Physiology of the Amphibian Heart
April 13, 2007
Introduction
The heart is one of the most important organs in the body. It is responsible for pumping blood throughout the body. This blood carries oxygen, nutrients, and signaling molecules (like hormones) to cells, and carries waste products away from cells. The heart is a pair of valved muscular pumps combined in a single organ.1 In mammals, there are four cardiac chambers, the two atria receive venous blood as weakly contractile reservoirs for final filling of the two ventricles which then provide the powerful expulsive contraction force that forces blood into the main arterial trunks.1 In the frog, however, there are only three chambers: two atria and one ventricle.6 The atria are in the upper part of the heart and the ventricles are in the lower part of the heart. They are separated from each other by a mixture of fibrous and fatty tissue, and two valves on each side which prevent blood from entering either chamber at the incorrect time.2 In mammals, the valve on the right side is called the Tricuspid valve, while that on the left side is called the Mitral valve.1
The heart has major blood vessels leading to it and away from it. The veins carry deoxygenated blood from the body systems to the right side of the heart; while the arteries carry oxygenated blood away from the heart to the body systems2. There are two exceptions to this rule in mammals: the pulmonary artery and pulmonary vein. The pulmonary artery carries deoxygenated blood away from the heart to the lungs, while the pulmonary vein carries oxygenated blood from the lungs to the heart. In frogs however there is one major artery that carries blood away from the heart: the Conus arteriosis.6 This branches off into two main trunks, one returns blood to the rest of the body, and one returns blood to the skin and lungs.6 In mammals, the right and left sides of the heart must be kept separate from each other because there cannot be a mixing of oxygenated and deoxygenated blood. This is accomplished by the interventricular septum, a muscular wall that separates the right and left sides of the heart.2 This structure is lacking in frogs so there is a mixing of oxygenated and deoxygenated blood.6
The muscles of the heart (and other parts of the body) contract to produce some sort of action, and in order to contract muscles need stimulus. This stimulus is in the form of an action potential. An action potential is a change in electrical potential from the resting potential of the neuronal cell membrane.5 It propagates itself along membranes at high velocity and in only one direction causing depolarization along the way. In muscles, this depolarization and action potential is stimulated by the release of neurotransmitters at the neuromuscular junction (the point of innervation of a muscle). In cardiac cells, depolarization causes an influx of sodium (or calcium) into the cell, leading to the sliding of muscle filaments which causes muscle contraction.3
Whenever heart muscles contract, this action is called systole and when the muscles relax, the action is called diastole.3 These muscle contractions are necessary to pump blood into the ventricles from the atria, and then out of the ventricles, through the arteries, to the rest of the body (left ventricle) or the lungs (right ventricle). The atria contract first to send blood into the ventricles, and then the ventricles contract to send blood out of the heart. This cardiac rhythm is maintained by the heart’s own internal conducting system. The atria and ventricles cannot contract at the same time because, simultaneous atrial and ventricular contraction may cause a backflow of blood from the left ventricle into the left atria thereby reducing the flow of available blood to the body systems leading to stroke or possible death.7 Also, atrial contraction against closed atrioventricular valves may generate more backflow of blood from the atria to the pulmonary veins.7 Cardiac muscle contractions are stimulated by the heart’s internal conducting system, which consists of: Sinoatrial node (SA node), Atrioventricular node (AV node), Atrioventricular bundle (AV bundle), Bundle branches, and Purkinje fibers.4
The SA node is located in the right atrium1 and it is here that an action potential is generated which then spreads along the walls of the right and left atria. The action potential then gets to the AV node located between the atria and ventricles. At the AV node there is a slight delay while both atria contract.4 This delay lasts about 0.1seconds and allows the ventricles to finish contracting before the next action potential moves down the heart.5 The action potential then continues down the AV bundle, which is a continuation of the AV node. The AV bundle then further divides at the beginning of the septum, and the branches travel along the wall of the septum to the apex (tip) of the heart. At the tip of the heart the branches then move upward along the walls of the ventricles (at this point they are called Purkinje fibers) and here the action potential triggers the contraction of ventricles.
