Lecture 4 -- Growth hormone + Calcium and Phosphorous

What are the two broad classes of hormones?

Water soluble hormones and fat soluble hormones.

How do water soluble hormones function?

They bind to receptors on the surface of the cell.

How do fat soluble hormones function?

They mostly bind to receptors in the cytoplasm or nucleus of the cell.

What are the other differences between water soluble hormone and fat soluble hormone?

• Water soluble hormone have a shorter half life; fat soluble hormone have a longer half life

• Water soluble hormone need to activate second messenger systems within the cell

• Water soluble hormone is freely soluble in blood plasma; fat soluble hormone is protein bound in plasma

• Water soluble hormone has rapid action; Fat soluble hormone has less immediate action

• Water soluble hormone mostly metabolised by liver and excreted in kidney

What is the advantages of carrier proteins for hormones?

1. Act as a hormone reservoir → Ready to go

2. Act as a hormone barrier → Not completely inactive → Still remain some ability to bind to it receptors + left some residual effect to prevent excessive hormonal action

3. Reduce loss of hormone → Protein bound means it cannot be filtered in the renal corpuscle

What are the disadvantages of carrier protein

1. Affected by disruptions in blood levels of carrier protein

2. Competition from other substances e.g. drugs for carrier proteins

What hormones does the anterior pituitary secrete?

FSH, LH, ACTH, TSH, GH, and Prolactin.

What is growth hormone also known as?

Somatotropin.

What are common inputs that increase growth hormone secretion?

1. Input from CNS signals

2. Strenuous physical activity

3. Starvation

4. Stress

5. Thyroid hormones, androgen and oestrogen

6. Increase amino acid concentration

7. Decrease glucose and fatty acid

8. Ghrelin from parietal cells and hypothalamus

What hormone mediates the growth-stimulating effects of growth hormone?

Insulin growth factor 1 (IGF-1) produced from the liver

How is growth hormone (GH) produced and regulated?"

Hypothalamus produces GHRH → Stimulate the anterior pituitary gland to release GH → GH cause liver to produce IGF-1 (Insulin growth factor) → IGF-1 has negative feedback on GH and stimulate GHIH (somatostatin) to inhibit the production of growth hormone

What are the function of growth hormone?

→ Stimulates elongation of bone + cartilage growth+ soft tissue growth + growth of body mass

→ Important for growth of mammary glands → Lactation
→ Inhibits tissue uptake and utilisation of glucose
→ Increase lipolysis + protein synthesis

How can we obtain vitamin D2 and D3?

→ Vitamin D2 is produced by plant → When we consume plant, we may ingest vitamin D2
→ Vitamin D3 is produced by the action of UV light

How is vitamin D₃ converted into the active form of vitamin D?

After UV penetrates into the skin, it converts 7 dehydrocholesterol into vitamin D3 → 1st hydroxylation happens in liver, which converts vitamin D3 into calcidiol → 2nd hydroxylation then happens in the kidney, which converts calcidol into calcitriol, the active form of vitamin D

Is calcitrol water soluble?

No. It binds to vitamin D binding globulin.

What does calcitriol do?

1. Increase absorption of Ca2+ from the small intestine

2. Increase renal absorption of Ca2+

3. Increase mobilisation of calcium from bone → Osteoclast consume → Dissolve in bone and release Ca2+

What is the role of osteoclasts in calcium homeostasis?

They mobilize calcium from bone into the bloodstream.

Where is parathyroid gland located?

Sits on the thyroid gland
Four in total → Two on each sides

Where is parathyroid hormone (PTH) produced?

It is made by the chief cells of parathyroid gland

What does parathyroid hormone do?

Detect small change of free calcium in parathyroid gland
→ Release PTH
→ Increase the concentration of calcium in blood plasma by
1. increasing renal absorption of calcium
2. increasing intestinal absorption of calcium
3. stimulating osteoclast activity to release calcium from bones.

What stimulates the release of calcitonin?

High levels of free calcium in the blood.

Where is calcitonin produced in the body?

Parafollicular cells of the thyroid gland

What does the thyroid gland do when there is a high level of calcium in plasma?

Thyroid gland produces calcitonin via parafollicular cell → Reduce level of calcium by
1. Decrease gut reabsorption of calcium
2. Decrease Ca2+ reabsorption = More excretion via urine
3. Inhibit the action of osteoclasts on bone

How does the regulation of phosphate differ from calcium?

Phosphate regulation mainly depends on urinary excretion while regulation of calcium depends on differential absorption of calcium from the small intestine

How does PTH affect serum phosphate levels?

It decreases serum phosphate levels and increases urinary excretion.

What is the overall effect of parathyroid hormone on calcium and phosphate homeostasis?

Increases plasma calcium levels while decreasing serum phosphate.

What metabolic changes occur during lactation regarding calcium?

Mammary cells extract significant amounts of calcium from extracellular fluid.

What effect does increased free calcium in the blood have on PTH secretion?

It decreases PTH secretion.

Which cellular structures absorb calcium from the intestines in response to active vitamin D?

Enterocytes in the intestinal lining.

What is parathyroid hormone related protein (PTHrP)

It has same function as PTH

What type of cell produce PTHrP?

Certain types of cancers and cancer producing cells

What happens if there is excessive production of parathyroid hormone-related protein (PTHrP)?

It can lead to hypercalcemia due to lack of negative feedback mechanism

What does hydroxyapatite consist of?

A complex of calcium and phosphate that can be found in bone

What is the primary source of phosphate in the body?

Bone (85%), primarily as hydroxyapatite.

What process occurs during eggshell production in birds regarding calcium?

Calcium deposits are made from the blood into the eggshell

What physiological demands increase calcium homeostasis needs during lactation and late pregnancy?

1. Lactation → Mammary cells extract large amount of Ca2+ from extracellular fluid

2. Fetal skeleton mineralization occur during late pregnancy.