| Appendix B Further Experimentation: Determining the Effects of an Exogenous Drug on the Embryonic Heart Rate |
Developmental and Physiological Aspects of the Chicken Embryonic Heart |
Instructors: To make this lab more inquiry-based and exploratory, students design their own toxicology or physiology experiment using the embryonic chicken heart as a model. The instructor may define what chemicals are available to the students. Students first define their hypothesis and experimental protocol, and get approval from instructor, before starting the experiment. After they carry out the experiment and collect the data, they write a paper describing their results, and may go even further by presenting their data to their peers in a Power Point presentation.
Students: Start by defining a clearly stated hypothesis. You can examine the effects of a single or multiple chemicals. Next, consider the following: how you will administer these drugs to the chicken embryo or isolated heart chambers, what concentrations you will use, what will be your control, what kind of data you will collect, how you will collect the data, how many replications you will carry out, and how you will represent these data in their final form. At the beginning of the lab, your protocol will be reviewed.
Here are some tips for setting up the experiment:
- Have a control group of 2 or more embryos.
- Have at least 2 embryos in each experimental group.
- Be sure to have a baseline heart rate for each embryo before exposing it to the drug.
- Prepare the dilutions of the drug with which you chose to work. Make each dilution in one of the test tubes and use the warm chick saline to create the dilutions. Store your labeled test tubes in the test tube rack that’s in the 40°C water bath.
- Remember, the temperature of the embryo or the solution that surrounds the embryo has a significant effect on the heart rate. So, keep the embryo and your solutions at a temperature as close to 37°C as possible.
- Design a table wherein you can record all of your data. Remember to include both in vivo and in vitro embryonic heart rates before you expose your embryos to the drugs.
The following is one example of what E. McCain added to her lab:
The heartbeat is under autonomic control so it does not need to be stimulated by nerves to contract. Thus, as you will see in lab today when you dissect the chicken heart out of the embryo, it will continue to beat independently. The pacemaker region of the embryonic and adult heart, in the atrial region, stimulates action potentials that spread through the ventricles, causing the atria(atrium) to contract slightly before the ventricle(s). The heart rate and strength of contractions are influenced by a variety of compounds, some of which are normally found in the body (endogenous) and some which are not normally found in the body (exogenous) and are considered to be drugs, pharmacological agents. In the last portion of the lab you will design and test the effects of various exogenous and/or endogenous compounds on the embryonic heart. We are interested in seeing if the adult and embryonic hearts respond in similar fashion to these compounds.
In the adult, the following compounds influence the heart in the following ways:
Calcium ions: An increase in extracellular calcium will increase the force of the contraction but decrease the heart rate. In addition, it can produce ectopic pacemakers in the ventricles, producing abnormal rhythms (Fox, 1999).
Acetylcholine/Carbachol: Acetylcholine is normally released by the vagus nerve (a parasympathetic nerve), binding to muscarinic acetylcholine receptors. Once these receptors are activated, through an inhibitory G protein, the potassium channels are open in the cardiac muscle cell membranes. This makes it much harder to depolarize the muscle cells, which contributes to the inhibitory effect of acetylcholine; heart rate and strength are reduced. So, at least in the adult heart, acetylcholine causes the muscle to relax. This is not what acetylcholine does in skeletal muscle!! Carbachol is an acetylcholine agonist and mimics its effect.
Atropine: This is definitely an exogenous chemical! It is derived from the nightshade plant (Atropa belladonna) and it blocks the acetylcholine receptors of the parasympathetic neurons. By blocking acetylcholine, the cardiac rate is increased. This also happens to be an antidote to nerve gas.
Caffeine: As we all know, caffeine is an exogenous chemical and a mild central nervous system stimulant that can increase the heart rate and strength in adults. The biochemical explanation for this is that it inhibits the enzyme (phosphodiesterase) that breaks down cyclic AMP. An increase in cAMP in the heart mimics the effect of epinephrine, increasing the strength and rate of the heart.
This page was last modified February 16, 2007.
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Copyright© 2007 Dr. Jacqueline McLaughlin and Dr. Elizabeth R. McCain
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