Summary Physiology Advanced Concepts

7/25/2019 Summary Physiology Advanced Concepts

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PAC Hertentamen Notes

Lecture 1 Endothelium function

Without endothelium, relaxation after contraction takes longer.Binding of acetylcholine to G/protein receptor leads to activation of IP3, which activates NO

synthase. NO goes into SMCs and activates guanylyl cyclase, which in turn converts GMP to

cyclic GMP. This activates protein kinase G, which leads to muscle relaxation.

eNOS (endothelial NO synthase) can be activated

through two different pathways:

1.

Ca2+/Calmodulin by acetylcholine and bradykinin

2. PI(3)K/Akt pathway by shear stress and

VEGF

Also leads to inhibition of proliferation and vascular

remodeling.

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Ach can lead to production of NO and vasodilation. When endothelial layer is damaged the

Ach binds directly to receptor on SMC and this causes vasoconstriction.

Vasoprotective effects of nitric oxide:

-

Dilates blood vessels;- Reduces oxidation of LDL cholesterol (prevents plaque formation);

- Reduces release of superoxide radicals (O2-);

- Reduces multiplication of SMCs;

o PDGF

o

bFGF

o

TGFβ

o Prostacyclin

o Shear stress

- Reduces monocyte stickiness (prevents plaque formation);

-

Reduces platelet stickiness.

Platelet activation


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Blood clotting inhibitory factors

Anti-thrombin binds to thrombin and thereby inhibits blood clotting.

Active protein C inhibits V conversion to Va and VIII conversion to VIIIa.


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Hyperemia: when blocking blood flow, after release the blood vessel should dilate. If not

endothelial dysfunction.

See lecture slide with endothelial dysfunction tests.

See course book 3.11 measurement of endothelial function page 21

3.13 Consequences of endothelial dysfunction page 23!

EndoPAT: hyperemic finger (established with cuff) and control finger. In dysfunction:

amplitude of signal is lower. (non-invasive)

Flow-mediated dilation test: place cuff on arm, block blood flow. Echo above occlusion.

Release of occlusion, measurement of lumen. Dilation should be seen. (non-invasive)

Venous occlusion plethysmography: clamp of vein but not artery. Cuff at wrist and upper

arm. With injection of compound see diameter change of vessels. (invasive)

Quantitave coronary angiography: cathether. Administration of Ach. Constriction when

endothelium is damaged.

Factors in endothelial dysfunctionAge, gender, race, genetic background, family history, dyslipidemia, hyperhomocysterinemia.

Inactivity, diet, smoking.

Diabetes and endothelial dysfunction.

Insulin activates eNOS via PI3K pathway.

Free fatty acids lead to ONOO- formation

Lecture 2 NOS

3 types of NOS - NOS1 = nNOS = neuronal NOS

-

NOS2 = iNOS = inducible NOS

- NOS3 = eNOS = endothelial NOS


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eNOS: NADPH donates electron to Fad. From FAD to FMN and generates oxygen radical

(when cofactor/substrate is missing, not beneficial). Calmodulin. NO and citrulline will be

produced in presence of all co-factors.

In case of:

-

Low L-arginine O2-; - Low BH4 (oxidative stress) ONOO-;

- Oxidative SH groups ONOO-, Zn loss.

Endothelial dysfunction leads to upregulation of NADPH oxidases, which produce O2-. When

combined with NO, this will form ONOO-. This causes more oxygen radicals and so on.

Calmodulin can displace the auto-

inhibitory loop of NOS and is then able

to become active.

LPS stimulates the uptake of L-arginine

by transporter. Also leads to production

of BH4. Pro-inflammatory cytokines

increase activity of iNOS so more

production of nitric oxide. Anti-

inflammatory cytokines activate arginase.

Can eventually downregulate iNOS.

KSRP normally binds iNOS. Makes it

targeted for degradation. But

proinflammatory cytokines cause TTP(tristetraprolin) to bind to KSRP. This


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causes stabilization of the mRNA, no degradation. iNOS is normally not constitutively active

in tissues.

eNOS activation

initially bound to cav. Increase in Ca displaces the cav. Then CaM binds to eNOS. Hsp90 binds to it, Akt will cause phosphorylation of eNOs. NO production increases along this series

of events.

