The Effects of Nitroglycerin on the Heart Rate of the 120-hour in vivo and in vitro Chicken Embryo

1
The Effects of Nitroglycerin on the Heart Rate of
the 120-hour in vivo and in vitro Chicken Embryo

• Adrienne Dorward, Susan Mette,
• Jacqueline McLaughlin, Ph.D.
• The Pennsylvania State University, Berks-Lehigh
  Valley College

2
Introduction

• The development of the embryonic chicken heart
  begins with a series of cellular migrations,
  fusions, and specific differentiations.
• The heart develops from the fusion of a pair of
  precardiac mesodermal tubes that give rise to
  four regions located anterior to posterior
  bulbus cordis, ventricle, atrium, and sinus
  venosus (McLaughlin and McCain, 1999).
• The 120-hour chicken embryo heart develops into 4
  chambers left and right atria, and left and
  right ventricles at this time the sinus venosus
  has become the mature pacemaker or the SA node,
  which regulates the electrical signals that cause
  muscle contractions in the embryonic heart.

http//www.lv.psu.edu/jxm57/chicklab/outline.html
introduction
3
Background

• Nitroglycerin (NG) is a common cardiac drug used
  to treat angina pectoris, and is also
  administered to prevent a myocardial infarction
  this drug lowers blood pressure.
• It is well known that NG relaxes vascular smooth
  muscle, venous more than arterial.

Figure 1A Chemical Structure of NG
C3H5N3O9
Diagram from www.chemfinder.cambridgesoft.com
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Background

• The dilation of venous smooth muscle promotes
  peripheral pooling of blood, which results in a
  lower venous return, and the amount of blood that
  the heart has to pump results in a decreased left
  ventricular end-diastolic pressure. This
  decreases the workload on the heart and the
  amount of oxygen as well (Physicians Desk
  Reference, 2001).

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Background

• When NG is administered in the body, enzymes
  within the cell convert it to the gas, nitrous
  oxide (NO).
• NO then functions as a hormone by stimulating an
  enzyme in the plasma membrane, which creates a
  second messenger.
• The second messenger relaxes nearby smooth
  muscle, which allows the blood vessels to dilate.
• In the heart, the dilation of the coronary
  vessels occurs.
• (Campbell, 2002).

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Purpose

• We investigated the effects of Nitroglycerin (NG)
  on the in vivo and in vitro 120-hour embryonic
  chicken heart rate before and after exposure to
  0.008, 0.08, and 0.8 NG concentrations.
• In a side study, we explored the effects of
  adding a 0.02 alcohol (Etoh) solution, a known
  vasodilator and depressant, after administering
  the 0.8 concentration of NG in both the in vivo
  and in vitro embryos.

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Purpose

• We hypothesized that exposure to NG would cause
  bradycardia on both the in vivo and in vitro
  heart rates, directly proportional to the NG
  concentration, due to vasodilation of the
  coronary vessels and increase in cardiac output.
• The addition of Etoh would enhance the effects of
  NG by further inducing bradycardia, leading to
  fibrillation and cardiac arrest because both are
  known vasodilators.

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Methods

• Prepared NG concentrations of 0.8, 0.08, and
  0.008 from an 80 NG liquid stock and chick
  saline.
• An Etoh concentration of 0.02 was available for
  use.
• Obtained four, 120-hour chicken embryos/eggs.
• Employed Cruzs (1993) window method.
• Determined in vivo heart rate for 15 seconds (5
  times) for each.
• Added 2 drops of a separate concentration of NG
  to each embryo.
• Determined subsequent heart rates.
• Added Etoh to embryo 4.
• Determined its heart rate.

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Methods

• Obtained three, 120-hour chicken embryos/eggs.
• Windowed the eggs using Cruzs (1993) window
  method.
• Explanted the embryos using Cruzs (1993)
  explantation method.
• Determined in vitro heart rate for each.
• Added 2 drops of a separate concentration of NG
  to each embryo.
• Determined subsequent heart rates for each.
• Added Etoh to embryo 3.
• Determined heart rate.

