Biological Oxygen Sensors

Question 1

What are some classical oxygen sensors in organisms?

Answer 1

1) Carotid body glomus (type I)
2) Pulmonary arteries (constrict in response to localised hypoxia)
3) Systemic arteries dilate
4) Neuroepithelial bodies, clusters of cells in airway of lumen. Innervated by vagus and release neurotransmitters.
5) Adrenomedullary chromaffin cells (release catecholamines)

Question 2

How do glomus cells, NEBs and AMCs respond to hypoxia?

Answer 2

Inhibit K+ channels, results in depolarization and activation of voltage-gated Ca channels which cause release of neurotransmitter (Lopez-Barneo, 2001). Contrasts with pulmonary and systemic smooth muscle which involves both voltage-dependent and independent Ca release and Rho-kinase dependent Ca sensitization.

Question 3

What are two broad categories for oxygen sensor mechanisms?

Answer 3

Ward categorized mechanisms as bioenergetic or biosynthetic (2008).

Question 4

Why is it better to cite P50 values instead of Km?

Answer 4

It’s better to use P50 because Enzymatic response to oxygen doesn’t always follow Michaelis-Menten mechanics (Ward, 2008).

Question 5

Does inhibiting oxidative phosphorylation have any effect on oxygen sensing?

Answer 5

Inhibiting oxidative phosphorylation trigger oxygen sensing pathways in pulmonary artery smooth muscle cells, glomus cells and AMCs but not NEBs.

Question 6

What are the main inputs into the electron transport chain?

Answer 6

Nicotinamide adenosine dinucleotide (NADH) and flavin adenine dinucleotide (FADH2).

Question 7

What is the Redox Hypothesis?

Answer 7

A postulate by Weir and Archer, that argued that decreased oxidative phosphorylation and ROS production makes the cell more reduced and inhibits redox-sensitive Kv channels. However more work suggests, it is decreased ROS alone that inhibits Kv channels instead of any change in the redox state of the channel.

Contrasts ROS hypothesis by Schumacker.

Question 8

What does Ward warn is an issue with ROS probes?

Answer 8

Probes have shown ROS increases and decreases in similar preparations (increase: Thompson, 2007) (decrease: Michelakis, 2002)

Question 9

What do NOX2 proteins do?

Answer 9

NOX stands for NADPH Oxidase. Produce superoxide from oxygen. Suggested that decrease in NOX and thus ROS during anoxia is a signal (Cross, 1986). However paradoxically NOX activity increases during hypoxia in some experiments because there is more NADPH around (reducing equivalents run out) (Marshall, 1996) (He, 2005).

Question 10

What is the P50 for NOX2? Why is it significant with regards to TASK-1 channels?

Answer 10

P50 for NOX is 1.7 kPa (Cross, 1986). Similar to the P50 of activation for TASK-1 channels: 1.6 kPa (Buckler, 2000).

Question 11

What role do heme oxygenases play in large conductance Ca2+ activated K channels (BKCs)?

Answer 11

Heme oxygenases breakdown heme to CO, biliverdin and Fe(II). HO-2 keeps BKCs active during normoxia. Has low P50 for Oxygen, 0.001 kPa (High affinity).

Question 12

What role do cytochrome P-450 monoxygenases play?

Answer 12

Possibly activated by arachidonic acid released during hypoxia, since ROS and hypoxia both activate Phospholipase A2. CYP may activate during anoxia to regulate fatty acid metabolism. (Ward, 2008)

Question 13

Why is HIF-1 inhibited during normoxia?

Answer 13

Prolyl hydroxylases domain proteins (PHD) which depend on oxoglutrate and Fe(II) hydryoxlate proline residues on HIF-1 which mark it for ubiquitination and subsequent degradation by von HIppel-Landau (vHL) factor (Maxwell, 1999).

Additionally FIH-1 and asparaginyl hydroxylase prevents transactivation of HIF-1 (Lando, 2002).

Question 14

What is the P50 of of Phd?

Answer 14

22 kPa (165 torr)

Question 15

What is the P50 of FIH-1?

Answer 15

9 kPa (67.50 torr)

Question 16

Does inhibiting mitochondria increase HIF-1 expression, since doing so reduces O2 consumption?

Answer 16

Blocking mitochondria with CN- increased HIF-1 expression during normoxia (Jiang, 1996).

Question 17

What did Gnaiger estimate cytosolic PO2 to be?

Answer 17

Gnaiger estimates cytosolic PO2 to be 1-2 kPa (1998, 2003).

Question 18

What is the median PO2 in systemic arterioles and then the capillaries?

Answer 18

It is 7 pKa in arterioles and falls to 3-4 kPa in the capillaries (Rumsey, 1991).

Question 19

Where does most O2 exchange occur, capillaries or arterioles?

Answer 19

Most O2 exchange seems to occur at arterioles instead of capillaries unlike previously though. This is because O2 gradient is non existent at capilarries, suggests no gas exchange. (Tsai, 2003) (Johnson, 2005) (Tsai, 2007).

Question 20

What is the PO2 of exchange vessels in carotid body?

Answer 20

3 kPa (Rumsey, 1991)

Question 21

What is one contributing factor to the O2 gradient in arteriole walls?

Answer 21

Oxygen is consumed by the endothelial lining of the arteriole itself.

Question 22

Since P50 of O2 sensitive tissues correlates with arterial PO2, what issues does this suggest for perfusate or superfusate experiments?

Answer 22

In vivo tissues have vasculature to handle O2 demands whereas perfusate and superfusate preparations have large diffusion distances to the core. Increased diffusion would make hypoxia induced changes appear at higher PO2 than they would in vivo. Same issues apply to cell-culture preparations.

Question 23

Why do the P50 values of Phd and HIF-1 initially suggest they are not O2 sensors?

Answer 23

Their P50 values are in the upper and lower extremes of physiological PO2 seen in arteries. May not be relevant during hypoxic events.

However, one must also look at degree of change. Despite a P50 above normoxia, Phd activity is still predicted to decrease by 50% when PO2 decreases from physiological PO2. Similarly NOX and FIH-1 are predicted to decrease greatly as well.

Question 24

How does Phd1 lower oxygen consumption?

Answer 24

Phd1 reduces oxygen consumption by reprogramming glucose metabolism from oxidative to more anaerobic ATP production via activation of the Ppar-alpha pathway.

Question 25

How does Ppar-alpha inhibit glucose entry into the TCA?

Answer 25

Ppar-alpha increases the expression of PDKs which inhibit the activity of pyruvate dehydrogenase complex required for the conversion of pyruvate to acetyl CoA.

Question 26

(Argones 2008)

Answer 26

Loss of Phd1 upregulates Pdk4 and Pdk1. Both restrict the entry of glycolytic intermediates into the TCA cycle by inhibiting PDC.

Question 27

(Beckenbach 1975)

Answer 27

Use something similar to piecewise analysis.

Salamanders of family Plethodintidae have no lungs and have lost structures needed for pulmonary ventilation.

Limited to bucco-pharyngeal surface. Body size can’t get too big as a result.

Large animals more sensitive to changes in O2.