Calcium and Phosphate Homeostasis and Derangement Flashcards
Where is most of the body’s calcium found?
99% of total body calcium found in mineralized osteoid of bone.
1% important in the cell (exchangeable pool)
What is calcium needed for inside the cell?
Needed for muscle contraction, gland secretion, peristalsis, complement activation, and enamel formation.
How much calcium intake daily?
1000 mg/d.
How is calcium excreted?
700 mg/d fecal
300 mg/d urine
so balance is 0 in a healthy adult. In children, it is positive and in elderly it is negative
Normal serum Ca levels?
8.7-10.2 mg/dl
Normal ionized Ca component levels?
4.8-5.2 mg/dl
Of the 1% of non-bone retained calcium, the majority is intracellular. How does the extracellular Ca+ exist?
roughly half is protein bound (and hence inactive) and ~half is in the free form (hence physiologically capable of involvement in processes).
Well, a small part of the free form is also complexed to proteins but a major part is in the ionized free form and this is the component that is active.
How does calcium enter enterocytes from the intestinal lumen?
TRPV6 channel through the brush border membrane along a favorable electrochemical gradient.
How is Ca released from the BL side of the enterocyte?
Pumped out of the cell at the BL side against a steep electrochemical gradient by the Ca2+-ATPase.
What happens when there is a major elevation of intracytoplasmic Ca2+ in the enterocyte?
Ca2+ leaves the cell using a Na+-Ca2+ exchanger instead on the BL.
What else controls Ca efflux (and influx to some extent)?
Calcitriol, which binds the vitamin D receptor (VDR) which promotes absorption
The late part of the distal convoluted tubule (DCT) and connecting tubule (CNT) play an important role in fine tuning renal excretion of Ca2+ and represents the bulk of ACTIVE reabsorption of Ca2+. How?
The epithelial Ca2+ channel (TRPV5) is primarily expressed apically in these segments and functions with calbindin- D28K (28K) on the apical surface and Na+/Ca2+ exchanger (NCX1), and the plasma membrane ATPase (PMCA1b) on the BL
55% of Ca2+ reabsorbed in the PT-passive
25% in the Thick ALH-mainly passive
Describe the absorption of Ca2+ in the distal segments
Upon entry via TRPV5, Ca2+ is buffered by 28K and diffuses to the basolateral membrane where it is released and extruded by a concerted action of NCX1 and PMCA1b.
What else does the BL membrane express?
In addition, the BL membrane exposes a parathyroid hormone receptor (PTHR) and the Na+/K+-ATPase consisting of the α-, β-, and γ-subunit.
What does PTHR do?
PTHR activation by PTH stimulates TRPV5 activity, and entered Ca2+ can subsequently control the expression level of the Ca2+ transporters.
What is BK2? Function?
At the apical membrane, a bradykinin receptor (BK2) is activated by urinary tissue kallikrein (TK) to activate TRPV5-mediated Ca2+ influx.
How is Ca2+ regulated In the cell?
entered Ca2+ acts as a negative feedback on channel activity, and 28K plays a regulatory role by association with TRPV5 under low intracellular Ca2+ concentrations.
What is Klotho? Function?
Extracellular urinary Klotho directly stimulates TRPV5 at the apical membrane by modification of the N-glycan, and intracellular Klotho enhances Na+/K+-ATPase surface expression that in turn activates NCX1-mediated Ca2+ efflux.
What are calcium sensing-receptors (CaSRs)? Function?
Receptor on cells. Activation of the CaSR by calcium stimulates phospholipase C, leading to increased IP3, which mobilizes intracellular calcium and inhibits parathyroid hormone (PTH) synthesis.
A decrease in serum calcium inhibits intracellular signaling, leading to increased PTH synthesis and secretion.
How does the body respond to increased plasma Ca2+?
- activation of CaSR
- leads to decreased PTH and increased fractional renal Ca excretion and calcitonin levels
What does decreased PTH/increased calcitonin lead to?
decreased mobilization of Ca from skeleton and soft tissue
and decreased renal 25-hydroxylation of vitamin D precursor
What is the main function of vitamin D?
Ca intestinal absorption
T or F. Serum Ca decrease slowly with age from infancy
T.
What is the distribution of phosphorus in the body?
- 85% bone
- 14% soft tissue
- 1% ECF
total 700g
Where is intracellular phosphorus stored?
in cell membrane phospholipids mainly
Uses of phosphorus in the cell?
- mediator of Pi transfer (ATP/ADP, etc.)
- DNA/RNA backbone linkage
- Ionized inorganic phosphate also serves as a buffer to maintain the proper pH of body fluids.
What is a very important use of phosphorus in RBCs?
The presence of 2,3-diphosphoglycerate in red blood cells facilitates the release of oxygen from oxyhemoglobin into body tissues.