Some phosphorylations cause increase of activity and some cause decrease of activity of

eNOS.

Guanylate cyclases (GCs) catalyse formation of cGMP from GTP.

GCs are activated by NO.

NO binding to the heme group disrupts interaction between His105 and Fe2+. This accelerates

the catalytic conversion of GTP to cGMP.

Reactive oxygen species impair this pathway by scavenging NO and by oxidizing sGC to its

NO insensitive state. Bay 58-2667 can bypass the impaired NO-sGC-cGMP pathway by

activation of the oxideixed and heme-free sGC.

Testosterone stimulates

endothelial cells to produce

NO and stimulates the

conformation of cGMP to

GMP by phosphodiesterase

5.

Phosphodiesterase 5

inhibitors helpen tegen

erectiestoornissen. Door dit

enzym te inhiberen zal er

meer cGMP aanwezig zijn relaxation of blood vessels.


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Guanylate cyclase stimulators/activators applications:

- Heart failure

-

Pulmonary hypertension

-

Systemic hypertension

- Atherosclerosis

-

Ischemia

S-nitrosylation Binding of NO to sulfate.

Changes in protein activity, localization, stability and interactions.

S-nitrosylation can inactivate caspase 3. (?)

NO exerts is biological effect mainly via:

-

Stimulation of sGC

- Nitrosylation of tyrosine & cysteine residues.

NO is a pluripotent biological messenger with an impact on many biological processesincluding:

- Muscle contraction/relaxation (nNOS + eNOS)

- Inflammation & host defense (iNOS)

-

Neural innervation (nNOS)

-

Mitochondrial respiration

- Apoptosis

- Development

Nitroglycerin is a drug that mimics the vasodilation like NO.

Fibrinolysis = prevention of blood clotting.

Anti-fibrinolytic = prothrombotic

tPA and uPA are serine proteases. They both convert plasminogen into plasmin, which

degrades fibrin and thus is pro-fibrinolytic (=anti thrombotic).

Smoking cigarettes may potentiate monocyte adherence to endothelial cells, via reactive

oxygen intermediates in the circulation. Smoke itself is full of radicals.

NO is responsible for the activation of matrixmetalloproteinases (MMPs). It can cause

beneficial things such as angiogenesis and vasculogenesis but also harmful things such as

ventricular dilatation. It depends on the concentration and compartmentation of the NO whicheffects will be reached.

The upregulation of metalloproteinase (MMP) expression in resident macrophages and

VSMCs cause extracellular matrix degradation plaque rupture.

The NOS form associated with the SR of cardiomyocytes is NOS1/nNOS. The ryanodine

receptor at the SR is activated by NO. It causes the Ca to be released but also to be stored

again. Inhibition of calcium channels causes hyperpolarization.

If the H4B concentration is suboptimal, eNOS produces, besides NO, superoxide anions (O2-●)

This reactive oxygen radical is toxic, since it is easily converted to H 2O2 (by superoxide

dismutase), and, in the presence of Fe2+

ions, to the free hydroxyl radical (HO●

). Ifcircumstances favor the formation of O2

-●, H2O2 and HO●, one speaks of oxidative stress.


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Competitive inhibitors of (e)NOS, of which asymmetric dimethyl L-arginine (ADMA).

In patients with atherosclerosis, hypercholesterolemia and hypertension, the plasma

concentration of ADMA is increased.

The higher the concentration of ADMA in comparison to L-Arginine the less NO is

made because most of the NOS is occupied by ADMA.

Other inhibitors are L-NAME and L-NMMA, which also resemble L-arginine in structure.

Thus, for optimal production of NO by endothelial cells several conditions should be met:

1

ligand-receptor interaction

2

activation of eNOS by Ca2+ -dependent calmodulin

3 optimal presence of H4B

4

sufficient supply of L-arginine, oxygen and NADPH, and

5

absence of inhibitors of eNOS.

