Question

Using the antagonist drugs atenolol (20mg/kg) and propranolol (20mg/kg) describe an experiment to differentiate the actions of the b-adrenoceptor agonist isoprenaline on b₁-adrenoceptors and b₂-adrenoceptors.

Answer 1

What are adrenergic receptors?

They are a type of G-protein coupled receptors, which are seen in the lining of smooth muscles lining the airway and in cardiac muscles. They facilitate the respiration process and also the contraction of cardiac muscles. There are alpha and beta-adrenergic receptors. Beta can be divided into B1,B2 and B3. They have different locations and have different functions. Agents that activate adrenergic receptors are called sympathomimetics. One that blocks adrenergic receptors are called sympatholytic.

The drugs used to bind and activate the receptors or enzymes are termed as agonists. Agonists can bind to the receptor more firmly than its natural ligands and the effect is that much increased. Thus, the drugs like Isoprenaline act as agonist and it increases the heart rate.

Isoprenaline is a agonist drug which is structurally similar to epinephrine. Thus it can easily bind to beta adrenergic receptors in place of epinephrine much tighter and thus increase the heart rate by increasing muscle contraction.

What are antagonists?

Antagonists are drugs or ligand molecules, which bind to enzymes or receptors and block them by not activating them. It is just opposite to the agonists. Adrenergic antagonists blocks adrenergic receptors and cease their activation and thus helps in decrease the heart rate. Antagonists like Atenolol and Propanolol helps in reduction of blood pressure, heart associated chest pain etc.. thus they are known as beta blockers.

Β-1 selective blockers- atenolol works on heart. They attaches to the adrenergic receptors and thus epinephrine or nor epinephrine cannot attaches to the receptors. Thus it decreases the heart rate and blood pressure. If antagonists like isoprenaline is present in the body , which increases the heart rate, it is inhibited or blocked by these drugs.

As an experiment, if a person with hypertension is drugged with a betta blocker and he is undergone a sympathetic signal like a shock or a psychological surprise, he doesn’t get affected much and his heart rate will remain the same. The isoprenaline has ligand effect on both beta 1& 2. So it a drug to asthma patients too. It vasodialate the muscles of lungs and helps in aeration. If a patient is drugged with this for asthma and then a b blocker is injected, the asthma symptoms remains the same.