Oxygen Sensing

Question 1

What is the effect of location of the level of “normoxia” ?

Answer 1

Oxygen cascade to mitochrondria, due to metabolic processes consuming oxygen. Large drop air -> alveoli due to dead space

Question 2

Why is oxygen sensing necessary?

Answer 2

Adapting tissues to high low PaO2 - normal homeostasis, birth, disease, altitude

VQ matching

Matching ventilation to metabolic needs

Question 3

Which cell types display the ability to sense oxygen?

Answer 3

Pulmonary circulation - HPV
Carotid body glomus - control of ventilation
Neuroepithelial cells
Kidney - EPO
Adrenal medulla - fetal adrenal chromaffin cells (release catecholamines ? needed for adaptation to birth)
Vascular system - angiogenesis (VEGF)

Question 4

Cellular targets following sensing of hypoxia?

Answer 4

EPO
Growth
Protective proteins, eg haemoxygenase and SOD
Angiogenesis and VEGF
Ion channels (direct and indirect)
Ca2+ handling mechanisms (eg SR release)

Question 5

What is the response of the carortid body glomus cells to hypoxia?

Answer 5

Type 1 cells inhibit K+ channels, depolarisaing and activating VGCC, release of NTs ATP, Ach, dopamine

Question 6

What are glomus type II cells?

Answer 6

Sheath of the carotid body

Question 7

When is hypoxic pulmonary vasoconstriction a problem?

Answer 7

In chronic hypoxia, entire lung becomes vasoconstricted

Question 8

What is the function of neuroepithelial cells?

Answer 8

Located at bifurcation of the small airways

Sense airway hpoxia and act in concert with HPV and CB especially during transition to air breathing in parturition

Question 9

Function adrenomedullary chromaffin cells?

Answer 9

In fetus and neonate, hypoxia causes the release of catecholamines.

Probably a protective function to support cardiac function during birth and transformation of airway epithelium for breathing air.

Question 10

Describe the function of HIF

Answer 10

Leads to transcription EPO and VEGF

Hydroxylation of proline residues by prolyl-hydroxylases labels HIF for degradation by von Hippel Lindau tumour suppressor. Action of HIF also inhibited by FIH.

Hypoxia inhibits PHDs as they use O2 as a co-substrate.

Question 11

What is the effector mechanism in the pulmonary artery after sensing hypoxia?

Answer 11

Release Ca2+ from ryanodine sensitive stores and Rho kinase mediated Ca2+ sensitisation.

MLCP is inhibited by Rho kinase. MLCP inhibits contraction.

Question 12

What is the mechanism of smooth muscle contraction?

Answer 12

Ca2+ binds to calmodulin which phosphorylates MLC to allow contraction.

Question 13

What is the effect on HPV of blocking Rho kinase?

Answer 13

Prevents the sustained vasoconstriction seen in control

Question 14

Effect of hypoxia on VGCC?

Answer 14

Inhibits K+ channels -> opening VGCCs

But HPV still seen to occur when L-type Ca channels blocked, RyR sensitive channels critical as are Ca sensitisation mechanisms

Other K+ channels may be involved in glomus cells - eg two-pore domain K+ channels (TASK)

Question 15

4 main hypotheses for hypoxia sensing?

Answer 15

1 - Mitochondria. ATP production/energy state. Redox hypothesis. Increased ROS hypothesis.

2. NADPH oxidases - Increase ROS. Decrease ROS.
3. Haemoxygenase2 - CO and K+ channels
4. Hydrogen sulphide - increases in hypoxia due to decreased oxidation

Each mechanism has a varying sensitivity.

Question 16

What is the effect of a mitochondrial inhibitor?

Answer 16

Mimics hypoxia in glomus cells

Question 17

Theories why inhibition of mitochondrial function might be linked to effectors?

Answer 17

1 - Redox and ROS hypotheses

2 - Altered energy state, the AMPK hypothesis

Question 18

What is hydrogen sulphide?

Answer 18

An endogenously produced gaseous signalling molecule which dilates the systemic, but constrict the pulmonary vasculature. Appears to mimic hypoxia.

Inhibits mitochondrial ETC.

Acts in similar manner to CO and NO

? not sensor for HPV

Question 19

What is haemoygenase-2?

Answer 19

HO-2 is co-located with BK channels in glomus cells. In normoxia, CO from HO-2 keeps channel open. Lack of O2 removes this so channels close, membrane depolarise and VGCC open -> Ca entry

Question 20

Evidence against HO-2 mechanism?

Answer 20

HO-2 KO mice do not lose hypoxia sensiong.

BK channels not involved in HPV and controversial in glomus cells.

Question 21

What does HO-2 produce?

Answer 21

Heme -> Biliverdin, Fe, CO

Question 22

What switches on AMPK?

Answer 22

Hypoxia
Hypoglycaemia
Ischaemia

Question 23

Evidence for AMPK hypothesis?

Answer 23

Inhibition of mitochondrial function increases ration AMP:ATP, activating aMPK which then regulated BKCa and TASK-3 through potentially direct phosphorylation -> depolarisation -> Ca release

Question 24

What is the redox hypothesis of HPV?

Answer 24

Hypoxia inhibits the mitochondria to reduce ROS, increase NADH and GSH. This inhibits Kv channels leading to depolarisation which then activates L-type Ca channels -> constriction

Question 25

What is the energy state and AMPK hypothesis?

Answer 25

Inhibition of mitochondrial oxidative phosphorylation. Increased ROS and decreased ATP production. Ros activates AMPK. Reduced ATP activated AMPK. AMPK leads to Ca2+ release from SR (RyR receptors) via cADPR. It also inhibits Kv channels -> depolarisation -> Ca release

Question 26

What is the elevate ROS hypothesis?

Answer 26

Inhibition of mitochondrial oxidate phosphorylation -> increased generation of SO from complex III -> Ca release from SR via AMPK/cADPR, activation NSCCs, Rho Kinase mediated Ca sensitisation, release from RyR sensitive stores

Question 27

What evidence is there againstREDOC and AMPK hypothesis?

Answer 27

Mitochondrial inhibitors should mimic hypoxia, but they actually block it

Question 28

Evidence for increased ROS hypothesis?

Answer 28

Antioxidants block HPV/elevation in Ca2+