Introduction

A common lab in physiology is to observe the effects different chemicals have on the heart rate of a frog or insect. The purpose of this lab is to first show the effects of epinephrine and acetylcholine on the heart. Furthermore, the purpose of this lab is to note what chemicals are agonists of epinephrine/acetylcholine and which are antagonists. In the past we have used water fleas or grasshoppers for this lab. However, the water fleas resting heart rate is too fast to count and the grasshoppers get mangled. Furthermore, students tend not to like to use cockroaches or frogs for this experiment for various reason. Therefore, we will be using a simulated frog heart.

First, you will find the resting heart rate of the “control” frog. You will then observe the effects of epinephrine and several epinephrine agonists or antagonists. One will have no effect since that receptor is not found on the heart. You will then repeat this with acetylcholine and its agonists/antagonists. Again, one will have no effect because its receptors are not found on the heart.

To find the heart rate, count the number of beats in 15 seconds and multiply that number by 4. The lab book will have you repeat 3 times to make sure your number is accurate. However, it is an animated gif so if you get the same number twice, just write it for the third try. You can practice on the heart to the right. Using the contracting ventricles as a guide, I got 8 beats in 15 seconds. 8 beats/15 seconds x 4 = 32 beats per 60 seconds.

Lastly, for each treatment, we are going to assume the heart was washed in amphibian Ringer's solution and allowed to return to normal.

Control

This is our control frog. We are witnessing the frogs baseline heartrate. We will use this heart rate to compare the effects of all the other substances.

Epinephrine

Epinephrine (hormone) and nor-epinephrine (neurotransmitter) are released by the sympathetic nervous system and bind to either alpha or beta receptors on the target organs. In the heart, they increase the leak of calcium into the cells while preventing potassium from leaving. This results in a faster heart rate. They also affect the AV node allowing the signal to pass through the heart faster and increase the force of contraction.

Alpha Blocker

Alpha blockers are sometimes given to reduce blood pressure. As the name implies, alpha blockers block alpha adrenergic (epinephrine ) receptors which stops the action of epinephrine/nor-epinephrine. But are alpha receptors found on the heart? If the heart slows down compared to the control, that means that alpha receptors are found on the heart. If the heart rate is the does not change from the control treatment, that means they are not found on the heart.

Beta Blocker

Beta blockers are sometimes given to reduce blood pressure. As the name implies, beta blockers block beta adrenergic (epinephrine ) receptors which stops the action of epinephrine/nor-epinephrine. But are Beta receptors found on the heart? If the heart slows down compared to the control treatment, that means that beta receptors are found on the heart.

Digitalis

Digitalis is a medicine that is used to help treat an irregular heartbeat. It blocks the sodium/potassium ATPase pump. As a result sodium and calcium build up in the cell causing the cells to reach threshold quickly. However, too much can cause the heart to contract too quickly and as a result, it doesn’t have time to fill. In fact, according to science direct, Digitalis has been used as a poison for hundreds of years. PS have fun counting.

Acetylcholine

Acetylcholine is found in the somatic motor, the parasympathetic motor, and the central nervous systems. It binds to either nicotinic or muscarinic receptors depending on the target organ. Since it is part of the parasympathetic motor division of the nervous system, we would expect it to slow the heart rate from the baseline.

Nicotine

Nicotine (as in from cigarettes) binds to nicotinic acetylcholine receptors and opens sodium channels causing an excitatory response. But are nicotinic receptors on the heart. If the heart rate slows from the control, it means that nicotinic receptors are found on the heart. If the heart rate is similar to the control, it means that nicotinic receptors are not found on the heart.

Muscarine

Muscarine is a toxin found in some mushrooms. It binds to muscarinic acetylcholine receptors which can cause an excitatory or inhibitory response depending on the sub type. But are muscarinic receptors on the heart. If the heart rate slows from the control, it means that muscarinic receptors are found on the heart. If the heart rate is similar to the control, it means that muscarinic receptors are not found on the heart.

Pilocarpine

Pilocarpine binds to muscarinic receptors. But is it an agonist or an antagonist. If the heart rate slows from control, it is an agonist. If it is remains the same, it is an antagonist.

Atropine

Atropine (belladonna) binds to muscarinic receptors. It has been used as a treatment for people coming out of surgery as well as the antidote to certain mushroom poisonings. It has also been used as a poison for hundreds of years. But is it an antagonist or an agonist? If the heart rate slows down, it is an agonist. If it speeds out, we will assume it is an antagonist and K.O.ed the vagus nerve which is supposed to slow down the heart.

Removed Heart

When doing this experiment on frogs, the last step is to always to remove the heart form the body. This is to prove that the heart is autorhythmic. However, we have established that I am using hand drawn animated gifs and therefore we won’t really prove anything. Instead let’s play a game. Let us assume that both the sympathetic and parasympathetic nervous systems were affecting the heart during the control treatment and we didn’t mess it up with any of our poisons. Do think the heart will be faster or slower now that we removed it? Count the BPM? Which branch of the autonomic nervous system was affecting it during the control treatment?