Calcium, Phosphate, Magnesium

Question 1

describe the relative proportion of Ca intracellularly vs extracellularly. why is this so?

Answer 1

very low intracellularly. so Ca can be used as a second messenger

Question 2

where is most of the Ca in the body found?

Answer 2

in the skeleton 99%, .5% in cells, .1% in ECF

Question 3

describe Ca in the plasma

Answer 3

plasma [Ca] is 2.5 mM.

50% is ionized
10% is w small anions
40% bound to albumin

Question 4

where is Ca kept within cells?

Answer 4

sequestered in organelles

Question 5

describe the mechanisms for removing intracellular Ca

Answer 5

1. Ca/Na exchanger

2. Ca ATPase

Question 6

describe the effect of hypocalcemia on APs

Answer 6

a decrease in membrane bound Ca lowers the threshold potential necessary for an AP d/t its effects on voltage gated Na channels

Question 7

what are the effects of hypocalcemia

Answer 7

cramps/tetany
intestinal cramps
dry skin/brittle nails
trousseaus sign
decreased BP
bronchospasms
seizures
arrhythmias w/ long QT

Question 8

what are the symptoms of hypercalcemia

Answer 8

muscle weakness
increased BP
decreased GFR
stones
lethargy
arrhythmias short QT

Question 9

how does absorption of Ca in the gut differ from Na and K?

Answer 9

Na and K are fully absorbed, but only 350 mg of Ca is absorbed.

there is also a secretory flux of ~150 mg Mg, leading to only a net 200 mg increase

Question 10

where does Ca absorption take place in the gut?

Answer 10

proximal small intestine- active absorption

Question 11

describe the relative proportion of Ca reabsorbed in each part of the nephron

Answer 11

70% PCT
20% ThickALOH
5-10% DT

Question 12

where is the site of regulation in Ca handling?

Answer 12

late DT

Question 13

describe Ca reabsorption in the PT?

Answer 13

passive and paracellular

Question 14

describe Ca reabsorption in the DT

Answer 14

transcellular and active

enter through a Ca channel and exit through a Ca ATPase or Na/Ca exchanger

in order to keep free Ca low in the cell, Ca binds to calbindin, allowing the diffusion gradient to stay

Question 15

describe the relationship between Na and Ca in different parts of the nephron

Answer 15

Na reabsorption in the PT drives paracellular Ca reabsorption and they are coupled together (Na is preferentially reabsorbed, leaving Cl behind, concentrating Cl, causing it to leave paracellularly, leaving a + charge behind, driving paracellular cation exit

in the DT, Na and Ca reabsorption is opposite. Ca relies on a Ca/Na exchanger for basal removal. Na is absorbed w/ Cl in the DT, which lessens the gradient. simultaneously, the Cl leaves through the basal Cl channel, creating a net positive effect discouraging Ca uptake

Question 16

how do thiazide diuretics effect Ca handling?

Answer 16

they inhibit primarily on the NaCl transporters in the DT, which have an inhibitory effect on Ca reabsorption.

thus, thiazide inhibitors increase Ca reabsorption

Question 17

how does acid/base status affect Ca?

Answer 17

1. alkaline settings lower plasma Ca b/c it binds to albumin spots no longer occupied by H
2. alkalosis opens Ca luminal channels in DT, causing reabsorption. opposite for acidosis

Question 18

where is the majority of phosphate?

Answer 18

bone 84$, cells 14%, 1% ECF

Question 19

what determines the proportion of P moving between ECF and ICF?

Answer 19

acid/base

base- causes P to move intracellularly via phosphofructokinase activity

Question 20

describe proportional P uptake in the nephron

Answer 20

85% is taken up in the PT cotransported w/ Na

5% in DT, not regulated

Question 21

what does PTH do and what stimulates it?

Answer 21

removes Ca and P from bone
upregulates Ca in DT
downregulates P in PT
activates calcitriol in PT

stimulated by low Ca

Question 22

1-alpha-hydroxylase

Answer 22

enzyme in PT that converts calcitriol to active form in response to PTH

Question 23

calcitriol

Answer 23

stimulates Ca reabsorption in the gut

Question 24

describe Ca regulation as a function of time

Answer 24

PTH- handles short term

renal conservation and calcitriol- long term

Question 25

describe P regulation

Answer 25

in the PT, there is a distinct Tm on Na/P cotransporters. the Tm is right at the resting splay point, so any extra P is excreted

PTH removes Na/P cotransporters, causing loss of P

Tm also regulated directly by P- hyperP causes a lowering of Tm and vice vresa

Question 26

FGF23

Answer 26

inhibits P reabsorption

Question 27

what is Mgs effect on K?

Answer 27

Mg inhibits apical K channels in the CD, reducing K secretion. Mg depletion can cause K depletion

Question 28

what is Mgs effect on Ca?

Answer 28

Mg inhibits Ca channels, so Mg depletion = Ca sparing

Question 29

60% of Mg is found in bone, but it is not readily exchangeable with plasma

Answer 29

ok

Question 30

describe proportional Mg reabsorption throughout the nephron

Answer 30

15% in PT- paracellular
75% in LOH- paracelullar
10% DT- active

Question 31

where is Mg uptake regulated?

Answer 31

early DT- regulated by Mg concentration

Question 32

how does the kidney keep these heavy ions from precipitating out?

Answer 32

1. manages their regulation at different sites
2. CaSRs in 2 places

• ThickALOH- basal side where they decrease Ca and Mg absorption when activated
• CD- luminal side- decrease ADH-mediated water reabsorption when activated, washing out Ca and Mg. also stimulates H secretion to prevent salt formation

Question 33

citrate is incompletely reabsorbed in nephron and usually in urine

Answer 33

ok