Calcium and phosphate homeostasis: renal contribution

Question 1

What 3 organ systems are involved with calcium and phosphate reabsorption/secretion?

Answer 1

Intestine
Bone
Kidneys

Question 2

What 3 things regulate calcium and phosphate movement through these organ systems?

Answer 2

Parathyroid hormone
Calcitriol
Calcitonin

Question 3

Where is most of the calcium stored in in the body?

Answer 3

Bone

Question 4

What is bone resorption mediated by?

Answer 4

Osteoclasts

Question 5

What is bone formation mediated by?

Answer 5

Osteoblasts

Question 6

How is calcium found in the ICF?

Answer 6

Sequestered in organelles eg smooth ER
Complexed with macromolecules

Question 7

What is 40% calcium bound to in plasma?

Answer 7

Albumin
Globulin

Question 8

What happens in acidosis?

Answer 8

Increased H+ displaces Ca2+ from proteins -> increasing plasma conc. of ionized Ca2+

Question 9

What happens in hypo/hyperalbuminemia?

Answer 9

Hypo - increases ionized Ca2+
Hyper - decrease ionized Ca2+

Question 10

What two factors does Ca2+ homeostasis depend on?

Answer 10

Total amount of Ca2+ in the body
Distribution between bone and ECF

Question 11

What cell types are found in the thyroid gland?

Answer 11

Follicular cells and C cells

Question 12

What do c cells have?

Answer 12

Ca2+ sensing receptors

Question 13

What do C cells secrete and why?

Answer 13

Calcitonin - reduces Ca2+ release when plasma calcium is high

Question 14

What is the predominant cell type in parathyroid glands?

Answer 14

Principal (chief) cells

Question 15

What do principal cells secrete and why?

Answer 15

PTH - promotes Ca2+ release from bone when plasma calcium is low

Question 16

How does Vitamin D produce calcitriol?

Answer 16

PCT contains alpha 1 hydroxylase which converts 25-Hydroxycholecalciferol (from Vit D) into calcitriol

Question 17

What does calcitriol cause?

Answer 17

Increased Ca2+ and Pi absorption from small intestine
Increased Ca2+ and Pi release from bone
Decreased Ca2+ and Pi excretion

Question 18

What are other actions of increased PTH?

Answer 18

Increases calcitriol levels
Acts mainly on bone

Question 19

How is Ca2+ reabsorbed in the PT?

Answer 19

Paracellular - by solvent drag and due to shift in luminal voltage to +ve
Transcellular - Chaperoned binding to Calbindin

Question 20

What transporter is expressed on basolateral side of PT?

Answer 20

Ca2+ATPase transporter (PMCa1b)

Question 21

How does the TAL absorb Ca2+?

Answer 21

Transcellular
Paracellular - no solvent drag
CORTICAL only

Question 22

How is Ca2+ reabsorbed in DCT and CD?

Answer 22

Hormonally
PTH stimulates Ca2+ opening channels on apical membrane

Question 23

Where is most of the phosphate stored in the body?

Answer 23

Bone

Question 24

What 4 forms is phosphate present in the ICF?

Answer 24

Acid (H2PO4-)
Alkaline (HPO4^2-)
Inorganic
Organic - ATP, ADP and cAMP

Question 25

What 2 ways is phosphate homeostasis dependent on?

Answer 25

Total amount in body Distribution between ICF and ECF

Question 26

What are the 3 main regulators of phosphate?

Answer 26

PTH Calcitriol Calcitonin

Question 27

How is phosphate reabsorbed in PT?

Answer 27

Transcellular

Question 28

Where in the kidneys is negligible amounts of phosphate reabsorbed?

Answer 28

LoH CD

Question 29

What transporter is expressed on the apical membrane for phosphate reabsorption in PT?

Answer 29

NPT2

Question 30

What hormone is Pi reabsorption decreased by?

Answer 30

PTH