calcitonin: Decreases activity of ruffled border. Dedifferentiation of osteoclasts. Osteoclasts quickly desensitize to calcitonin Down regulation of calcitonin receptors. Not relevant in minute to minute regulation of Calcium Protection against excessive bone resorption.

*Regulation of Calcitriol*

Calcitriol inhibits the expression of 1α-hydroxylase expression Calcitriol stimulates the expression of 24 hydroxylase (the first step in PTH degradation)

*Know the mechanisms of action of Vitamin D.*

Calcitriol largely affects the intestine, bone, kidney, and parathyroid gland. It primarily increases intestinal absorption of Ca, facilitates PTH-mediated Ca reabsorption in the DCT, and suppresses synthesis of PTH from the parathyroid gland. Also plays a role in regulation of bone resorption and formation. 2 Mechanisms: 1. Altering gene transcription via nuclear recptor proteins 2. Rapid responses from interaction with membrane receptors --> signal transduction

Actions of Vitamin D on Intestinal Transport of Calcium

Calcitriol-VDR receptor stimulates gene expression Ca2+ channels ECaC aka TRPV5, TRPV6 Calbindin Na+/PO4 transporter Effect stimulate Ca/Pi absorption

Biosynthesis of Vitamin D

D3 (cholecalciferol) formation requires uv light Biologically inert Binds vitamin D carrier 25-OH-D3 Biologically inactive High affinity for vitamin D carrier protein Long half-life 15 days Dietary supplement along with D3 25-OH-D3 --> 1,25-(OH)2-D3 active form *** Rate-limiting*** *PTH stimulates 1α hyroxylase* Low affinity for vitamin D carrier protein Short half-life, 15 hours

*Describe how estrogens and androgens influence bone remodeling.*

Estrogens inhibit bone resorption Decrease: M-CSF; RANKL; IL-6; TNFa Estrogen deficiency Osteoclast activity dominates over osteoblasts. Menopause, ovaries don't produce estrogen Increased cytokine release Increased osteoclastic activity and bone resorption Osteoporosis

Effects of Sex Steroids on Bone Resorption

Estrogens inhibit bone resorption Decrease: M-CSF; RANKL; IL-6; TNFa Estrogen deficiency Osteoclast activity dominates over osteoblasts. Menopause, ovaries don't produce estrogen Increased cytokine release Increased osteoclastic activity and bone resorption Osteoporosis

*Know the regulation of PTH.*

High Ca2+ inhibits PTH release CaSensing GPCReceptor EC Ca2+ binds receptor Coupled to Gαi (inhibits AC) Coupled to Gαq (stimulates PLC) cAMP stimulates PTH release Cytosolic Ca2+ inhibits PTH *PTH secretion very sensitive to Ca2+ decrease*

1st step in calcitriol degradation

Inactive metabolites 24,25-(OH)2-D3 1,24,25-(OH)2-D3 calcitroic acid

Effects of PTH on Kidney

Increase Ca2+ resorption in distal tubule Increase phosphate excretion in proximal tubule Increase expression of 1α-hydroxylase in proximal tubule

*Know the integrated actions of PTH/VitD to a hypocalcemic challenge.*

Increased PTH Increased Ca++ from the kidney Increased Phosphate excreted Increased Calcitriol Increased Ca++ and PO4 absorbed from the intestine Increased Ca++ and PO4 from bone Increased serum Ca++ PO4 stays the same

Risks associated with too much Ca2+ and Vitamin D

Kidney stones Calcification of soft tissue Heart disease Bone Fractures

*Know the mechanisms of action of PTH.*

Kidney: Increase Ca2+ resorption in distal tubule Increase phosphate excretion in proximal tubule Increase expression of 1α-hydroxylase in proximal tubule (activates vitamin D) Bone: Stimulate Osteoblasts to release M-CSF and RANK-L --> differentiation of osteoclasts Increase Ca++ resorption, Increase PO4 resorption Activates vitamin D

Osteoporosis

Loss of bone tissue leads to brittle bones and kyphosis Prevalent in the elderly, especially women Loss of estrogens/androgens Vitamin D deficiency decreased outdoor activities, suntan lotion, don't drink milk. "at least one-third of all osteoporotic fractures have a nutritional basis."

Hormonal Response to Hypocalcemic Event

Low serum Ca --> PTH --> increase Ca resorption in kidney, dump Phosphate --> PTH stimulates formation of calcitriol --> helps bring Ca in through the intestine --> then together PTH and calcitriol act on the bone to stimulate bone resorption --> act on osteoblast --> cytokines --> act on osteoclast Sum effect: increase serumCa, no change in serum phosphate

Calcitonin

Lowers blood calcium and Pi Not essential for calcium balance in humans Synthesized in thyroid gland Parafollicular or C cells Encoded in CGRP gene Major action on osteoclasts in bone

Effects of PTH on Bone

Osteoblasts couple bone resorption and reformation. PTH stimulates osteoblasts to release M-CSF and RANKL M-CSF and RANKL stimulate Differentiation of osteoclasts Secrete acid and endopeptidases Growth factors released from bone activate osteoblasts. Osteoblasts secrete Collagen, Ca2+ vesicles Alkaline phosphatase (cleaves PPi)

Effects of Vitamin D on Bone

Osteoblasts couple bone resorption and reformation. Calcitriol stimulates osteoblasts to release M-CSF and RANKL In the absence of PTH Decreased sensitivity to Vitamin D 30-100-fold more VitaminD In the absence of VitaminD Decreased sensitivity to PTH

Osteoclasts Erode Bone Matrix at the Ruffled Border

Osteoclasts attach to bone surface at ruffled border through integrins Carbonic anhydrase II generates H+ H+ pumped out at ruffled border H+ "dissolves" hydroxyapatite, Ca2+, phosphate crystals in bone. Ca2+ and Pi are released by transcytosis. Integrins keep H+ high by "sealing" ruffled border to bone Bisphosphonates disrupt integrin interaction.

