Artem has a doctor of veterinary medicine degree.
Designing A Drug
If you were to design a product that blocks sunlight, what would you call it? Sunblock would be about right, now wouldn't it? So if you were to design a drug that blocks a type of receptor, called a beta receptor, what would you call it? A beta blocker sounds about right.
That's what this lesson is about, the basics surrounding cardioselective (heart-selective) and non-cardioselective beta blockers.
An Adrenergic Pharmacology Brief
To understand the main differences between cardioselective and non-cardioselective beta blockers, we need to make sure you know some very basic concepts pertaining to adrenergic pharmacology. The adrenergic system is the one that uses neurohormones and neurotransmitters called epinephrine (adrenaline) and norepinephrine (noradrenaline).
These biochemicals land on specific receptors of your body to exert their effects. Some of these receptors are called beta receptors, aka beta-adrenergic receptors. While both norepinephrine and epinephrine act on beta receptors, it is mainly epinephrine that is in charge of stimulating beta receptors, so we'll ignore norepinephrine for simplicity's sake.
There are three major types of beta receptors stimulated by epinephrine: beta-1, beta-2, and beta-3. And here's where you have to wake up for this lesson's sake!
- Beta-1 receptors are found in the heart.
Memory aid: You've got 1 heart, so beta-1 receptors are found in that 1 heart. When epinephrine stimulates beta-1 receptors, the heart rate increases, and the force of contraction of the heart increases as well. Excessive stimulation of beta-1 receptors can lead to irregular heart rhythms, or arrhythmias. Beta-1 stimulation also causes the release of an enzyme called renin from the kidneys. The release of renin sets off a cascade of steps that increases a person's blood pressure.
- Beta-2 receptors are found in the smooth muscle of the body, especially the smooth muscle surrounding the airways of the lungs and the blood vessels of your skeletal muscles.
Memory aid: You've got 2 lungs and 2 types of blood vessels (arteries and veins) so beta-2 receptors are found in the smooth muscle of these 2 places. When epinephrine stimulates beta-2 receptors, the airways and the blood vessels (especially in skeletal muscle) dilate. That is to say, they open up or expand.
- Beta-3 receptors. Not relevant to our lesson. These are found on fat cells of your body.
(Non-)cardioselective Beta Blockers
Now that you've got that down pat, let's move on to the blocking part of beta-blockers. Some medications can block the effects of epinephrine on beta receptors. Hence the name beta blocker, beta antagonist, or beta-adrenergic antagonist. They're synonymous terms.
Beta blockers are used to treat a wide variety of conditions, including high blood pressure and all sorts of heart problems like heart failure or arrhythmias. Some of the beta blockers are non-cardioselective beta blockers. Based on what you just learned, what do you think that means? It means they don't specifically target beta-1 receptors. Instead, they target and thus affect both beta-1 and beta-2 receptors.
Examples of non-cardioselective beta blockers include:
So what do you think these medications do, generally speaking? They block beta-1 and beta-2 receptors. When they block beta-1 receptors, they block the effect of epinephrine on the beta-1 receptors and thus:
- Decrease the heart rate and contractility of the heart (thus reducing the workload on a potentially failing heart).
- Minimize arrhythmias.
- Decrease a person's blood pressure since the release of renin is minimized.
But since such drugs also block epinephrine's effects on beta-2 receptors, they run the risk of constricting a person's airways within the lungs, which may be risky in cases where the patient has asthma or some other lung disorder.
Cardioselective Beta Blockers
So, what do you do in cases where you have a patient with a lung problem and a heart condition that needs a beta blocker? Well, you give them a cardioselective beta blocker, which targets beta-1 but not beta-2 receptors. This way, you avoid the risk of constricting their airways, but you can still treat their high blood pressure or heart condition. Names of such cardioselective (beta-1) blockers include:
Beta receptors are mainly stimulated by epinephrine. Two main types of beta receptors exist:
- Beta-1, which are receptors found in the heart. Their stimulation increases heart rate, contractility (and thereby increases the workload on the heart), the potential for an arrhythmia, and (through a series of steps involving the kidneys) raises blood pressure.
- Beta-2, which are receptors found in the smooth muscle of the body, especially the smooth muscle surrounding the airways of the lungs and the blood vessels of your skeletal muscles. Their stimulation opens up the airways and dilates the blood vessels, namely the ones in the skeletal muscles.
Beta blockers reverse these effects. Non-cardioselective beta blockers, like propranolol or nadolol, helps minimize arrhythmias, blood pressure, heart rate, and the workload on the heart. But they also constrict the airways and may be dangerous for some people with lung disorders.
Cardioselective beta blockers, like atenolol and esmolol, avoid the latter problem by targeting and affecting just the beta-1 (heart) receptors.
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