Smoking induces oxidative stress, as found by increased levels of 8-epi-PGF2α in urine, an F2

isoprostane product of lipid peroxidation. Smoking cigarettes may potentiate monocyteadherence to endothelial cells, via reactive oxygen intermediates in the circulation.

The defect in the endothelial function in diabetics might be due to diminished production of

NO, or more likely to increased quenching of NO by advanced glycosylation products

associated with diabetes, or by oxygen-derived free radicals.

The precise mechanism by which hyperhomocysteinemia impaires endothelial function is not

known, but a deficiency of folic acid may be responsible as folic acid is a precursor of H4B.

The active form of folic acid, 5-methyltetrahydrofolate (MTHF), was able to restore NO

production and depress O2-● production in partially H4B-depleted eNOS in vitro.

In patients with CHF, plasma levels of tumor necrosis factor- (TNF) are elevated. TNF

has been reported to downregulate expression of eNOS in endothelial cells, and to upregulate

expression of iNOS in macrophages, VSMCs and endothelial cells. Moreover, TNF

increases the production of O2-● by VSMCs.

In CHF, vascular angiotensin-converting enzyme (ACE), that produces angiotensin II and

inactivates bradykinin, is upregulated.

Treatment of endothelial dysfunction - Anti-oxidants;

-

Inhibitors of ACE activity: By inhibition of ACE activity, the availability ofangiotensin II falls and the availability of bradykinin increases. Also the oxidative

stress improves, and endothelial cells have enhanced release of tPA and less release of

PAI-1;

- Smoking cessation; - Lipid-lowering therapy (statins are able to upregulate eNOS expression);

- L-arginine;

- H4B suppletion;

- Estrogen suppletion;

- eNOS gene transfer;

- Folic acid reduces homocysteine levels. Hyperhomocysteine causes increased ADMA

levels and therefore less NO production.


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NOS3Thus, sarcolemma-bound NOS3 inhibits the L-type Ca2+ channel and attenuates the β-

adrenergic receptor-stimulated increase in myocardial contractility.

Angiotensin-converting enzyme (ACE) inhibitors have been shown to enhance NOS3

expression and NO bioavailability.

NOS2Generally spoken, NOS2-derived NO is considered to have detrimental effects on the myocardium.

NOS1

Myocardial NOS1 stimulates SR Ca2+ release and reuptake, facilitating Ca2+-induced Ca2+

release and potentiation of the cardiac force-frequency, probably by S-nitrosylation of

calcium-handling proteins.

Myocardial NOS1 is normally localized at the sarcoplasmic reticulum (SR) membrane

vesicles, where it influences the activities of calcium-handling proteins.

Like many signaling proteins, the activity of NOS1 can be regulated by phosphorylation. If phosphorylated by calcium calmodulin-dependent protein kinase II (CaCMKII), NOS1

activity diminished, but after phosphorylation by protein kinase C (PKC) NOS1 activity

demonstrated a modest increase

mtNOS Translocated NOS1 that is posttranslationally modified.

mtNOS is localized at the inner membrane of the mitochondrion and functions as an

important regulator of cellular respiration.

In kidneys that have undergone ischemia and reperfusion mtNOS activity is upregulated. The

increased mitochondrial NO production causes the generation of intramitochondrial

peroxynitrite, which in turn leads to cytochrome c release and apoptosis.

S-Nitrosylation

Nitrosylation of cystine residues (i.e. S-nitrosylation) and nitration of tyrosine residues.

Cellular proteins that may undergo S-nitrosylation - L-type Ca2+ channel

- Kv1.5 channel

- Ca2+-activated ATPase of the sarcoplasmic reticulum (SR)

-

the ryanodine receptor-2 of the SR

Apoptosis

NO exerts anti-apoptotic effects by S-nitrosylating and thereby inhibiting caspases 3 and 9.

Also, S-nitrosylation of Bcl-2, an inhibitory regulator of the mitochondrial apoptotic death

pathway, increases its stability and thereby suppresses apoptosis.

Denitrosylation

While protein tyrosine nitration is irreversible, both reduced thioredoxin (Trx) and S-

nitrosoglutathione reductase (GSNOR, converts S-nitrosoglutathione to reduced glutathione)

can denitrosylate nitrosylated cysteine residues.


