Effects Of Sympathomimetic And Parasympathomimetic Agents Biology Essay


Describe the effects on the isolated guinea pig atria and ventricles of: isoprenaline and propranolol and acetylcholine and atropine

Equipment set up

The equipment was already set up and the bath volume used was 50ml . Each time drugs were added the final bath concentration was calculated using the formula below :

Final organ bath concentration = Volume of stock solution (ml)/50 X Concentration of Stock solution (M)

Methods as described on the lab book.


Unfortunately the guinea pig atria examined did not present any SA node. Attempts to recover an SA node were made by adding increasing amounts of isoprenaline and atropine.

Experiment 1a: Effects of Isoprenaline

Isoprenaline is a sympathomimetic beta adrenergic agonist

It has structure similar to adrenaline but acts selectively onbeta receptors, activating β1 and β2 receptors equally

Below is a trace that shows the effects of isoprenaline at three different final bath concentrations.

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As it can be seen from the graph attached the tissue shows a baseline SA node at around 0.9-1 mN. After the addition of the first amount of isoprenaline it was recorded activity showing response of 1.1-1.2 mN. The tissue was washed out and when the second dose was applied it showed an activity of 1.2-1.4mN. Finally following the addition of the third dose the maximum responce the tissue showed was 1.5mN.

It is clearly shown that the addition of isoprenaline increased the activity of the tissue leading to a maximum response. This clinical result agrees with the theory for isoprenaline as this drug is sympathomimetic. Sympathomimetic drugs are substances that mimic the effects of the sympathetic nervous system in that case increase the heart activity. Isoprenaline acts on cardiac β1 receptors and β2 receptors on skeletal muscle arterioles and has positive inotropic and chronotropic effects on the heart.

Experiment 1b: Effects of Isoprenaline after Propranolol

As it is mentioned above, the addition of isoprenaline acts on beta receptors causing excitation. In the second experiment there is addition of propranolol. Propranolol is a non-selective beta-adrenergic receptor blocking agent. Propranolol is a competitive antagonist which specifically competes with beta-adrenergic receptor stimulating agents (such as isoprenaline) for available beta-receptor sites. Propranolol shows antiarrhythmic effects producing beta-adrenergic blockade, which appears to be its principal antiarrhythmic mechanism of action.

From the graph attached it can be seen that the addition of propranolol does not increase the SA node blocking the effect of the isoprenaline as it competes for the same beta receptors. As an antagonist it has to be noted that propranolol on its own does not have any remarkable effect on the tissue and its action is only when isoprenaline is present. Antagonists do not maintain the ability to activate a receptor.

So propranolol has affinity but no efficacy for the adrenergic receptors. The potency of propranolol is defined by its IC50 value. This can be calculated by determining the concentration of propranolol needed to elicit half inhibition of the maximum biological response of the isoprenaline.

Exercise 2a: Effects of Acetylcholine

Acetylcholine is the neurotransmitter produced by neurons referred to as cholinergic-neurons.

Acetylcholine is synthesized in certain neurons by the enzyme choline acetyltransferase from the compounds choline and acetyl-CoA. The enzyme acetylcholinesterase converts acetylcholine into the inactive metabolites choline and acetate.

This enzyme is abundant in the synaptic cleft, and its role in rapidly clearing free acetylcholine from the synapse is essential for proper muscle function. Certain neurotoxins work by inhibiting acetylcholinesterase, thus leading to excess acetylcholine at the NMJ, thus causing paralysis of the muscles needed for breathing and stopping the beating of the heart

In the peripheral nervous system acetylcholine plays a role in skeletal muscle movement, as well as in the regulation of smooth muscle and cardiac muscle. Acetylcholine, while inducing contraction of skeletal muscles, instead inhibits contraction in cardiac muscle fibers. There are two main classes of acetylcholine receptor (AchR), nicotinic acetylcholine receptors (nAChR) and muscarinic acetylcholine receptors (mAChR). They are named for the ligands used to activate the receptors. Cholinergic receptors, muscarinic 2 slow heart rate reduce contractile forces of atrium reduce conduction velocity of AV node in CNS. Ach optimises the action of parasympathetic system (that slows down the heart rate)

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Looking at the trace after the increasing doses of acetylcholine the effects on the cardiac tissue are now obvious. More specifically, the addition of acetylcholine slows down the heart rate and the force. Especially in the FBC3 the force is kept to minimum and the SA node activity is very low confirming the theory about the acetylcholine.

Experiment 2b: Effects of acetylcholine after atropine

As it was mentioned before Acetylcholine receptors can be muscarinic and nicotinic. Especially for the muscarinic receptors M2 that act inhibiting the heart rate an agonist is Acetylcholine and antagonist is Atropine. Therefore, it is expected that the addition of atropine will have reverse effects of that of Ach ones. Indeed looking at the trace it can be deducted that the addition of atropine has a positive effect on the heart rate stimulating the tissue expressing a high contractile force which is totally different from the effect of the Ach. Atropine is an anticholinergic drug that blocks the Ach in the CNS and PNS increases the SA node through the AV node of the heart. Atropine as a competitive antagonist of the muscarinic acetylcholine receptors it lowers down the parasympathetic activity. Atropine on its own increases the heart rate by blocking the vagus nerve of the parasympathetic system on the heart rate. Due to this effect is used in the cases of bradycardia and cardiac arrest.

Clinical uses of the drugs used in the experiment


Isoprenaline can be used in case of anaphylactic shock instead of adrenaline. Note that isoprenaline injection is available from special-order' manufacturers or specialist importing companies.


Propranolol can be used for hypertension taken orally initially 80 mg twice daily, increased at weekly intervals prn; and then stick to 160-320 mg daily

By iv injection for arrhythmias and thyrotoxic crisis, 1 mg over 1 minute; if it is required repeat at 2-minute intervals however maximum total dose 10 mg


Acetylcholine chloride is an ocular peri-operative drug and is used for cataract surgery, penetrating keratoplasty, iridectomy, and other anterior segment surgery requiring rapid complete miosis.


Atropine is a cycloplegic drug that is used in anterior uveitis mainly to prevent posterior synechiae and to relieve ciliary spasm.

It is also indicated for Intra-operative bradycardia, by intravenous injection


The traces that meant to be attached, are the given sample ones that can be found on the Blackboard

A copy of these has also been submitted to the second word document file.


As it can be seen from the experiments and the data given isoprenaline increases the heart rate having similar to sympathetic system effects while propranolol inhibits its action as its antagonist. On the other hand Ach has similar to parasympathetic system effects slowing down the heart rate while atropine acts against acetylcholine firing the heart rate.