Category Archives: Beta Antagonists

Metoprolol: Go For the Beta-1!

Metoprolol (Lopressor*) is the prototype for second-generation, or what I like to call souped-up, beta blockers.  Although propanolol is widely used as well, metoprolol has a feature that makes it a safer choice as an antihypertensive.  Right now (and I do mean 12:00 pm on Saturday, May 25, 2013) it is considered the first line drug for hypertension.

Remember the features of an “ideal drug”?  The one that can never exist?  Well, metoprolol is the more ideal answer to some of propanolol’s shortcomings.  The biggest problem with propanolol is that it’s not selective.  Remember, it is a beta-1 and beta-2 antagonist, so it can cause bronchoconstriction by blocking beta-2!

Metoprolol is selective-ish** for the beta-1 receptors.  Woot!  So it’s safer for more patients.

Let’s have a brief overview of the features of a beta-1 antagonist.  For reference, check out the adrenergic receptors section (and don’t forget we are talking about antagonists!).

Negative Chronotropic:  Beta blockers slow down the heart rate

Negative Inotropic:  Beta blockers make the heart a better pump by easing the strength of contractions, thus conserving it’s oxygen supply and energy.

Negative Dromotropic:  Beta blockers slow the rate of conduction so the heart doesn’t spaz out as much. They are actually classified as antidysrhythmics as well, which we will get to later.

All of these things culminate in Reduced Cardiac Output.  Remember that when you have hypertension, there is a lot of resistance in the blood vessels that the heart has to overcome!  So reducing the cardiac output reduces the pressure against the arterial walls, thus reducing blood pressure.  Yeehaw!

*Metoprolol is actually one of the few drugs with a useful (this may be harsh, but some of them are seriously confusing) trade name.  Drugs that constrict blood vessels or cause a rise in blood pressure are called pressors.  I think of the “pressing in” of the arterial walls.  So, Lopressor reminds me that there is low pressor activity, or low pressure.  So you know it’s an antihypertensive!  I still don’t like using trade names, but this one is handy. That, and people use them all the time in clinical settings.  Sigh.

**Yes, selective-ish.  Don’t forget that NO drug is ever entirely selective.  It always affects something else.  But pretty selective is as good as we can get!

Propanolol: LOL!

Get it?  LOL?  Bahahahaha!  Sigh.  Fine.  Well that’s a great way to remember beta blockers, because every last one of them has a generic name that ends in -lol.  This is a great example of why memorizing the generic names of things, instead of the trade names (these are the capitalized ones I put in parentheses), helps you immensely with categorizing drugs like lightning!

Propanolol (Inderal) is the prototype of 1st Generation Beta Blockers.  Propanolol is still prescribed for many things, including hypertension, angina pectoris, and stage fright.  Although it is very effective, it is not the number one drug of choice.  This is because, bless its heart, propanolol is non-selective. It likes both beta-1 and beta-2 just the same!  Can you think of a reason why this might be a problem?

Let’s start with the main therapeutic features of propanolol:

You already know it’s effects on the heart, because you know what the beta-1 receptors do:

1.  Negative Chronotropic (slows the heart rate)

2.  Negative Inotropic (eases the force of contraction)

3.  Negative Dromotropic (slows the rate of conduction)

And the effect of beta blockade on the blood vessels:

4.  Secondary Vasorelaxation (or reducing the amount of resistance from the blood vessels)

You may also remember that there are beta-1 receptors on the kidneys, so:

5. Renin inhibition (stop kidney/heart-controlled vasoconstriction)

But, propanolol also inhibits beta-2!

6.  Bronchoconstriction (anti-bronchodilation, to be more exact)

7.  Uterine relaxation (For our current purposes, just keep this one somewhere in your mind where you store those “fun facts”.  In general, no one gives much of a hoot about beta blockers’ effects on the uterus.  Sigh.)

Those are all the main effects of propanolol on your body!  In summary, they help relieve high blood pressure by loosening the blood vessel walls and stopping renin release, AND help make your heart a better pump by slowing it, letting it relax, and stopping its “go” signal.  All of this adds up to two main therapeutic uses:

1.  Reduced Cardiac Output, which with a little help, leads to…

2.  Antihypertension

But wait, what about the side effects?  For now, just think about that question.  See if you can come up with four possible adverse effects of propanolol, just by looking at what it does!

Beta Blockers: Take a Chill Pill

Beta antagonists, which pretty much everyone just calls beta blockers (and which one of my favorite teachers calls “The Wall”), reduce cardiac output by affecting the heart.  Some of the newest ones, like carvedilol, actually cause vasodilation (can you guess how?), but our prototypes, propanolol and metoprolol, do not.  They are more correctly secondary vasorelaxers (which is a phrase I just made up).  Remember, beta-1 receptors increase cardiac output by making the heart work harder. This also forces more blood through the blood vessels and causes them to resist.  When they are blocked, the heart doesn’t pump blood quite as forcefully.  So, a pure beta blocker, among other things, prevents the smooth muscle from feeling the need to squeeze the heck out of your arteries.  This allows blood to flow more freely!

You may also remember that beta blockers are negative dromotropic, that is, they block some of the electrical conductivity of the heart at the sinoatrial (SA) and atrioventricular (AV) nodes.

Don’t forget that beta-2 is really important for the lungs!  So beta blockers that are non-selective, that is, block both beta-1 and beta-2 receptors, can bronchoconstrict.  We have to be very careful that no one with breathing problems gets these by mistake!

In this section is your overview of three of the various sympatholytics:  the three generations of beta blockers.