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ONOO- can (i) oxidize protein sulfhydryl groups and (ii) nitrate polypeptides at tyrosine

residues.

Nitration can also occur through the oxidation of nitrite, which forms nitrogen dioxide (NO2).

The interaction of NO with transition metals is fundamental to NO signaling.

Apoptosis

There are two pathways leading to apoptosis:

- the death receptor (or extrinsic) pathway induced by a ligand of one of these

receptors;

- mitochondria-dependent (or intrinsic) pathway, in which loss of mitochondrial

outer membrane potential leads to the release from this organelle of pro-apoptotic

factors including:

o

cytochrome c

o endonuclease G

o

apoptosis-inducing factor (AIF)o second mitochondria-derived activator of caspases (SMAC/DIABLO)

o serine protease HrtA2/Omi.

Inhibitors of apoptosis proteins (IAPs) that can inhibit the apoptotic process. Release of these

substances often occurs during opening of the mitochondrial permeability transition (MPT)

pore.

Explain how a cell that starts an apoptotic process can die from necrosis.

When the MPT channel for the release of cytochrome c stays open for too long this could

happen. The mitochondria will swell and causes loss of membrane potential. This induces

necrosis.Apoptotic bodies are ‘the moving boxes’. Apoptosis requires energy. If mito’s are completely

destroyed: necrosis. BAX and BAK are inhibited by BCL-2. When BCL-2 falls away the membrane of

mitochondria becomes permeable.

See lecture slide question 6 – apoptosis vs. necrosis for differences between apoptosis

and necrosis.

ADMA

ADMA is a natural NOS inhibitor.

Increased levels of ADMA leads to:

-

Reduction in NO bioavailability

-

Endothelial dysfunction- Hyperaggregrability of platelets

Increased ADMA levels are, in part, caused by reduced activity of its breakdown enzyme

dimethylarginine dimethylaminohydrolase.

Man-made inhibitors of NOS isoenzyme(s)

General NOS inhibitors

-

L-NG-nitroarginine methyl ester (L-NAME)

-

L-NG-monomethyl arginine (L-NMMA)

- 7-nitroindazole

-

N5-(1-iminoethyl)-L-ornithine (L-NIO)Specific NOS2 inhibitors


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- 1400W

-

Aminoguanidine

- N6-(1-iminoethyl)-L-lysine (L-NIL)

Specific NOS1 inhibitors

-

S-methyl-L-thiocitrulline (SMTC)

-

Vinyl-L- N -5-(1-imino-3-butenyl)-L-ornithine (L-VNIO)

-

S-ethyl- N -[4-(trifluoromethyl)phenyl]-isothiourea (EPIT)

DDAH degradates ADMA. DDAH activity is extremely sensitive to factors as oxidative stress and

inflammation, which decreases its activity and thus favors further increase of ADMA.

The clearance of ADMA from plasma occurs in both the liver and the kidneys.

ADMA plasma levels are increased in case of:

-

Oxidative stress (which inhibits DDAH)

- Renal failure

-

Hypercholesterolemia

- peripheral arterial occlusive disease (PAOD)

-

Plasma levels of ADMA seem to fall during normal pregnancy but are increased in women

with pre-eclampsia.

-

(Pulmonary) hypertension

- Congenital heart disease

-

Hepatic failure

-

Hypercysteinemia

- Diabetes

-

Hyperthyroidism

-

Myocardial ischemia


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Lecture 3 Shear stress and NO formation

At ageing compliance aorta goes down. Therefore the pressure difference goes down. And

thus velocity goes up and therefore Re goes up. Change of atherosclerosis goes up as a result

of this (more turbulence).

Shear stress goes up: KLF2 is activated. It acts on promoter on eNOS gene. More NO in cell.

It decreases Endothelin-1.

Lecture 4 Angiogenesis

3 different ways of making new blood vessels:

1. Vasculogenesis

2. Angiogenesis

3. Arteriogenesis

Vasculogenesis: de novo formation of blood vessels. Mesenchyme cells into primitive blood

cells and endothelial cells. Primitive blood cells have a nucleus.