Daily intake of phosphorus
800-1500 mg
How well is phosphorus absorbed?
At intake levels below 10 mg/kg/day, 80% to 90% is absorbed, while at intake levels above 10 mg/kg/day, only about 70% is absorbed via a Na-Pi transporter (we don’t know what the BL transporter is).
There are also paracellular routes so intake cannot be fully stopped with drugs or disease
Where in the GI does phosphorus uptake occur?
Absorption occurs throughout the intestinal tract, but primarily in the jejunum.
Again, A variable amount of dietary phosphorus (10% to 30% of the ingested amount) is excreted through the GI tract.
Keep in mind that it is the same transporter that is responsible for kidney reabsorption as is used in GI absorption
What is the usual phosphorus urinary excretion in percentage of filtered load?
As blood is filtered through the renal glomerulus, 85% to 95% of phosphorus is partially resorbed at the PT and is not further reabsorbed until the distal tubule. The usual net load of phosphorus excreted is 5% to 15% of the filtered load.
Not any in the ascending limbs
T or F. Hyperphosphatemia is rare
T. Usually won’t occur until the GFR declines to less than 25 mL/min in patients with chronic renal insufficiency,
Normal phosphorus levels?
2.5-4.5 mg/dl above GFR of 25 ml/min
Normal excretion routes of phosphorus
assume 1400g daily intake
900g urine
500g fecal
How is phosphate absorbed into enterocytes?
Phosphate enters the enterocyte (influx) through the brush border membrane by the Na+-Pi cotransport system, with a stoichiometry of 2:1, operating against an electrochemical gradient.
How does Phosphate exit at the basolateral side?
possibly occurs by passive diffusion or (more probably) by anion exchange.
Renal handling of phosphate
Tubular reabsorption of phosphate is saturable
With GFR>40 ml/min, TmP varies proportionately with GFR (TmP/GFR constant)
With GFR less than 40, the kidneys cannot remove enough phosphate and hyperphosphatemia ensues
How do we get vitamin D?
diet and sun (pro hormone in these forms)
Metabolism of vitamin D.
Cholesterol is converted to 7-dehydrocholesterol which is converted to vitamin D3 via sunlight and diet gives vitamin D2 from plants and yeasts and vitamin D3 from fatty fish
Both Vitamin D2 and D3 bind to vitamin D binding protein (VDBP) and circulate to the liver.
What happens once the vitamin D3 and D2 complexes reach the liver?
In the liver, vitamin D is hydroxylated by liver 25-hydroxylase (CYP27A1) to 25(OH)D, commonly referred to as calcidiol.
What happens to calcidiol?
Calcidiol is then further metabolized to calcitriol by the tissue 1α-hydroxylase enzyme (CYP27B1) in the PT of the kidney.
The active metabolite 1,25(OH)2D3 (calcitriol) acts principally on the target organs of intestine, the parathyroid gland, osteoclast precursors, and the kidney.
OR it can be converted to 24,25(OH)2D which a very mildly active form of vitamin D (not common)
Can activated vitamin D3 be inactivated/neutralized?
Yes, Calcitriol is metabolized to the inert 1,24,25(OH)3D through the action of the 24,25-hydroxylase enzyme (CYP24).
Calcidiol is similarly hydroxylated to 24,25(OH)2D.
What receptors in the PT tubular cell initiates endocytosis of DBP/25-OH-D3 complex?
megalin and cubulin. And then CYP27B1 acts
This is found in target cells (and the kidneys) that are capable of activating caltriciol from the inactive form
What happens to activated calcitriol?
It circulates bound to DBP. 1,25(OH)2D3 is then taken up by target cells and targeted to intracellular D-binding proteins (IDBP) to mitochondrial 24-hydroxylase or to the VDR.
What does the 1,25(OH)2D3-VDR complex do?
It heterodimerizes with the retinoic acid receptor (RXR) and binds to specific sequences in the promoter regions of the target gene.
The DNA bound heterodimer attracts components of the RNA polymerase II complex and nuclear transcription regulators.
What is the initial PTH precursor product of the parathyroid?
A 84-aa protein that gets cleaved into N-terminal, C-terminal, and mid-region fragments
What do first-generation PTH assays detect?
Full-length PTH (1-84) in addition to PTH fragments.
These assays include radioimmunoassays (RIAs) that use antisera specific to the amino-terminal (N-RIA), middle (MID-RIA), or carboxyl-terminal (C-RIA) regions of PTH.
What do second generation “intact PTH” assays detect?
full-length PTH (1-84) and non–(1-84) PTH fragments.
What do third generation “intact PTH” assays detect?
Third-generation PTH assays (biointact PTH) detect only full length PTH (1-84).
What does PTH bind to?
the PTH1R (GPCR)- increases cAMP, PKA, and then PKC to increase intracellular calcium (widely expressed in several tissues including the kidneys)
Where is PTH1R found?
many tissues including the kidney and osteoblasts
How does PTH regulate serum Ca levels?
