Endocrine System: Calcium and Phosphate Metabolism

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Questions and Answers

What is the effects of 1,25-dihydroxycholecalciferol on the parathyroid glands?

Decrease preproPTH mRNA (correct)

Increase phosphate reabsorption

Increase preproPTH mRNA

Has no effect on preproPTH mRNA

What is the effect of increased plasma phosphate levels on PTH secretion?

Stimulates PTH secretion

Inhibits PTH secretion (correct)

Increases calcium absorption

Has no effect on PTH secretion

What is the role of magnesium in parathyroid function?

Has no effect on parathyroid function

Required to maintain normal parathyroid secretory responses (correct)

Inhibits PTH release

Stimulates PTH secretion

What is the effect of magnesium deficiency on calcium levels?

Occasionally causes hypocalcemia Signup and view all the answers

What is the effect of low plasma PO4(3–) levels on 1,25-dihydroxycholecalciferol production?

It increases 1,25-dihydroxycholecalciferol production Signup and view all the answers

What is the effect of high plasma PO4(3–) levels on 1,25-dihydroxycholecalciferol production?

It inhibits 1,25-dihydroxycholecalciferol production Signup and view all the answers

What is the effect of calcitonin on calcium levels?

Decreases calcium levels Signup and view all the answers

What is the effect of 1,25-dihydroxycholecalciferol on 1α-hydroxylase?

It inhibits 1α-hydroxylase activity Signup and view all the answers

What stimulates calcitonin secretion in the thyroid gland?

All of the above Signup and view all the answers

What is the typical location of the parathyroid glands in humans?

Four glands, two embedded in the superior poles and two in the inferior poles of the thyroid Signup and view all the answers

What is the effect of gastrin on calcitonin secretion?

Stimulates calcitonin secretion Signup and view all the answers

What type of cells are abundant in the parathyroid gland and responsible for synthesizing and secreting PTH?

Chief cells Signup and view all the answers

What is the half-life of calcitonin in humans?

Less than 10 minutes Signup and view all the answers

What is the molecular weight of human PTH?

9500 Signup and view all the answers

What is the number of amino acid residues in the preproPTH molecule?

115 Signup and view all the answers

What happens to the preproPTH molecule when it enters the endoplasmic reticulum?

A leader sequence is removed from the amino terminal Signup and view all the answers

What is the general formula for hydroxyapatites found in bone?

Ca10(PO4)6(OH)2 Signup and view all the answers

Which hormone increases bone formation?

Insulin Signup and view all the answers

What is the approximate total blood flow to the bone in adult humans?

200-400 mL/min Signup and view all the answers

What percentage of bone in the body is made up of trabecular bone?

20% Signup and view all the answers

Which of the following hormones can cause osteoporosis?

Thyroid hormone Signup and view all the answers

What is the function of canaliculi in compact bone?

To deliver nutrients to bone cells Signup and view all the answers

What is the primary function of compact bone?

To protect vital organs Signup and view all the answers

Which of the following is a characteristic of trabecular bone?

High surface-to-volume ratio Signup and view all the answers

What percentage of bone mass is being remodeled by bone-remodeling units in the human skeleton at any one time?

5% Signup and view all the answers

What is the primary function of ionized calcium in the body?

Cell signaling and muscle contraction Signup and view all the answers

Which hormone increases plasma calcium levels and decreases phosphate levels?

Parathyroid hormone Signup and view all the answers

What is the main site of phosphate storage in the body?

Bones Signup and view all the answers

What is the estimated renewal rate for compact bone per year?

4% per year Signup and view all the answers

What is the primary mechanism by which 1,25-dihydroxycholecalciferol regulates calcium and phosphate levels?

Transcriptional mechanisms Signup and view all the answers

What is the estimated number of bone-remodeling units in the human skeleton at any one time?

2 million Signup and view all the answers

What is the primary factor influencing the remodeling of compact bone?

Gravity and stresses on the skeleton Signup and view all the answers

What is the primary mechanism by which nutrients are provided to compact bone?

