"Medical Physiology Parathyroid Hormone Joseph Spadaro" 3/23

Question 1

How does acidosis increase free ionized calcium?

Answer 1

The lower pH blocks Ca binding to albumin.

Conversely, alkalosis may bind too much ionized calcium.

PO4 also binds to calcium and can lower free ionized calcium levels, acting like a buffer.

Question 2

Why does higher than normal albumin suggest a falsely positive hypercalcemia?

Answer 2

Total calcium may be higher, because there is more albumin to bind to, but ionized calcium, the true indicator for hypercalcemia, may be normal.

Question 3

T/F: Patients with FHH, familial hypercalcemic hypocalcuria have high serum calcium but no symptoms of hypercalcemia.

Answer 3

True.

Patients with FHH have higher than normal calcium serum levels, but this does not effect their PTH secretion, which remains normal.

Question 4

What are the symptoms of hypocalcemia?

Answer 4

Learning retardation, apnea (children)
Tetany, siezure, muscle cramps, numbness, spasms
Long QT interval, reduced cardiac output
Weak bone development, osteomalacia

Question 5

What are the symptoms of hypercalcemia?

Answer 5

Fatigue, anorexia, delirium, coma
Headache, intracranial pressure, muscle weakness
Hyperpolarization leading to reduced neural responses
Bradiacardia, short QT internal
Polydipsia, Polyuria, HTN, calculi

Question 6

In normal physiology, when is PTH high?

Answer 6

PTH secretion from the parathyroid gland (chief cells) increases when serum calcium is low.

The targets are (1) the kidney (fast), and (2) the bone (slow).

Question 7

What are the 3 effects of PTH on the kidney?

Answer 7

PTH:

1. increases Ca absorption from the distal tubule
2. inhibits PO4 absorption from the proximal tubule
3. increases vitamin D synthesis (to its active form)

Question 8

In general, PTH effects on bone are slow. What is the fast effect of PTH on bone?

Answer 8

Osteocytic osteolysis

Question 9

How does PTH influence osteoclast activity?

Answer 9

PTH’s main effect is on cells of osteoblastic lineage. These cells secrete paracrine agents is RANK-L that cause their maturation into active multi-nucleated osteoclasts, which resorb bone on the surfaces. This process slows in response to normalizing calcium levels.

Question 10

How does the calcium sensor on the parathyroid gland function?

Answer 10

The calcium sensor on the parathyroid gland is a GPCR. Other inputs for PTH maintenance include high PO4 and low Mg, which both increase PTH release.

Question 11

Would high active vitamin D increase or decrease PTH release?

Answer 11

Decrease

Question 12

Where does the majority of calcium re-absortion occur in the kidney?

Answer 12

60% of daily calcium re-absorption occurs in the proximal tubule, via active transport, but this is not where PTH exerts its main effect on calcium. PTH affects the 9% of calcium normally re-absorbed in the distal tubule, through a channel.

Question 13

Low PTH would have what effect on the proximal tubule?

Answer 13

Low PTH would allow more re-absorption of PO4 in the proximal tubule.

Question 14

How does vitamin D act to support calcium reabsorption in the DCT?

Answer 14

Vitamin D acts on the basolateral side to put re-absorbed calcium into the blood, so that there is a gradient for movement of more calcium from the lumenal side. Calcium moves through the calbindin transporter.

Question 15

Why is it important for PTH to decrease PO4 re-absorption from the PCT?

Answer 15

Ultimately, PTH increases result in both high serum calcium and phosphate due to bone resorption and calcium/PO4 absorption from the small intestine. Therefore wasting PO4 in the PCT keeps the PO4 from getting too high.

Question 16

The two major targets of vitamin D are:

Answer 16

The two major targets of vitamin D are:

1. small intestine
2. bone

In both instances, vitamin D (active) acts to cause calcium and phosphate to travel from the lumen into the plasma.

Question 17

What is the effect of vitamin D on the parathyroid gland?

Answer 17

Vitamin D negatively feeds back to the parathyroid gland to reduce PTH release, acting as a “buffer” to low calcium levels.

