Freed – endothelial dysfunction 5-24-18

“A Tale of Two Studies: Flow-Induced Dilation and Vasoplegia in the Human Microcirculation”

Julie K. Freed, MD, PhD

**summary by Avnee Kumar, MD

*Video does not start till 04:30

Clinical Pearls

• Causes of Vascular Inflammation and Dysfunction
  • Obesity
  • Aging
  • Cancer

Study #1: Flow-Induced Dilation (FID) and Vascular Hemostasis

• Increased vascular flow in a health human adult causes conversion of endothelial nitric oxide (NO) to NO, thereby vasodilating smooth muscle cells
  • NO decreases inflammation, thrombosis, and atherosclerosis.
• Increased vascular flow in a diseased (i.e., CAD, or risk factors for CAD) human adult causes conversion of endothelial NO (eNO) to NO. This NO binds to reactive oxygen species from the mitochondria. Bound NO converts to hydrogen peroxide (H₂O₂), thereby vasodilating smooth muscle cells.
  • H₂O₂ increases inflammation, thrombosis, and atherosclerosis.
• You can study vascular reactivity in human arterioles using discarded tissue from OR
  • Requires careful dissection of arterioles (100-200 microns) within 48 hours after removal from human body
  • Using various products, you can determine if a vessel is vasodilating in response to H₂O₂ or NO
    • L-NAME inhibits NO synthase (in the first diagram, L-NAME inhibits dilation à the process is NO-mediated à healthy tissue)
    • Catalase causes the breakdown of H₂O₂ (in the second diagram, Catalase inhibits dilation à the process is H₂O₂-mediated à diseased tissue)

Using human arterioles, L-NAME, and Catalase, the study determined that certain factors make a healthy arteriole diseased (or vasodilate using H₂O₂, like an arteriole with coronary artery disease)

• High intraluminal pressure
  • Inhibition of Mitochondrial Telomerase (mTERT) via BIBR-1532
  • Creation of extracellular vesicles which is stimulated by ceramide
• What is Ceramide?
  • A sphingolipid
  • Induces mitochondrial dysfunction
  • Ceremide synthesis can be inhibited with exogenous compound GW4869
    • If ceramide is inhibited in a diseased arteriole, NO-dependent vasodilation is restored
  • Ceramide is broken down into sphingosine by neutral ceramidase
    • If neutral ceramidase is promoted by lab-made Adenovirus Ad-GFP (thereby INCREASING ceramide breakdown), NO-dependent vasodilation is restored in a diseased vessel
    • If neutral ceramidase is inhibited by exogenous Ceranib-1 (thereby DECREASING ceramide breakdown), H₂O₂ dependent vasodilation is induced in a healthy vessel

Ceramide eventually becomes Sphingosine-1-Phosphate (S1P) a compound which actually promotes NO-mediated vasodilation and mitochondrial biogenesis

• Ceramide, by increasing H₂O₂, is bad for vasodilation. Sphingosine-1-Phosphate, by increasing NO is good for vasodilation
  • The S1P:ceramide ratio may influence flow-induced dilation

Study #1 Key Points

• Vasoactive mediations can profoundly affect vascular homeostasis
  • A transition from NO to H₂O₂ dependent FID occurs between health and disease
  • The transition to H₂O₂ as the mediator of FID can occur via multiple mechanism (i.e. high pressure, telomerase, vesicles, ceramide metabolism)
  • The intracellular S1P:ceramide ratio may determine the mediator of FID
  • Neutral ceramidase plays a key role in determining the S1P:ceramide ratio
  • Patients treated with chemotherapy are likely to develop cardiac disease within 5 years of treatment
    • Even though ceramide can cause H₂O₂ dependent vasodilation, it is used as a cancer drug
    • Even though S1P can cause NO mediated vasodilation, a S1P inhibitor is used as a cancer drug

Study #2: The Vasoplegia Story

• Vasoplegia is refractory hypotension at a MAP <50 mmHg with normal or elevated cardiac index, and reduced systemic vascular resistance
  • Observed after cardiopulmonary bypass, liver transplant, and sepsis
  • Mechanism is unknown
  • Can be treated with methylene blue
    • Transient effect
    • Side effect of methemoglobinemia
• Streptococcus and Neisseria produce hydrogen sulfide which produces vasodilation
  • 12-24 hours after sepsis, there is an increase in plasma concentrations of hydrogen sulfide [2]
  • Endogenous hydrogen sulfide protects bacteria against antibiotic toxicity [3]
  • Exogenous hydrogen sulfide is a potent vasodilator
• Incubation with N. Lactamica induces vasoplegia
  • Inhibition of hydrogen sulfide production restores vasoconstrictive response
  • Treatment with hydroxocobalamin restores vasoconstrictive response
  • Scavenging of hydrogen sulfide with cobalamins may reverse vasoplegia
• Cyanokit (Hydroxocobalamin, Vitamin B12)
  • FDA approved for cyanide toxicity
  • Can also increase blood pressure in healthy patients
• Study #2 Key Points
  • Vasoplegia is a clinical problem
  • Hydrogen Sulfide may be a key player in vasoplegia (produced by bacteria and hypoxia)
  • Scavenging of hydrogen sulfide with B12 (cobalamin) may rescue vasoplegic vessels

Overall Summary

• Patients are getting sicker
  • Vascular dysfunction will lead to poor outcomes
  • Ceramide metabolism and hydrogen sulfide scavenging with B12 are crucial factors in human microcirculation

References

[1] Beyers et al. AJP Heart 2014;307:11, H1587-H1593

[2] Gaddam et al. J of infection 2017;75(4):293-300)

[3] Konstantin Shatalin et al. Science; 334:986-990)