Calcium, Phosphate, Magnesium Flashcards
describe the relative proportion of Ca intracellularly vs extracellularly. why is this so?
very low intracellularly. so Ca can be used as a second messenger
where is most of the Ca in the body found?
in the skeleton 99%, .5% in cells, .1% in ECF
describe Ca in the plasma
plasma [Ca] is 2.5 mM.
50% is ionized
10% is w small anions
40% bound to albumin
where is Ca kept within cells?
sequestered in organelles
describe the mechanisms for removing intracellular Ca
- Ca/Na exchanger
2. Ca ATPase
describe the effect of hypocalcemia on APs
a decrease in membrane bound Ca lowers the threshold potential necessary for an AP d/t its effects on voltage gated Na channels
what are the effects of hypocalcemia
cramps/tetany intestinal cramps dry skin/brittle nails trousseaus sign decreased BP bronchospasms seizures arrhythmias w/ long QT
what are the symptoms of hypercalcemia
muscle weakness increased BP decreased GFR stones lethargy arrhythmias short QT
how does absorption of Ca in the gut differ from Na and K?
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
where does Ca absorption take place in the gut?
proximal small intestine- active absorption
describe the relative proportion of Ca reabsorbed in each part of the nephron
70% PCT
20% ThickALOH
5-10% DT
where is the site of regulation in Ca handling?
late DT
describe Ca reabsorption in the PT?
passive and paracellular
describe Ca reabsorption in the DT
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
describe the relationship between Na and Ca in different parts of the nephron
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
how do thiazide diuretics effect Ca handling?
they inhibit primarily on the NaCl transporters in the DT, which have an inhibitory effect on Ca reabsorption.
thus, thiazide inhibitors increase Ca reabsorption
how does acid/base status affect Ca?
- alkaline settings lower plasma Ca b/c it binds to albumin spots no longer occupied by H
- alkalosis opens Ca luminal channels in DT, causing reabsorption. opposite for acidosis
where is the majority of phosphate?
bone 84$, cells 14%, 1% ECF
what determines the proportion of P moving between ECF and ICF?
acid/base
base- causes P to move intracellularly via phosphofructokinase activity
describe proportional P uptake in the nephron
85% is taken up in the PT cotransported w/ Na
5% in DT, not regulated
what does PTH do and what stimulates it?
removes Ca and P from bone
upregulates Ca in DT
downregulates P in PT
activates calcitriol in PT
stimulated by low Ca
1-alpha-hydroxylase
enzyme in PT that converts calcitriol to active form in response to PTH
calcitriol
stimulates Ca reabsorption in the gut
describe Ca regulation as a function of time
PTH- handles short term
renal conservation and calcitriol- long term
describe P regulation
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
FGF23
inhibits P reabsorption
what is Mgs effect on K?
Mg inhibits apical K channels in the CD, reducing K secretion. Mg depletion can cause K depletion
what is Mgs effect on Ca?
Mg inhibits Ca channels, so Mg depletion = Ca sparing
60% of Mg is found in bone, but it is not readily exchangeable with plasma
ok
describe proportional Mg reabsorption throughout the nephron
15% in PT- paracellular
75% in LOH- paracelullar
10% DT- active
where is Mg uptake regulated?
early DT- regulated by Mg concentration
how does the kidney keep these heavy ions from precipitating out?
- manages their regulation at different sites
- 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
citrate is incompletely reabsorbed in nephron and usually in urine
ok
