Ch.5 Calcium Regulation

What cells synthesize and secrete parathyroid hormone (PTH)?

Chief cells of the parathyroid glands.

Where are the parathyroid glands anatomically located?

On the posterior surface of the thyroid gland.

What are the primary target organs of parathyroid hormone?

Bone and kidney, with indirect effects on the intestine.

What is the primary physiologic effect of parathyroid hormone?

An increase in plasma calcium concentration.

How does parathyroid hormone affect phosphate levels?

It increases renal phosphate excretion.

Through what receptor does PTH exert its effects?

Parathyroid hormone receptor (PTHR1), a G protein–coupled receptor.

What is the primary function of osteoblasts?

Bone formation through secretion of organic matrix and facilitation of mineralization.

What is the primary function of osteoclasts?

Bone resorption through breakdown of mineralized bone matrix.

From what cell lineage are osteoclasts derived?

Bone marrow–derived cells.

How do osteoclasts resorb bone?

By secreting hydrogen ions and proteolytic enzymes that dissolve hydroxyapatite and degrade organic matrix.

Which hormones primarily regulate bone remodeling?

Parathyroid hormone, vitamin D, and calcitonin.

What is the primary regulator of parathyroid hormone secretion?

Plasma ionized calcium concentration.

How does hypocalcemia affect PTH secretion?

It stimulates parathyroid hormone release.

How does hypercalcemia affect PTH secretion?

It suppresses parathyroid hormone release.

What is the calcium-sensing receptor (CaSR)?

A G protein–coupled receptor that detects extracellular calcium levels.

What is the role of the CaSR in parathyroid chief cells?

Inhibition of parathyroid hormone secretion when calcium levels are high.

How does severe hypomagnesemia affect PTH secretion?

It paradoxically inhibits PTH release.

What is the primary source of vitamin D in humans?

Synthesis in the skin from 7-dehydrocholesterol via ultraviolet radiation.

Where does the first hydroxylation of vitamin D occur?

In the liver, forming 25-hydroxycholecalciferol.

Where does the final activation of vitamin D occur?

In the kidney.

What enzyme converts vitamin D to its biologically active form?

1α-hydroxylase.

Which hormone stimulates renal activation of vitamin D?

Parathyroid hormone.

What is the biologically active form of vitamin D?

1,25-dihydroxycholecalciferol (calcitriol).

What are the major target organs of vitamin D?

Intestine, kidney, bone, and parathyroid gland.

What is the primary effect of vitamin D in the intestine?

Increased calcium absorption.

How does vitamin D affect parathyroid hormone levels?

It suppresses PTH synthesis and secretion.

What are the two mechanisms of action of vitamin D?

Genomic and nongenomic mechanisms.

How does PTH influence vitamin D production?

It stimulates renal 1α-hydroxylase to increase calcitriol synthesis.

How does calcitriol regulate PTH secretion?

It suppresses parathyroid hormone synthesis and release.

What type of feedback relationship exists between PTH and vitamin D?

Negative feedback.

How do elevated calcium levels affect PTH and vitamin D activation?

They suppress both PTH secretion and vitamin D activation.

What are the consequences of excess parathyroid hormone?

Hypercalcemia, increased bone resorption, and phosphate wasting.

What results from parathyroid hormone deficiency?

Hypocalcemia and increased neuromuscular excitability.

What skeletal disorder results from vitamin D deficiency in children?

Rickets.

What skeletal disorder results from vitamin D deficiency in adults?

Osteomalacia.

What are the consequences of excess vitamin D?

Hypercalcemia, soft tissue calcification, renal damage, and cardiac arrhythmias.

What cells produce calcitonin?

Parafollicular (C) cells of the thyroid gland.

What stimulus triggers calcitonin release?

Elevated plasma calcium concentration.

What are the primary target organs of calcitonin?

Bone and kidney.

What is the main physiologic effect of calcitonin?

Decrease in plasma calcium levels.

Why does calcitonin play a minor role in calcium homeostasis?

Its effects are weaker than those of parathyroid hormone and vitamin D.