Mechanism of Ischemia/Reperfusion Injury

Question 1

Overview Illustration

Answer 1

Question 2

Characteristics of I/R Injury

Answer 2

• Increased reactive oxygen species

*mitochondria = incomplete reduction of O2

*oxidases (e.g., NADPH; Nox 1 to 4) in tissue

*Uncoupled endothelial nitric oxide synthase (eNOS)

• Initiate lipid peroxidation

*cause new cell damage; injured cells release cytokines

• Leukocyte chemotaxis

*cytokines released (TNFa) attract leukocytes and initiate ROS release from tissue within 15min. reperfusion following prolonged ischemis (i.e. >30min.)

*IgM antibodies accumulate in ischemic tissues and activate the complement pathway (i.e. C5a)

*Activated leukocytes release LTB4

• Decreased endothelial derived nitric oxide

Question 3

Time Course of Events in Myocardial I/R

Answer 3

• 2-5min reperfusion: endothelial dysfunction = decreased endothelial derived NO/increased ROS
• 10min. reperfusion: upregulation of P-selectin expression on coronary endothelium promoting polymorphonuclear (PMN) leukocyte rolling on endothelium
• 20min. reperfusion: increased adhesion of PMNs to coronary endothelium
• 30min. reperfusion: PMNs transmigrate from the endothelium and being to accumulate within the myocardium, where they can release SO, which contributes to cardiac contractile dysfunction

Question 4

Types and Experimental Models of I/R Injury

Answer 4

• Clinical I/R injury- Ischemic heart disease (IHD)

*myocardial infarction

*unstable angina pectoris

*chronic IHD w/ heart failure

*coronary angioplasty/bypass

*organ transplantation

• Experiment I/R injury

*acute ischemia vs. preconditioning

*organ/cellular responses to experimental I/R injury

*the role of eNOS

Question 5

Treatments that Attenuate I/R Injury

Answer 5

Preconditioning

• Biochemical response is mediated by protein kinase C (PKC) epsilon activation involving activation of mitochondrial Katp channels

Postconditioning

• PKC epsilon inhibition during reperfusion is assoc. w/ restoration of postreperfused organ function; attenuates H2O2 release and enhances endothelial-derived NO bioavailability in part by inhibiting uncoupled eNOS

Question 6

Role of eNOS in I/R Injury

Answer 6

• Under normal conditions eNOS produces NO from L-arginine w/ the necessary co-factor tetrahydrobiopterin (BH4)
• Under conditions when BH4 is oxidized to dihydrobiopterin (BH2), eNOS may produce SO from molecular oxygen as a substrate; also termed eNOS uncoupling
• When the ratio of BH2 to BH4 is increased, eNOS uncoupling can be another source of ROS

Question 7

Different Effects of eNOS Coupling/Uncoupling and Regulators on I/R Injury Summary Chart

Answer 7