Haversian canals containing blood vessels Signup and view all the answers

What is the approximate percentage of type I collagen in bone matrix?

90% Signup and view all the answers

What is the structure of type I collagen?

A triple helix of three polypeptides Signup and view all the answers

How many collagen genes have been identified to contribute to distinct trimeric collagens?

40 Signup and view all the answers

What is the process by which cartilage is transformed into bone during fetal development?

Endochondral bone formation Signup and view all the answers

What is the function of the epiphysial plate during growth?

To lay down new bone on the end of the shaft Signup and view all the answers

What is the relationship between the width of the epiphysial plate and the rate of growth?

The width of the epiphysial plate is directly proportional to the rate of growth Signup and view all the answers

What is the structure formed around each haversian canal in compact bone?

A concentric layer of osteons Signup and view all the answers

Study Notes

Calcium and Phosphate Metabolism

Production of 1,25-dihydroxycholecalciferol is increased by low plasma PO4 (3-) levels and inhibited by high plasma PO4 (3-) levels
1,25-Dihydroxycholecalciferol formation is also controlled by a direct negative feedback effect on 1α-hydroxylase, a positive feedback action on the formation of 24,25-dihydroxycholecalciferol, and a direct action on the parathyroid gland to inhibit PTH expression

Parathyroid Glands

Humans usually have four parathyroid glands, two embedded in the superior poles of the thyroid and two in its inferior poles
Each parathyroid gland is a richly vascularized disk, about 3 × 6 × 2 mm, containing two distinct types of cells: chief cells and oxyphil cells
Chief cells synthesize and secrete PTH, contain a prominent Golgi apparatus, endoplasmic reticulum, and secretory granules
Oxyphil cells contain oxyphil granules and large numbers of mitochondria in their cytoplasm, increase in number with age

Synthesis and Metabolism of PTH

Human PTH is a linear polypeptide with a molecular weight of 9500, containing 84 amino acid residues
PTH is synthesized as part of a larger molecule containing 115 amino acid residues (preproPTH)
1,25-Dihydroxycholecalciferol acts directly on the parathyroid glands to decrease preproPTH mRNA

Calcitonin

Calcitonin is a Ca2+-lowering hormone produced by the parafollicular cells of the thyroid gland
Calcitonin secretion is increased when the thyroid gland is exposed to a plasma calcium level of approximately 9.5 mg/dL
β-Adrenergic agonists, dopamine, and estrogens also stimulate calcitonin secretion
Gastrin, cholecystokinin (CCK), glucagon, and secretin have been reported to stimulate calcitonin secretion, with gastrin being the most potent stimulus

Bone Physiology

Bone is a special form of connective tissue with a collagen framework impregnated with Ca2+ and PO4(3-) salts, particularly hydroxyapatites
Bone is involved in overall Ca2+ and PO4(3-) homeostasis, protecting vital organs, and providing rigidity for locomotion and support against gravity
Old bone is constantly being resorbed and new bone formed, permitting remodeling that allows it to respond to stresses and strains
Bone is a living tissue that is well vascularized and has a total blood flow of 200–400 mL/min in adult humans

Bone Structure

There are two types of bone: compact (cortical) bone and trabecular (spongy) bone
Compact bone makes up the outer layer of most bones and accounts for 80% of the bone in the body
Trabecular bone makes up the remaining 20% of bone in the body, is made up of spicules or plates, and has a high surface-to-volume ratio
Nutrients diffuse from bone extracellular fluid (ECF) into the trabeculae, but in compact bone, nutrients are provided via haversian canals, which contain blood vessels

Bone Growth

During fetal development, most bones are modeled in cartilage and then transformed into bone by ossification (enchondral bone formation)
Exceptions include the clavicles, mandibles, and certain bones of the skull, which form bone directly (intramembranous bone formation)
During growth, specialized areas at the ends of each long bone (epiphyses) are separated from the shaft of the bone by a plate of actively proliferating cartilage, the epiphysial plate

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Description

Learn about the regulation of calcium and phosphate metabolism in the endocrine system, including the production of 1,25-dihydroxycholecalciferol and its feedback mechanisms.