Increased Ca2+ binds to the CaSR and causes IP3 to release SR calcium to decrease PTH. If Ca is low, this is blocked and PTH acts to
increase bone resorption,
increase renal calcium reabsorption and decrease Pi reabsorption,
and up regulate 1a hydroxylase in the kidney, thereby increasing gastrointestinal calcium and Pi absorption.
FGF (Fibroblast growth factor) exerts its effect in what fashion?
paracrine via binding to one of four distinct FGF receptor tyrosine kinase gene products (Fgfr1 to Fgfr4)
What does FGF binding to its receptors require?
heparin
Are any FGFs heparin independent? Why?
The FGF19 subfamily members (consisting of FGF19, FGF21, and FGF23) are heparin independent, resulting from unique structural features permitting them to circulate and act as endocrine factors:
FGF19: energy and bile acid homeostasis;
FGF21: glucose and lipid metabolism;
FGF23: phosphate and vitamin D homeostasis
Where is FGF23 found in the genome?
12p13
What is the function of FGF23?
- Phosphaturic
- Suppressor of 1-alpha hydroxylase in the kidney
- Reduced renal phosphate reabsorption
- Prevents soft tissue calcification
Where is FGF23 mostly expressed?
Predominately expressed in osteocytes, but also ventrolateral thalamic nucleus, central venous sinusoids and thymus.
What is an important cofactor for FGF23 action? Function?
Klotho - single pass TM protein necessary for FGF23-medicated receptor activation (stimulated by high calcitriol levels)
decreases Pi PT reabsorption and 1a hydroxylase activity
What is the difference between intermittent and constant production of PTH?
intermittent PTH release promotes serum Ca+ increase but constant release can have a counter effect
What are the parts of bone?
- crystaline part
- amorphous part
- water (30%)
the crystalline part is responsible for the exchangeable pool in the body
When the amount of Ca2+ in the GI lumen is very high OR there is very low blood Ca2+ (aka there is a large gradient), what happens?
Paracellular absorption dominates (cant be saturated) over TRPV6. Because you want to absorb everything you can
How do the kidney react to large increases in serum Pi? can they reabsorb it all?
No, tubular reabsorption of phosphate is saturable
When GFR>40 ml/min, the amount reabsorbed is proportional that amount being taken in from the GI (i.e. the more taken up, the more excreted). When GFR is less than 40, more phosphate is reabsorbed and hyperphosphatemia ensues
What are the regulators of renal phosphate handling?
- PTH
- Dopamine
- Phosphatonins (FGF-23, FGF-7)
FGF-23 Function
Secreted by osteocytes in response to high phosphorus/ vitamin D3 signals. Binds to the heterodimer of FGF receptor and Klotho and the binding causes reduction of the NaPi cotransporter on the lumenal surface of the PT and down regulates 1a hydroxylase to reduce serum Pi levels
Cleaved by furins when phosphate levels are normal to inactivate it.
What cells can block vitamin D production?
melanocytes in skin
T or F. While the kidneys are responsible for the vast (~90%) of vitamin D activation, there are target cells in the body that are capable of producing their own
T.
What are some things that decrease PTH secretion?
- calcium via CaSR
- calcitriol
- FGF23 (direct inhibition of PTH mRNA expression)
What are some things that increase PTH secretion?
- high phosphorus (pathologic in chronic kidney disease)
- low calcium
- FGF23 (decreased calcitriol synthesis)
What are the effects of PTH (from low Ca2+)?
- increased 1a hydroxylase activity to increase calcitriol which increases GI Ca and PO4 absorption
- increased bone turnover to increase serum Ca and PO4
- increased renal Ca reabsorption and decreased PO4 reabsorption
SO the end game is increased Ca and NORMAL/slight decrease in PO4*
What are the effects of low vitamin D?
-low vitamin 25(OH) D3 leads to low Ca2+ levels leading to increased PTH secretion and thus, dropping of phosphorus from the kidney
Note that the high levels of PTH can stimulate kidney 1a hydroxylase to make calcitriol from the low amounts of dicitriol as a compensatory mechanism
THIS CHANGES IN CKD
What stimulates production of FGF23?
1,25(OH)2D, possible PTH(could be cause or effect), bone metabolism
Adverse effects of FGF23?
- inflammation
- vascular disease
- increased RAAS
- metabolic disorders via KLOTHO downregulation
What does FGF23 do to KLOTHOS?
down regulates it through negative feedback
What happens to FGF23 levels in CKD?
increased as phosphorus levels rise dramatically
How does PTH increase bone turnover?
It binds to a receptor on osteoBLASTS to increase RANK and downregulate its inhibitor osteoproteregin. RANK then binds to RANKL to promote osteoclast formation