This entire process is completed in a few seconds, and as one cycle is ending another one begins. Although the heart has its own internal conducting system, it is also innervated by two major nerves from the spinal chord. These are: the vagus nerve, and the phrenic nerve.1 These nerves serve two opposite purposes. The vagus nerve is part of the parasympathetic system of nerves, this system is responsible for the “rest and digest” response, and so it releases the neurotransmitter acetylcholine to “calm” the heart i.e. slow heart rate.4 The phrenic nerve is part of the sympathetic system, this system is responsible for the “fight or flight” response, and so it releases the neurotransmitter norepinephrine to “stimulate” the heart. i.e. increase heart rate. 4
These two systems (parasympathetic and sympathetic) are part of the peripheral nervous system. The peripheral nervous system influences heart rate by increasing or decreasing the force of atrial and ventricular contractions and speeding up or slowing down the rate of conduction of cardiac muscle fibers. Hormones can also act on increasing or decreasing heart rate. The adrenal glands of the kidneys secrete the hormone epinephrine (also known as adrenaline) whenever the sympathetic nervous system is active.4
The purpose of this experiment is to determine the effect of the hormone epinephrine, and the neurotransmitters atropine and acetylcholine on the heart. The hypothesis states that: since epinephrine is released in response to the sympathetic system, it will increase the heart rate; the neurotransmitter acetylcholine will decrease heart rate since it is normally released by the parasympathetic system; and the neurotransmitter atropine will increase heart rate since it decreases parasympathetic activity.9 Materials and Methods
Procedures were followed exactly as detailed in Foundations of Biology: Cell and Organ Physiology (Faculty of the Department of Neurobiology & Behavior, pp 15-7)2 with the following exceptions.
1. Only one frog was used, due to a lack of frogs.
Results
In general, data showed that the hormones and neurotransmitters worked on the frog heart. Epinephrine increased the heart rate of the frog from 48 beats/min to 54 beats/min. After administering epinephrine, acetylcholine was administered. Acetylcholine decreased the frog’s heart rate from 54 beats/min to 12 beats/min. After administering acetylcholine, atropine was added. Atropine increased the frog’s heart rate from 12 beats/min to 48 beats/min. After administering atropine, acetylcholine was administered. The heart rate showed no change.
The ECG readings and transducer readings obtained during the experiment also show the effects of these substances on the heart. On the ECG sheets, “Vertical” represents the readings obtained by the transducer. “Vertical 2” represents the readings obtained by the ECG. Appendix 1-6
ECG 1 shows the normal heart rate of the frog before any substances were added and also shows the strength of ventricular contractions as recorded by the transducer. The normal heart rate was 48 beats/min; the transducer shows the strength of ventricular contractions to be 0.3 while that of the atrial contractions was 0.25.Appendix-1
ECG 2 shows the readings obtained after the addition of epinephrine. The waves of the ECG appear more rapidly, and the height of the contractions has increased. Heart rate increased to 54 beats/min, atrial contraction height increased to 0.35, and ventricular contractions increased to 0.5. Appendix-2
ECG 3 shows the readings obtained after the addition of acetylcholine. ECG 3 shows the readings obtained at the initial addition of acetylcholine. The waves of the ECG are very few, far apart. Heart rate has decreased to 12 beats/min, atrial contraction height decreased to 0.2, and ventricular contraction height increased to 0.6. Appendix-3
ECG 4 and 5 show the readings obtained after the addition of atropine. ECG 4 shows the readings obtained immediately after the addition of atropine. Heart rate increased to 18 beats/min, atrial contraction height remained at 0.2, and ventricular contractions remained at 0.6. Appendix-4 ECG 5 shows the readings obtained a few minutes after the addition of atropine. The waves of the ECG appear more rapidly than before, and the height of the contractions has increased. Heart rate increased to 48 beats/min, atrial contraction height increased to 0.35, and ventricular contractions increased to 0.45. Appendix-5
