Calcium Homeostasis

Question 1

Intracellular & extracellular calcium ions are essential for several normal biochemical processes such as…

Answer 1

• Neuromuscular/synaptic excitation
• Blood coagulation
• Hormone secretion
• Enzyme activity
• Fertilization

Question 2

Major Sites for Regulation of Calcium Homeostasis

Answer 2

Bone*
• Osteoblasts and osteoclasts
• Turn-over of bone tissue

Kidney*
• Ca?+ absorption from glomerular filtrate

Blood plasma
• Free and bound Ca2+
- *calcium exchange

Gut*
• Active and passive, Ca2+ absorption, and bile/fecal excretion

Muscle cells & others
•Ca2+ uptake for multiple cell functions

Question 3

PTH involvement

Answer 3

• Peptide produced by the parathyroid gland
• Increases serum Ca2+ mainly by promoting osteoclast activity
• Secretion of PTH largely determined by serum Ca2+ concentration
• Parathyroid seen in terrestrial amphibians onwards
• Two pairs in the neck, one often with the thyroid gland
• Two cell types: chief cells (produce PTH) and oxyphils (function unknown)
PTH = increased serum Ca2+ = binds to Ca2+-sensitive receptors — decrease in serum Ca2+ promote PTH secretion in renal tubules & C cells!

Question 4

PTH: Regulation of Secretion
• Secreted continuously, not stored!
• Hypocalcemia — PTH secretion ______
• Hypercalcemia — PTH secretion ______
• Vitamin D3 — PTH secretion ______

Answer 4

• Secreted continuously, not stored!
• Hypocalcemia — PTH secretion +
• Hypercalcemia — PTH secretion -
• Vitamin D3 — PTH secretion -

Question 5

Calcitonin

Answer 5

• Peptide produced by thyroid C cells
• Secreted in response to hypercalcemia, short acting
• Vitamin D3 inhibits calcitonin secretion
• CT lowers serum Ca2+ by inhibiting osteoclast activity
• CT also reduces Ca2+ reabsorption by the nephron (distal tubules)

Question 6

Calciferols (vitamin D)

Answer 6

• Synthesized from 7-dehydrocholesterol with UV light in skin + obtained from dietary sources
• In dogs and cats vitamin D3 is essential vitamin (only from diet)
• Cholecalciferol converted to more active forms in the liver and kidneys
• Dihydroxy metabolites produced in the liver and kidney enhance absorbance of Ca2+ by the gut and reabsorption of these ions by the nephron
• Both PTH and calcitonin increase vitamin D3
synthesis

Question 7

Other key hormones involved

Answer 7

GH and IGF-1, Thyroid Hormones
• Increase bone mineralization and growth

Estrogens/ Androgens
• Oppose PTH actions by slowing turnover. of bone (deficiency leads to osteoporosis)

Glucocorticoids
• Oppose calciferol actions in the GI tract

Question 8

Hyperparathyroidism (causes, treatment & complications)

Answer 8

• Hypercalcemia
• Increased bone resorption (fractures)
• Mineralization of soft tissues
• Increased thirst and urination (Ca2+ blocks ADH effects)

• Primary: hyperplasia, adenoma, genetic
• Secondary: kidney failure, vitamin D deficiency

• Primary: surgical, radiation therapy, bisphosphonates, healthy and balanced diet
• Secondary: chronic kidney failure treatment
• Complications are rare, mainly kidney stones and osteoporosis

Question 9

Hypoparathyroidism (causes, treatment & complications)

Answer 9

• Hypocalcemia
• Muscular weakness, ataxia
• Cardiac arrhythmias

• Surgery and injuries to the neck
• Resultant autoimmune disease
• Di-George syndrome (developmental)
• Low magnesium levels

• High-calcium diet and vitamin D supplements
• IV drip if necessary (muscle spasms, heart rate)
• Kidney stones, cataracts, dental problems, stunted growth

Question 10

Hypercalcitonism

Answer 10

All symptoms of hypoparathyroidism but weaker
Hypocalcitonism — Not recognized

Question 11

Rickets

Answer 11

• Malnutrition resulting in severe calcium and vitamin D deficiency that occur in young animals (before epiphyseal closure)
• Leads to bone malformations, enlarged and
“weakened” joints (mainly carpal and tarsal), costochondral swellings (“rickety rosary”), soft skull

Question 12

Osteomalacia

Answer 12

Results of vitamin D deficiency and low dietary calcium in older individuals

Question 13

Milk fever typically occurs in dairy cows during the ________________ period

Answer 13

periparturient

3-10% of dairy cows may be affected; may occur in beef cattle

Question 14

______________________ usually affects medium-sized dogs with large litters, approximately 3-4 weeks after whelping-untreated may be a life-threatening condition

Answer 14

Eclampsia (puerperal tetany)