Exam 2 (Lecture 14) - Calcium Homeostasis

Question 1

What are the main physiological functions of Ca2+?

Answer 1

1) Muscle contraction
2) Hormone and neurotransmitter release
3) Second messenger
4) Coagulation
5) Structural component of bone

Question 2

Describe which processes in gut, kidney, and bone contribute to the levels of Ca2+ in plasma and extracellular fluid.

Answer 2

1) Gut:
- absorption and secretion; more absorption than secretion for excretion in feces

2) Kidney:
- filters and reabsorbs most of it; but some is excreted in urine

3) Bone:
- accretes and resorbs a fairly constant amount and serves as the storage site

Question 3

Which three hormones contribute to the regulation of calcium homeostasis? Which of them increase and which decrease its concentration in plasma?

Answer 3

1) Parathormone (PTH)
- increases Ca2+

2) Vitamin D
- increases Ca2+

3) Calcitonin
- decreases Ca2+

Question 4

Describe the chemical nature of parathormone, which cells secrete it, what stimulates its secretion (including how this physiological stimulus is detected) and what are its main physiological effects.

Answer 4

1) Protein hormone secreted by chief cells of parathyroid gland

2) Triggered by a decrease in serum ionized calcium
- detected by calcium-sensing receptors in membranes of chief cells

3) Stimulates bone resorption and calcium renal reabsorption

4) Increases renal vitamin D activation and decreases renal phosphate reabsorption

Question 5

Describe how vitamin D is synthesized, how its synthesis is regulated, and what are calcium-related functions of this vitamin/hormone.

Answer 5

Synthesized from cholesterol to D3; then goes to liver; then to kidney where PTH stimulates its synthesis

Functions:
- increases intestinal calcium and phosphate absorption
- increases renal calcium and phosphate reabsorption
- increases bone resorption

Question 6

Describe the chemical nature of calcitonin, which gland secretes it, how is its secretion regulated and what are its main physiological functions.

Answer 6

1) Peptide secreted by the thyroid gland by parafollicular cells in response to hypercalcemia

2) Triggers:
- high plasma calcium
- gastrin

3) Functions:
- decreases of intestinal calcium absorption
- decreases resorption of bone
- decreases renal reabsorption

** Most likely protects bones of mother against loss of calcium during pregnancy

Question 7

What are the main signs of hypercalcemia, what are common causes, and how is it treated?

Answer 7

1) Decreased neuromuscular excitability

2) Causes:
- Primary hyperparathyroidism (e.g. tumor)
- Vitamin D toxicosis

3) Treatment:
- Volume expansion (dilute it out)
- Loop diuretics

Question 8

What are the main manifestations of hypocalcemia? What could cause this condition? How is it treated?

Answer 8

1) Increased neuromuscular excitability, coagulopathies, eclampsia, milk fever

2) Causes:
- Primary hypoparathyroidism
- Renal failure
- Pregnant or lactating dogs or cows; demand for milk calcium depletes their plasma levels, and their homeostatic mechanisms can’t keep up

3) Treatment:
- IV calcium gluconate
- Oral calcium
- Vitamin D

Question 9

How (and why) does calcium affect neuromuscular excitability?

Answer 9

Calcium blocks sodium channels
- low calcium makes membrane depolarization easier
- high calcium makes membrane depolarization more difficult