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!