ca2+ and phosphate

Question 1

calcium and phosphate levels are regulated by what three main compounds

Answer 1

• parathyroid hormone
• calcitonin
• vitamin D

Question 2

where is calcitonin created and secreted from

Answer 2

thyroid gland

Question 3

where is parathyroid hormone created and secreted from

Answer 3

parathyroid

Question 4

what does humoral secretion of hormones means

Answer 4

• hormone secretion set by concentration of substance in blood and whether it is above or below the setpoint

Question 5

what triggers parathyroid hormone release

Answer 5

low plasma Ca2+ triggers hormone release from parathyroid gland

Question 6

PTH is synthesized in what cells

Answer 6

• chief cells
  
  • ca2+ receptors are located on chief cells

Question 7

PTH is synthesized as

Answer 7

preproPTH

Question 8

how does increases in extracellular CA2+ inhibit PTH secretion

Answer 8

1. ca2+ binds to receptor coupled to G protein -> PLC activated -> IP3 increases Ca2+ which inhibits release of granules containing PTH
2. increased Ca2+ also inhibits PTH synthesis

Question 9

where is parathyroid gland located

Answer 9

four parathyroid glands lie on the posterior side of the thyroid

Question 10

what is the main effect of PTH

Answer 10

• promote bone resorption releasing both Ca²⁺ and PO₄ into ECF
• also promotes Ca2+ reabsorption at the kidney

Question 11

PO4 released from bone has what effect on free Ca2+

Answer 11

• PO4 released during bone resorption will bind with Ca2+ in the ECF limiting rise in free Ca2+
• therefore, PTH inhibits PO4 reabsorption at the kidney to limit the rise in ECF PO4

Question 12

PTH actions on the kidney

Answer 12

1. inhibit PO4 reabsorption
2. stimulates Ca2+ reabsorption

• net effect: fall in plasma PO4 and rise in plasma free Ca2+

Question 13

vit D has affects on what organs

Answer 13

• bone
• kidney
• intestine

Question 14

vit D actions on intestine

Answer 14

• activated vit D stimulatues Calbindin synthesis, which then increases intestinal absorption of Ca2+ and PO4

Question 15

the small intestine absorbs Ca2+ by two mechanisms. which one is not under control of Vit D

Answer 15

• the passive, paracellular absorption of Ca2+ that occurs throughout the small intestine

Question 16

the small intestine absorbs Ca2+ by two mechanisms. which one IS under control of Vit D

Answer 16

• active, transcellular absorption of Ca2+ that occurs in the duodenum
• active form of vit D: D-25-dihydroxyvitamin D stimulates all three of the following steps
  
  • ca2+ enters cell across apical membrane through a channel
  • inside cell, Ca2+ in buffered by binding proteins and is taken up into intracellular organelles
  • Ca2+ exits cell through a Na-Ca exchanger

Question 17

what enzyme is responsible for making vit D active

Answer 17

1-alpha-hydroxylase

Question 18

where can vit D come from

Answer 18

• 7-dehydrocholesterol acted on by UV light -> cholecalciferol
• cholecalciferol from diet

Question 19

actions of vit D on kidney

Answer 19

• promotes Ca2+ reabsorption from distal tubule
• negative feedback: inhibits 1-hydroxylase required for activation of vit D

Question 20

actions of vit D on bone

Answer 20

• direct effect: mobilize Ca2+ from bone (bone resorption)
• indirect effect: promotes bone mineralization by providing more Ca2+

Question 21

PTH has what 3 effects on the kidney

Answer 21

1. protmoes Ca2+ reabsorption and thus an increase in plasma Ca+
2. inhibits Pi reabsorption
3. promotes the hydroxylation of 25-hydroxyvitamin D, thereby creating the active metabolite of vit D

Question 22

PTH effect on bone

Answer 22

promotes net bone resorption and hence increases plasma Ca2+

Question 23

PTH actions of intestine

Answer 23

• the active form of vit D, produced indurectly as the result of PTH) enhances CA2+ absorption

Question 24

calcitonin is produced by what cells

Answer 24

parafollicular cells (C cells) of thyroid gland

Question 25

effects of calcitonin

Answer 25

* \*\*\*decrease in plasma Ca2+ levels * inhibits bone resorption of ca2+ and phosphate * stimulates excretion of Ca2+ and PO4 in urine

Question 26

what is rickets

Answer 26

**vitamin D deficiency in childhood** * insufficient ca2+ and PO4 to mineralize growing bone * hypocalcemia from decresed Ca2+ and PO4 absorption from gut * growth failutre and skeletal anomalies * elevated PTH

Question 27

tx of rickets

Answer 27

proper diet and vit D supplements

Question 28

what is osteomalacia

Answer 28

* deficiency of vit D in adulthood * causes: dietary deficiency, intestinal sx, lack of sunlight, chronic renal failure * presentation * soft, weak bones, frequent fx

Question 29

tx of osteomalacia

Answer 29

* vit D * calcium * phosphate supplements

Question 30

what is hypocalcemic tetany

Answer 30

* decreased extracellular ca2+ destabilizes neuronal membranes, lowers threshold * peripheral nerves begin to discharge spontaneously * skeletal muscles contract tetanically (successive contractions that are fused together)

Question 31

drinking a lot of colas can have what effect on ca2+ concentration

Answer 31

* excess dietary intake of PO4 can lead to decrease in plasma Ca2+ * PO4 inhibits activation of vit D * decreased plasma Ca2+ -\> increased PTH secretion * leads to phosphaturia * ca2+ and PO4 mobilized from bone due to elevated PTH

Question 32

clinical signs of hyperparathyroidism

Answer 32

* "**stones, bones, groans"** * hypercalcemia * hypophosphatemia * osteoporosis * kidney stones * muscle weakness * decreased muscle excitability

Question 33

clinical presentation of hypoparathyroidism (usually seen post thyroidectomy)

Answer 33

* hypocalcemia * hyperphosphatemia * tetany * convulsions * tx: calcium and vit D

Question 34

what is pseudohypoparathyroidism

Answer 34

* G protein receptors on target tissues not functional * disturbances * elevated PTH * hypocalcemia * hyperphosphatemia * growth abnormalities