ECG 6 shows the readings obtained when acetylcholine was added after the addition of atropine. The waves of the ECG appear almost the same as before, and the height of the contractions has not been affected. Heart rate remained the same at 48 beats/min, and the strength of atrial and ventricular contractions remained the same. Appendix-6
The following table shows a summary of the data obtained for the various substances used. The substances are arranged in the table in the order in which they were administered to the heart. For the Acetylcholine readings, the |Conditions|Heart |Strength of |Duration of contraction, |Other | | |Rate |contraction |relaxation, & AV interval |changes| | | |(measured by |(seconds) | | | | |height) | | | |Control |48 |Atrial Contraction:|Duration of Atrial |N/A | | |beats/m|0.25 |Contraction/Ventricular | | | |in |Ventricular |Relaxation: 0.5 | | | | |Contraction: 0.3 |Duration of Ventricular | | | | | |Contraction/Atrial | | | | | |Relaxation: 0.9 | | | | | |AV interval: 1.4 | | |Epinephrin|54 |Atrial Contraction:|Duration of Atrial |N/A | |e |beats/m|0.35 |Contraction/Ventricular | | | |in |Ventricular |Relaxation: 0.45 | | | | |Contraction: 0.5 |Duration of Ventricular | | | | | |Contraction/Atrial | | | | | |Relaxation: 0.70 | | | | | |AV interval: 1.15 | | |Acetylchol|12 |Atrial Contraction:|Duration of Atrial |N/A | |ine |beats/m|0.2 |Contraction/Ventricular | | | |in |Ventricular |Relaxation: 0.4 | | | | |Contraction: 0.6 |Duration of Ventricular | | | | | |Contraction/Atrial | | | | | |Relaxation: 1.0 | | | | | |AV interval: 1.4 | | |Atropine |48 |Atrial Contraction:|Duration of Atrial |N/A | | |beats/m|0.35 |Contraction/Ventricular | | | |in |Ventricular |Relaxation: 0.5 | | | | |Contraction: 0.45 |Duration of Ventricular | | | | | |Contraction/Atrial | | | | | |Relaxation: 0.75 | | | | | |AV interval: 1.25 | | |Acetylchol|48 |Atrial Contraction:|Duration of Atrial |N/A | |ine after |beats/m|0.35 |Contraction/Ventricular | | |Atropine |in |Ventricular |Relaxation: 0.5 | | | | |Contraction: 0.45 |Duration of Ventricular | | | | | |Contraction/Atrial | | | | | |Relaxation: 0.75 | | | | | |AV interval: 1.25 | |
Table 1
In this table, the duration of the heart chamber contractions and relaxations are shown. All data was obtained from the ECG readings (see Appendix 1-6).
The table also shows the duration of the AV interval (Atrio- ventricular) interval. Before any substances were added, the AV interval had a duration of 1.4 seconds. After the addition of epinephrine, the AV interval decreased to 1.15 seconds. On the addition of acetylcholine, the AV interval increased to 1.4 seconds. After the addition of atropine, the AV interval decreased again to 1.25 seconds. On the final addition of acetylcholine, the AV interval remained at 1.25 seconds.
Discussion
As seen from the results, the data obtained agreed with the hypothesis concerning the effects of the different substances on the frog heart. The hormone epinephrine worked as expected to increase heart rate. There was an increase in the strength of ventricular and atrial contractions. The reason for this increase in heart rate and heart contractions is that in the body, the secretion of the hormone from the adrenal glands on top of the kidneys is stimulated by the sympathetic nervous system. As mentioned in the Introduction, this is the branch of the peripheral nervous system that stimulates the “fight or flight” response in the body. 4 So, the addition of external epinephrine on the frog heart acted as expected and increased the frog heart rate and contraction strength. It also decreased the AV interval to a duration of 1.15 seconds. Appendix-2 This is because an increase in heart rate means an increase in the speed of atrial and ventricular contraction. So, a decrease in the AV interval shows that both chambers are contracting faster than usual.
After adding epinephrine, acetylcholine was added. Acetylcholine is a neurotransmitter secreted by the parasympathetic pre-ganglionic neurons. 4 The parasympathetic system is responsible for the “rest and digest” response in the body. So, the action of acetylcholine to reduce heart rate also supports the hypothesis. Acetylcholine reduced the heart rate from 54 beats/min to 12 beats/min (Table 1). Also there was a decrease in the strength of atrial contractions, but an increase in the strength of ventricular contractions. It would be expected that the strength of both atrial and ventricular contractions would decrease, however this could be a topic for further exploration in a new experiment. The almost immediate decrease in heart rate observed in the experiment is not uncommon.