Occurs in embryogenesis.

Arteriogenesis: increase size of already existing blood vessels. Gives faster increase in blood

flow than angiogenesis. Radius to the power 4th.

Angiogenesis: making new blood vessels out of already existing blood vessels. Angiogenesis

is induced by ischemia or hypoxia. VEGF is induced by hypoxia.

Steps:

1. Basement membrane breakdown by MMPs induced by VEGF.

2.

Endothelial cell migration nascent vascular sprouts

3. Endothelial cell proliferation sprout elongation

4.

Vascular maturation (SMC, pericyte recruitment)

Angiogenesis occurs during embryogenesis, in the uterus,

hair growth and wound healing.

Signaling

HIF1 alpha is transcription factor always produced but

normally degraded. When hypoxia occurs the HIF1alpha

activates production of VEGF.

VEGF receptor possibilities:VEGF family member A is very common. E for

angiogenesis of lymphe system.

VEGFR2 important for activation fase of endothelial.

1 for maturation of vessel

3 for lymphe angiogenesis


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VEGF co-receptors:

- HSPG

-

Neuropilin

Tip cells have sensors, can sense concentration of VEGF. Migrate to highest concentration.VEGF causes one cell to make more Notch and other cell to make more DII4. DII4 cell will

become tip cell. The other cells become stalk cells.

Monocytes are essential for joining tip cells together.


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Lecture 5 Endothelial progenitor cells in vascular injury and repair

RAA system

BM-derived cells can become endothelial cells.

CD34+ cells.Endothelial stem cells are needed for angiogenesis since stromal cells can only divide about 4

times due to telomere shortening.

CD34 cells can adhere via P-selectin.

Sheared CD34+ cells express KDR via PI3K pathway.

Aspirin inhibits CD34+KDR cell differentiation.

CD34+KDR cells are a marker for vascular injury.

- Circulating CD34 may function as endothelial progenitor cells to prevent endothelial

senescence with aging

-

CD34+ cells home through P-selectin (EC/Platelets) and upregulate KDR, CXCR4

from endosomal storage organelles after firm adhesion (no premature differentiation,

polarization)

- The vasculogenic potential of circulating CD34+ cells may be underestimated by

approximately 100 fold since <1% of the circulating CD34+ cells co-express KDR

- Circulating CD34+/KDR+ are generated after homing to platelet rich sites of vascular

injury. Percentage of circulating CD34+/KDR+ over CD34+ cells may reflect vascular

injury

Monocyte is important for vessel maintenance and repair. Monocytes make TNF when

recruited, and that triggers the formation of tip cells. Macrophages however direct theconnections. They can anostomose. Macrophages help to form the pattern of the vessels. They

promote tip cell fusion. Without the VEGF recruited macrophages the blood vessels won’t

branch and form good structure.

- mEPC are trained monocytes that may play a role in arteriogenesis (remodeling)

- mEPC may be reduced in CVD due to a phenotypic change secondary to systemic

chronic inflammation- This phenotypic

change may be causal

to impaired collateral

formation


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Lecture 6 Perivascular adipose tissue

PVAT releases:

- NO (vasodilation)

- Angiotensin (vasoconstriction)

- Adipocyte derived relaxing factor (ADRF) (vasodilation)

-

Adipokines (leptin, adiponectin) (vasodilation)

Net vasodilating.

Also releases pro inflammatory cytokines (TNF-alfa, IL-1, MCP-1)

And radicals.

Induction of smooth muscle cell differentiation and proliferation.

In obesity, increase in inflammatory cytokine release, decrease of ADRF release.

Treatment atherosclerosis:

1. Lower blood pressure: β- blocker (↓ heart rate and vasorelaxation)

2. Lower plasma cholesterol: Statins

3.

Lower plasma cholesterol: Activate BAT

4. Lower inflammation: Statins, anti-oxidants, MCP-1 inhibitors

Lecture 7 NO in health and disease

NOS inhibitors to treat cardiogenic shock (L-NNMA).

Excessive NO formation during cardiogenic shock usually results mainly from NOS2

induction in vascular tissue.