Question 18

How does vitamin D affect osteoclast activity?

Answer 18

It acts on osteoblast receptors, and induces cell signaling to convert blasts to clasts.

Question 19

Cholecalciferol, a vitamin D precursor, is also known as:

Answer 19

Vitamin D3, and is inactive until it becomes 1,25-OH2-vitamin D aka calcitrol.

D3 comes from cholesterol in the skin, and needs UV light to proceed down the path of vitamin D synthesis. Vitamin D synthesis occurs most notably in the kidney,but there is redundancy of this system in other tissues. The enzyme critical to this conversion to active vitamin D is 1-alpha-hydroxylase, the activity of which in creased when PTH acts on the kidney.

Question 20

How does vitamin D increase calcium and phosphate absorption from the small intestine?

Answer 20

Vitamin D increases the synthesis of the calbindin transporter.

Technically the increase in phosphate absorption is not needed or desirable.

Question 21

How does Rickets cause bone malformation?

Answer 21

Rickets is a chronic deficiency of vitamin D and/or the dietary deficiency of calcium/phosphorous during early development. The poor mineralization of bone results in bowing due to gravity of body on top of the legs. Children also have abnormal growth plates.

In adults, poor mineralization results in osteomalacia.

Question 22

The following symptoms and labs suggest what dx?
high serum calcium
low serum phosphate
high serum PTH
high alk phos
hypercalciuria
kidney stone

Answer 22

Primary hyperparathyroidism, such as that from a PTH secreting adenoma. Too much calcium resulted in the precipitation of calcium in the form of a kidney stone.

Question 23

PTH secretion in response to low calcium has what effect on urinary cAMP?

Answer 23

Increases urinary cAMP

Question 24

The following symptoms and labs suggest what dx?
Serum calcium high
serum phosphate low
serum PTH low
alk phos very high
lethargy
ongoing cancer

Answer 24

Humoral hypercalcemia of malignancy. The tumor cells are secreting a PTH analog (PTH-rp). The real PTH was inhibited by the high calcium.

Question 25

What is the difference between primary and secondary hyperparathyroidism?

Answer 25

Primary hyperparathyroidism is high PTH, such as a PTH-secreting adenoma.

Secondary hyperparathyroidism is mainly a hypocalcemia, perhaps associated with low vitamin D, renal failure, diet etc.

Question 26

What is the defect in hypoparathyroidism?

Answer 26

Low calcium, but no appropriate PTH response, perhaps from surgical disruption of the parathyroid gland, or a genetic defect.

Pseudohypoparathyroidism is similar to this in that they are both related to hypocalcemia. In this dx, there is a genetic defect in the GPCR PTH receptor in the kidney. The PTH is appropriately elevated.

Question 27

What is the life cycle of osteoclasts?

Answer 27

Osteoclasts form from monocytes of the blood or bone marrow. They have a lifespan of only a few days, during which time they participate in bone breakdown and remodeling. They do this by secreting acidic molecules to dissolve the mineral, and proteases to digest and phagocytize the collagen matrix. Mature osteoclasts do not divide.

Question 28

What elements influence RANKL to form new and activate existing osteoclasts?

Answer 28

Growth factors
Hormones
Cytokines
RANK –> binds to osteoblasts –> ostoeblasts release RANKL.

Question 29

What is the feedback mechanism for RANKL?

Answer 29

Osteoblasts produce OPG, osteopreotegerin, which binds to and inhibits RANKL.

Question 30

How do glucocorticoids affect bone remodeling?

Answer 30

Glucocorticoids inhibit calcium uptake from the small intestine, which can lead to low calcium and therefore brittle bones.

Question 31

For healthy adults, what is the main way bone strength is maintained?

Answer 31

Mechanical loading. The osteocyte and canaliculae are highly sensitive to fluid shear. Osteocytes reduce the presence of sclerostin, which inhibits bone formation with bone loading is weak.

Question 32

How does estrogen influence bone remodeling?

Answer 32

Osteoblasts have estrogen receptors, and the effect of estrogen is to reduce resorption (hence postmenopausal women have more bone resorption leading to osteoporosis).