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Data Interpretation

• In all trials, a decrease in heart rate was noted
  after the specific NG concentrations were added
  for both in vivo and in vitro.
• A decrease in heart rate was noted after adding
  the Etoh in the in vivo embryo, but the heart
  rate increased in the in vitro embryo.
• Periods of bradycardia, fibrillation,
  tachycardia, and other arrhythmias were noted
  with all trials, except the 0.008 NG
  concentrations.

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Results

• The average heart rate for chick embryo 1 in
  vivo was 138 bpm.
• The average heart rate after adding 0.008 NG was
  96 bpm.
• The heart rate decreased by 42 bpm after NG was
  added.

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Results

• The average heart rate for chick embryo 2 in
  vivo was 134 bpm.
• The average heart rate after adding 0.08 NG was
  117 bpm.
• The heart rate decreased by 17 bpm after NG was
  added.

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Results

• The average heart rate for chick embryo 3 in
  vivo was 146 bpm.
• The average heart rate after adding 0.8 NG was
  120 bpm.
• The heart rate decreased by 26 bpm after NG was
  added.

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Results

• The average heart rate for chick embryo 4 in
  vivo was 105 bpm.
• The average heart rate after adding 0.8 NG and
  0.02 Etoh was 53 bpm.
• The heart rate decreased by 52 bpm after both NG
  and Etoh were added.

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Results

• The average heart rate for in vitro chick embryo
  1 was 106 bpm.
• The average heart rate after adding 0.008 NG was
  90 bpm.
• The heart rate decreased by 16 bpm after adding
  the NG.

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Results

• The average in vitro heart rate for chick embryo
  2 was 64 bpm.
• The average heart rate after adding 0.08 NG was
  36 bpm.
• The heart rate decreased by 28 bpm after adding
  the NG.

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Results

• The average in vitro heart rate for chick embryo
  3 was 89 bpm.
• The average heart rate after adding 0.8 NG was
  58 bpm.
• The average heart rate after adding 0.8 NG and
  0.02 Etoh was 84 bpm.
• The heart rate decreased by 31 bpm after adding
  the NG, but increased 26 bpm after adding the
  Etoh.

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Conclusion

• Bradycardia did occur for all concentrations of
  NG in both the in vivo and in vitro embryos,
  which supports our hypothesis.
• The heart rate of the in vitro embryo decreased
  in direct proportion to the increase in NG
  concentration, but did not in the in vivo embryo.
• The discrepancy may be due to the irregularities
  in heart rate, such as fibrillation and other
  arrhythmias that were noted with the greater
  concentrations.

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Conclusion

• The side study of adding 0.002 Etoh after the
  administration of the 0.8 NG solution produced
  mixed results.
• The heart rate of the in vivo embryo decreased
  significantly, however, the in vitro heart rate
  increased, but was still lower than the control.
• This increase may be due to the observed constant
  fibrillation that occurred after the
  administration of Etoh, which also supports our
  hypothesis.

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Conclusion

• Overall, NG proved to be a vasodilator and
  decreased HR.
• The findings in this experiment correlate with
  the known effects that this drug has on the human
  heart.
• NG indirectly creates a secondary messenger that
  causes the dilation of coronary vessels. This
  vasodilation decreases the workload on the heart,
  lowers blood pressure, and ultimately results in
  a lower heart rate.
• In all trials, the heart muscles were relaxed at
  some point.

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Future Experiments

• Use lower concentrations of NG to see if
  bradycardia will still occur.
• Use higher concentrations of NG to test if too
  much will induce cardiac arrest.
• Administer the Etoh before the NG to see if NG
  will increase the effects of Etoh for a side
  study.
• Use caffeine, a known stimulant, instead of Etoh
  to see if the caffeine would reverse the effects
  of the NG for a side study.

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References

• McLaughlin, J.S. and McCain, E.R. Development
  and physiological aspects of the chicken
  embryonic heart. 1999. http//www.lv.psu.edu/jxm
  57/chicklab/(12 Feb. 2003).
• Physicians Desk Reference (55th ed.). 2001.
  Public Medical Economics Co., Montvale, New
  Jersey,1604 pages. ISBN 1-56363- 375-2 book.
• Campbell, N.A. and Reece, J.B. Biology (6th ed.).
  2002. Benjamin Cummings, New York, 1038 pages.
  ISBN 0- 8053-6624-5 book.