BH4’s cardioprotective effect against ischemia/reperfusion (I/R) involves the opening ofmitochondrial ATP-sensitive K+ channels through increased NO production.

This inhibits MTP opening and the release of pro-apoptotic factors such as cytochrome C.

NO protects against adriamycin cardiotoxicity Anthracyclins. Cardiotoxic. Kills not only tumorcells but also cardiomyocytes, induces

oxidative stress, apoptotic pathways. Replacement rate is very low, too low for this extent of

apoptosis. Fibroblasts come instead but they cannot contract. NO protects against Adriamycin

cardiotoxicity.

- Possibly the oxidative stress imposed by adriamycin treatment reduces the

bioavailability of NO through its reaction with oxygen radicals.- SOD (superoxide dismutase) overexpression leads to a reduction in superoxide levels

and an increase in NO bioavailability.

SOD induces O2- O2 + H2O2

In heart transplantation, graft rejection can be reduced by treating with iNOS inhibitors. Leads

to less apoptosis.


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Renal physiology

Effective moles (proteins, charged electrolytes)

Ineffective moles (glucose, urea etc).

Anion gap = [Na] plasma – ( [Cl] plasma + [HCO3-] plasma )

ENaC channels = Na channels located at the apical surface. Provides tranepithelial transport.

SERCA pump = Ca+2 pump located on ER, SR etc. Sequester cytosolic Ca2+ in intracellular

stores.

PMCA = calcium pump on cell surface which is activated by Ca-CaM complex. High [Ca]

means many Ca-CaM complexes and therefore low Km of PMCA.

NCX = Na-Ca exchanger on cell surface. Ca in, Na out.

Pumps page 130

Ultrafiltration = movement of water out of the cell due to hydrostatic pressure difference.

Regulatory volume increase (RVI) = Cell response to shrinkage by activating different types

of solute uptake mechanisms. Na-H exchanger, Cl-HCO3- exchanger, Na/K/Cl exchanger.

Regulatory volume decrease (RVD): In response to cell swelling, the cell activates solute

efflux mechanisms. Cl- channels, K+ channels, K/Cl cotransporter.

Effective osmolality = tonicity = 2xNa + glucose/18

Total osmolality = 2xNa + glucose/18 + BUN/2.8

BUN = blood urea nitrogen = concentration of nitrogen in blood contained in urea.

290 mOsm = isosmolal

Isotonic = effective osmolality is the same.

Page 139

0,9% NaCl = isotonic solution = 290 mOsm

Adding water: new osmolality = 290 * (old volume/new volume)

40% of the added water ends up extracellular and 60% intracellular.

Effective circulating volume = blood volume that is necessary to achieve adequate perfusionof key organs.

Total body osmoles = osmolality * total body water

Vasopressin = AVP = ADH = anti diuretic hormone

Reduced effective circulating volume, detected by baroreceptors, stimulates RAA system.

Baroreceptors also reduce renal blood flow and induces AVP release and ANP release

(reducing Na+ excretion).

Angiotensin stimulates Na-H exchanger and lowers renal plasma flow.


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Vasopressin = AVP = ADH

Lecture 9 Volume regulation

ECF volume is regulated via Na excretion.It is sensed by baroreceptors which send signals to the brain.

Osmolality is regulated by the hypothalamus. Receptors control thirst and AVP

(=ADH=vasopressin) release.

Pathways regulating ECV• Renin-angiotensin-aldosteron system (RAAS)

• Sympathetic nervous system

• Atrial natriuretic peptide (ANP)

• Antidiuretic hormone (ADH)

Angiotensin II levels are regulated by renin. Renin release is regulated by the JGA granular

cells, which can sense a decrease in effective circulating volume in three ways:

1. Decreased systemic blood pressure

2. Decreased NaCl concentration

3. Decreased renal perfusion pressure (renal baroreceptors)

Renin release is also stimulated by decreased [Ca]. In addition, also prostaglandin E2 and I2

and endothelin stimulate renin release.

Angiotensin II inhibits renin release (negative feedback).