The effects of the parasympathetic nervous system usually occur faster than that of the sympathetic nervous system.8 The effects of acetylcholine also do not last very long. Although in this experiment there was not enough time in between substance applications to see this property of acetylcholine, usually after acetylcholine stimulates a response in the target cell it is quickly broken down by acetylcholinesterase.4 Also noticeable from Table 1, is the increase in the duration of the AV interval. On addition of epinephrine the interval was 1.15 seconds, then after the addition of acetylcholine the duration increased to 1.4 seconds. This shows that the rate of contraction of the heart chambers decreased on addition of acetylcholine. This provides further evidence that the activity of acetylcholine in this experiment supports the hypothesis.
After the addition of acetylcholine, atropine was added. Atropine is a plant hormone that reacts in animals to further decrease parasympathetic activity in the body.9 When atropine was initially added, heart rate increased slightly to 18 beats/min, then after a longer period of time heart rate increased to 48 beats/min which was the exact normal heart rate of the frog. This delay in the effect of atropine might be due to the fact that it is a plant hormone which is initially unrecognized by the frog heart’s muscular synaptic receptors. It might also be due to the fact that atropine, although not a neurotransmitter, works in the same way as the sympathetic nervous system neurotransmitters which also take some time to take effect.8 This activity of atropine in restoring the normal heart rate is one of the properties that makes it useful in the medical setting. It is used in patients with bradychardia (slow heart rate) to increase their heart rates and restore normality.9 From Table 1 it can be seen that, Atropine decreased the strength of ventricular contraction and increased the strength of atrial contraction.
The newly observed levels are similar to those observed during the activity of epinephrine. Atropine also decreased the duration of the AV interval. It decreased from 1.4 in the presence of the acetylcholine to 1.25 in the presence of atropine. This shows that the rate of contractions increased. Again, the activity of atropine in the experiment supports the hypothesis of the experiment.
Finally, acetylcholine was added after the addition of atropine. This addition had no effect on the heart rate, AV interval duration, or strength of contractions. This is due to the fact that atropine acts as a competitive antagonist of acetylcholine.9 It does this by binding to acetylcholine receptor sites on cell membranes so that acetylcholine cant bind to those sites, rendering acetylcholine useless. Also, since atropine is manufactured by plants and not locally in the body, the hormone acetylcholinesterase cannot breakdown atropine to stop its activity. So, when acetylcholine was added after atropine, there was no effect on the heart. Atropine has to be removed from the body in other ways not involving acetylcholinesterase, and this can take some time. If atropine is administered in large enough doses it causes the heart muscles contract irregularly and rapidly leading to fibrillation and possible death.9
Overall, the results obtained from the experiment support the hypothesis stated in the introduction. Epinephrine and atropine increased heart rate, and Acetylcholine decreased heart rate. This experiment could be performed with greater accuracy by increasing the number of frog hearts used. The results obtained could provide further support for the hypothesis and expose more information about the effects of these hormones and neurotransmitters on heart rate. Also, the disparity in the activity of acetylcholine on the atria and ventricles can be further tested to determine the cause.
References
1 Standring, S. Department of Anatomy and Human Sciences. King’s College, London.Gray’s Anatomy, 39th Edition. Churchill Livingstone: 2004. 1005-10
2 Netter, F. Netter’s Atlas of the Human Body. Barron’s Educational Series: 2005. 75-7.
3 Campbell N.A. and Reece J.B. Biology: Sixth Edition. Benjamin Cummins: 2002. 876.
4 Pearson Education, Inc. Interactive Physiology CD. Nervous System 1, Cardiovascular System. Benjamin Cummins: 2005. 4/12/07.
5 Lerner, L.K and Lerner, B.W. World of Anatomy and Physiology. Thomson Gale: 2002. 5, 10, 12.
6 Department of Neurobiology and Behavior. Stony Brook University. Foundations of Biology: Cell and Organ Physiology. Pacific Crest: 2007. 92- 8
7 Israel, C.W. and Hohnloser, S.H. Acute Severe Cardiac Decompensation During Cardiac Resynchronization Therapy: What is the Cause?. Pacing and Clinical Electrophysiology: June 2006. 29 (6), 632-636. . 4/12/07.
8 Andriessen, Peter. Maastricht University, The Netherlands. Exploring The Baroreceptor Reflex Function in Neonates.
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9 Atropine – Wikipedia, the free encyclopedia. Wikimedia Foundation, Inc. Atropine. . 4/12/07.