Differentiation & Activation of Osteoclasts

Osteoclasts express receptors for macrophage colony stimulating factor and RANKL Osteoblasts secrete the ligands M-CSF and RANKL. Osteoblasts secrete OPG which neutralizes RANKL.

3 Major hormones Actions of Hormones that Regulate Calcium Balance

PTH -- elevates Ca in ECF Vitamin D (calcitriol) -- elevates Ca Calcitonin -- lowers Ca

Regulation of PTH

PTH gland—2 pair behind thyroid gland --Removal causes lethal tetany PTH keeps ECF Ca2+ high Acts *directly* on bone and kidney Acts *indirectly* on gut Ca2+ decrease stimulates PTH --stimulates Ca2+ resorption Kidney --inhibits Pi resorption Kidney --stimulates Ca2+ resorption Bone Ca2+ increase inhibits PTH PTH stimulates formation of active Vitamin D in the kidney

PTH Activation of Osteoblasts

PTH has receptors on the osteoblasts --> releases cytokines to regulate osteoclast, stimulates M-CSF --> stimulates to express RANKL

Effects of PTH on Proximal Tubule Cell "I'm not going to ask you about Nerf"

PTH inhibits (HPO4)2-resorption Na/Pi co-transporter (PT) Transports Na+/Pi into cell Stabilized in membrane by NERF On Blood side, PTH stimulates cAMP/PKA PKA phosphorylates NERF Endocytosis of PT Increase Pi excretion 1α-hydroxylase expression via CREB 1α-hydroxylase rate limiting in Vitamin D synthesis 1α-hydroxylase 2-4 hour half-life

Effects of PTH on PTC

PTH inhibits (HPO4)2-resorption Na/Pi co-transporter (PT) Transports Na+/Pi into cell Stabilized in membrane by NERF On Blood side, PTH stimulates cAMP/PKA PKA phosphorylates NERF Endocytosis of PT Increase Pi excretion 1α-hydroxylase expression via CREB 1α-hydroxylase rate limiting in Vitamin D synthesis 1α-hydroxylase 2-4 hour half-life

*Know the biosynthesis of PTH.*

PTH is synthesized as a prepropeptide that is rapidly cleaved to yield pro-PTH and then the mature form of PTH (84 amino acids)

*Know the down-regulation of PTH secretion.*

PTH secretion is down regulated by high levels of Ca++ in the ECF. *Lecture*: Ca++ binds to the CaSensing GPCReceptor --> Phospholipase C --> Increased Cytosolic Ca2+ --> Cytosolic Ca2+ inhibits** PTH *Book*: Ca++ binds to the CaSensing GPCReceptor --> Phospholipase C --> Increased Cytosolic Ca2+ --> Activates Phospholipase A2 --> arachadonic acid cascade --> leukotrienes --> *degrade intracellular PTH* --> less PTH released *PTH secretion very sensitive to Ca2+ decrease*

Effects of PTH on Ca2+ Resorption in the Distal Tubule Cell

PTH stimulates GPCR cAMP/PKA Luminal membrane, PTH Stimulates Ca2+ channel insertion TRPV5/TRPV6 (ECaC) Basolateral membrane, PTH Stimulates Na+/Ca2+ exchanger Stimulates Ca2+ ATPase Effects stimulate net flux of Ca2+ from lumen to blood side

*Know the regulation of Vitamin D synthesis and secretion of the active form.*

PTH stimulates the activity of 1α hyroxylase. Vitamin D, and high levels of calcium suppress the activity of 1α hyroxylase.

Biosynthesis and Secretion of PTH

Paradox**: Hi Ca2+ inhibits PTH release CaSensing GPCReceptor EC Ca2+ binds receptor 2 G proteins-- Coupled to Gαi (inhibits AC) Coupled to Gαq (stimulates PLC) cAMP stimulates PTH release Gαi inhibits** cAMP PTH release inhibited Cytosolic Ca2+ inhibits** PTH Gαqstimulates PLC/ cytosolic Ca2+ PTH release inhibited

*Know the regulation of PTH synthesis.*

The active form of vitamin D, calcitriol, down-regulates PTH by decreasing PTH gene expression and overall synthesis.

*Processing of PTH*

Weird hormone: *The degradation of this hormone is part of its regulation* Peptide hormone Preprohormone stages *"What you should know is, There are proteolytic fragments of the mature active PTH that are made in the chief cells of the parathyroid gland* *Ca stimulates the cleavage of the hormone -- rendering it inactive*.

1α-hydroxylase

activates vitamin D to calcitriol

The Process of Bone Remodeling

proteases are important to degrade the collagen in bone Resoption and formation is tightly coupled: Osteoblasts release cytokines --> activate osteoclasts

Where is PTH fastest acting?

the kidney