872: angiotensin actions - Aldosterone release


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- Vasoconstriction

- Enhanced tubuloglumerular feedback

-

Enhanced Na-H exchange

-

Renal hypertrophy

- Thirst & AVP release

Renal sympathetic nerve activity:

- Increased vascular resistance

-

Increased Na reabsorption

-

Enhancing renin release

Reduced GFR and enhanced Na reuptake increase effective circulating volume

ADH = AVP = Vasopressin - Increases distal nephron water permeability (water retention). AQP2 channels in the

collecting duct

- Promotes Na retention.

-

Released when osmolality of ECF increases

ANP- Release is inhibited by decreased effective circulating volume.

Effective circulating volume feedback loop page 870

In case of dehydration, hyperosmolality makes the glumerulosa cells less sensitive to ANG II

which leads to a decrease in aldosterone release. Therefore, the kidneys fail to retain Na+.

Renin release is stimulated by:

- Decrease in blood flow in afferent arterioles of kidney (sense decrease in Ca)

- Decrease in renal sympathetic nerve activity induced by central baroreceptors (sense

decrease in pressure)

-

Changes in [Na] in macula densa

Activators: cAMP, β-adrenergic agonist, endothelin, PGE2 and PGI2

Reduction: Ang II, ADH, NO, high plasma [K].

Aldosterone stimulates sodium reabsorption in principal cells.It also induces K+ secretion in ICT and CCT in principal cells.

It stimulates the Na/K pump and increases its number.

Stimulation of ENaCs negative lumen K+ secretion

The amount of K+ excreted as a result of aldosterone is highly dependent on the amount of

Na retained, due to the fact that Na retention leads to low luminal flow which inhibits K+

excretion. Thus, the more Na retained, the less K secreted, although still inversely related.

ADH: reabsorption of water(locations!). Urea permeability in medullary collecting duct.

Na/K/Cl cotransporter in thick ascending limb is stimulated. Stimulates potassium secretion

and thus sodium reabsorption since these are coupled.

At high concentration ADH leads to vasoconstriction.ANP release is stimulated by volume expansion


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ADH release is stimulated by volume contraction and increased osmolality. Sensitivity

depends on volume.

Angiotensin II and ANP have opposite effects.

Aldosterone actions

ANP


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Lecture 10 Acid-base regulation

K+ depletion leads to:

1. Inability of the kidney to concentrate urine

2. Intracellular acidosis

3.

Extracellular alkalosis.4.

Enhancement of renal ammonium excretion

K is distributed intracellular via:

- Insulin

- Epinephrine (β-adrenergic blocker)

- Aldosterone

… by stimulating Na-K pump.

Acidemia leads to hyperkalemia since tissues release K+.

Extracellular acidosis leads to loss of cellular K+ (hyperkalemia) because the resulting

intracellular pH decrease lessens the binding of K+ to nondiffusible intracellular anions.

Kidney

Most filtered K is reabsorbed in the proximal tubule via:

1. Solvent drag. Active natrium transport drives fluid reabsorption which takes K along.

2. Electrodiffusion. In the late proximal tubule the transepithelial voltage is positive

which drives K reabsorption.

Medullary collecting duct reabsorbs K+ due to:

-

High lumen [K+ a]s a result of water reabsorption and upstream K secretion.

- Active H-K pumps

Amiloride blocks ENaCs. This leads to high luminal [Na], and therefore less electrochemical

gradient for K+ to be excreted K-sparing diuretic

Thiazide diuretics inhibit Cl- uptake by Na/Cl cotransporter. This secondary inhibits K+

efflux in principal cell through the K/Cl cotransporter due to high luminal [Cl-] and thus

reduces K+ secretion.

Loop diuretics inhibit Na/K/Cl cotransporter in TAL hypokalemia

Also, Na stays in lumen less solvent drag of K. On top of that more water secretion high

luminal flow more K secretion. Also higher sodium load to principal cells which leads to K

secretion due to negative lumen when Na leaves lumen through ENaC.

