The effects of low altitude on isolated mammalian arteries, intact murine circulation, and as a therapy for myocardial infarction and hindlimb ischemia in mice

• Author / Creator

  Shahid, Anmol

• Humans residing at low and moderate altitudes (500 m - 3000 m above sea level) have a lower risk of dying from many cardiovascular diseases in comparison to sea level residents [1-3]. We hypothesized the lowered barometric pressure at these altitudes enhances vasodilation in the systemic vasculature through reduced external compressive forces on arteries. This could promote an anti-artherogenic environment and protect altitude dwellers from cardiovascular diseases. To explore potential mechanisms behind the altitude benefit, we investigated how normoxic low altitude (through small reductions in barometric pressure) affects arterial function ex vivo and hemodynamics in vivo. We also investigated the value of normoxic low altitude simulation in treating ischemic disorders such as myocardial infarction (MI) and peripheral arterial disease (PAD) in mice.
  We studied the effects of normoxic low altitude simulation on isolated resistance arteries of healthy C57BL6 mice (13.3±1 weeks) ex vivo using a pressure myograph system enclosed in a hypobaric chamber. Mesenteric arteries (n=14) were exposed to barometric pressures of 754 mmHg, 714 mmHg and 674 mmHg, and we used stepwise manipulation of perfusion pressures or flow rates to assess myogenic tone in the presence or absence of inhibitors of major endogenous vasodilators (i.e., L-NAME and Meclofenamate). We observed clear and immediate increases in vessel diameter at 714 mmHg and 674 mmHg (20.9±9.3% and 28.2±8.6%, respectively) compared to 754 mmHg (p<0.01) when perfusion pressure was increased. Flow-mediated vasodilation was enhanced under altitude simulation conditions with and without L-NAME and Meclofenamate. Vascular resistance was reduced significantly at 674 mmHg vs. 754 mmHg (2.14±0.60 mmHgmin/µL vs. 3.21±0.49 mmHgmin/µL, p<0.05).
  We then catheterized the left ventricle of healthy C57BL6 mice (n=8) and generated pressure volume loops during consecutive acute exposures to 754 mmHg, 714 mmHg, and 674 mmHg to study the in vivo hemodynamic effects of normoxic low altitude simulation. We found that total systemic vascular resistance was reduced with normoxic low altitude simulation (10.09±0.15 mmHgmin/µL at 754 mmHg vs. 8.11±1.45 and 8.18±1.24 mmHgmin/µL at 714 mmHg and 674 mmHg, respectively; p<0.05). We also observed significant increases in stroke volume and cardiac output from 754 mmHg to 714 mmHg and 674 mmHg (p<0.05).
  Then, to study the effect of normoxic low altitude simulation on a disease model of myocardial infarction, we performed left-anterior descending artery (LAD) ligation on three-month old C57BL6 mice. Treatment group mice (n=13) were placed in an altitude chamber to recover from surgery for 3-hours daily at 714 mmHg for 1 week, and controls (n=12) were only exposed to 754 mmHg at this time. Echocardiographic evaluation of left ventricular function was performed on Day 1 and Day 8. Ejection fraction improved by 14.2±5.3% in treatment group mice (p<0.01 versus Day 1) but did not change for control group mice on Day 8. Cardiac output and stroke volume increased by 11.48±3.9 mL/min and 14.33±8.3 µL respectively in treatment group mice (p<0.01 versus Day 1), while control mice showed no significant improvement. Infarct size was significantly smaller in treatment group mice.
  Lastly, we performed femoral artery ligation to generate a model of PAD in three-month old C57BL6 mice. Control group mice (n=10) recovered at 754 mmHg (control) for 14 days. Treatment group mice (n=15) were placed in a low altitude simulation chamber (at 714 mmHg) to recover from surgery for 3-hours daily for two weeks. Hindlimb perfusion imaging was performed using a laser doppler line scanner for all mice prior to the surgery, and on days 1, 3, 7 and 14 post-surgery. At two weeks, ischemic reserve was significantly higher in the treatment group mice vs. the control group mice (0.50±0.13 vs. 0.20±0.06; p=0.01). Treatment group mice also had higher limb function scores and showed better ability to walk at two weeks.
  We conclude that normoxic low altitude simulation through reduced barometric pressure noticeably increases vascular diameter in isolated vasculature and improves cardiac output and stroke volume in vivo in healthy mice. Furthermore, we found that normoxic low altitude simulation can be used to improve cardiac function and reduce infarct size after an MI and improve ischemic limb blood flow in mouse models of PAD. We conclude that these changes are mediated by a reduction in the compressive forces experienced by arteries when barometric pressure is reduced at low altitudes and not hypoxia mediated mechanisms.

• Subjects / Keywords

  • moderate altitude
  • altitude hemodynamics
  • lowered barometric pressure
  • low altitude

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-hhsc-xm45

• License

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