Hyperkalemia leads to:- Na/K pump stimulation in basolateral membrane

- Aldosterone release via glumerulosa cells in the adrenal cortex

-

Inhibition of Na & water reabsorption which increases luminal flow and Na delivery

Amplifies basolateral membrane of principal cells

Hypokalemia leads to:

- Low plasma [K] suppresses [K] secretion by:

- Reducing basolateral K uptake

- Reducing aldosterone release

-

Stimulation of K reabsorption in intercalated cells through H/K pumps.

-

MCD enhances H-K pump activity and paracellular K permeability. Amplifies apical membrane of intercalated cells.


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pH influences Na/K pump and apical K channels Decreasing extracellular pH inhibits Na/K pump and thus K+ secretion.

Low pH also reduces permeability of apical K channels.

Epinephrine stimulates K+ uptake in extrarenal tissues and thus lowers [K] filtration andtherefore K secretion.

AVP stimulates K secretion by:

1.

Increases apical Na conductance depolarization higher driving force for K to

diffuse from cell to lumen.

2. Increases apical K permeability.

However, it also reduces luminal flow which inhibits K secretion.

ANG II leads to K excretion.

Lecture 11 Acid-base excretion

H+ is bound to buffers such as phosphate, creatinine or urate in urine.

The kidney itself produces ammonium (NH3) to form NH4+.

Kidney also synthesizes HCO3- to form H2O + CO2

NH3/NH4+ is a non-titratable acid because the pK is too high (9).

Kidney can decrease acid excretion by decreasing excretion of titratable acids and NH4+.

Decreasin production of HCO3-.

The reaction H2CO3 H2O + CO2 is very slow. Carbonic anhydrase (CA) splits HCO3-

in CO2 and OH-. OH- is then neutralized by secreted H+.

Apical membranes of H+ secreting tubules are highly permeable to CO2.

H+ excretion is regulated by:

- Amount of buffer. This depends on the plasma concentration and GFR.

- pK of the buffer. Most effective is pK between GFR and final urine pH.

- pH of urine. At low pH, creatinine becomes a much more effective buffer.

Most filtered HCO3- is reabsorbed in proximal tubule (80%). Another 10% is reabsorbed inTAL. Rest is reabsorbed by the distal nephron (DCT to IMCD)

PKC activates Na-H exchanger

PKA and PTH inhibit Na-H exchanger.

Aldosterone stimulates H-ATPase pump.

Carbonic anhydrase inhibitors decrease Na, HCO3- and water reabsorption. diuretic.

NH3 production from glutamine leads to formation of 2H+ and 2 alfa-KG. The alfa-KG

contributes in gluconeogenesis which also produces 2 HCO3-.TAL reabsorbs a lot of NH4+ via Na/K/Cl cotransporter, in which NH4+ can replace K.


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For each amino group secreted in urea, the liver consumes one HCO3-.

For each excreted NH4+ by glutamine pathway, the proximal tubule produces one HCO3-

After a few days, metabolic alkalosis leads to a shift in intercalated cells: less alfa and more

beta cells. Beta cells have opposite apical vs basolateral distribution of H+ pumps and Cl-HCO3- exchangers, they secrete HCO3- into lumen.

Increased GFR leads to enhanced HCO3- reabsorption and therefore prevents metabolic

alkalosis.

Adrenal insufficienty leads to metabolic acidosis:

Glucocorticoids (cortisol) enhance Na-H exchange.

Mineralocorticoids (aldosterone) stimulate H excretion via

-

Apical H+ pump

- Basolateral HCO3-Cl exchanger

-

Enhanced Na reabsorption in collecting ducts

Diuretics and H+ secretion: Inhibitors of H+ secretion:

-

CA inhibitors

-

K+ sparing diuretics

Stimulators:

- Thiazide diuretics

- Loop diuretics

Contraction alkalosis:

During ECV contraction, [ANGII] and [norepinephrine] increase, which leads to Na

reabsorption. As a result of that, more H+ will be secreted due to the Na-H exchangers.

Also [aldosterone] increases which leads to stimulation of the HCO3-Cl exchanger due to Na

reabsorption

Secretion of ammonium occurs in proximal tubule.

Ammonium is reabsorbed in the ascending limb of Henle’s loop

Inotropy = contraction

Lusitropy = relaxation

Chromotropy = heart rate

Dromotropy = conduction velocity

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