Updated: Sep 4, 2020
This month I have the pleasure of introducing salbutamol: it’s inspirational.
Salbutamol is a “selective beta 2 (β2) adrenoreceptor agonist”. That's quite a mouthful, but hopefully it will make more sense by the end of this blog!
Mechanism of Action
Your lungs resemble a tree.
The trunk is akin to your trachea (or windpipe), while the branches are like the bronchi, which gradually reduce in size as each branch bifurcates. Let’s say our tree is a Cherry Blossom, where every fine branch is adorned with a delicate flower. Well, our bronchi are adorned with delicate gas-filled sacs known as alveoli, which are situated adjacent to capillaries (slender blood vessels). Each alveolus is responsible for exchanging blood gases with the atmosphere i.e. excreting waste gases and drawing in oxygen.
Wow! But how do the lungs work?
You’ll be well aware that, in a healthy adult, the rhythmic rise and fall of your chest coincides with the inspiration and expiration of air, respectively. That’s because when you expand your chest, you contract your diaphragm and the intercostal muscles located between your ribs. This forces your lungs to inflate due to the negative pressure created. Conversely, by relaxing your intercostals and diaphragm, you allow your ribcage to return to its original position under its own weight, thereby forcing air from the lungs.
So my breathing is controlled by these muscles?
Not quite. Your breathing is controlled by the “respiratory centre” in your brain and is under the command of the “sympathetic nervous system” (SNS). (“Sympathetic” may sound like a bit of an unusual title – it is actually a reflection of the fact that the SNS is sympathetic to your basic survival needs, because it manages your “fight or flight” responses.)
When you are placed in a stressful situation, your body releases chemicals known as catecholamines (these include hormones like adrenaline aka epinephrine). These catecholamines get your body ready to act to the urgent situation: to fight, or to flee for your life. The catecholamines "stimulate" your SNS, which has a variety of effects, including: pupil dilation, faster and harder heartbeat, sweating, blood vessel dilation, urinary retention and bronchiole dilation. These are basically the sort of things that you’re thankful for if you have to run or fight effectively, but which are a nightmare if you’re public speaking!
Where does salbutamol come into all this?
As you can see, catecholamines cause a number of different effects, which is partly because they act upon a number of different receptors. In this blog, we’re just going to concentrate on the β2 adrenoreceptor. Salbutamol is an agonist of this receptor i.e. it activates it.
The β2 adrenoreceptor is predominantly located in the lungs - more specifically on the smooth muscle tissue of the bronchi. When activated, these receptors cause the bronchi smooth muscle to relax, which in turn causes them to expand (or to "dilate"). In addition – and very usefully for respiration – activation of the β2 adrenoreceptor also reduces the lung’s hypersensitivity to allergens, thereby inhibiting the release of certain physiological hormones e.g. histamine.
As you will have guessed, salbutamol is used in respiratory conditions that are characterised by a shortness of breath: asthma and COPD (chronic obstructive pulmonary disease), which includes bronchitis and emphysema.
Unusually, salbutamol is sometimes used therapeutically for of one of its side effects rather than to induce bronchodilation.
Hyperkalaemia (high potassium levels in the blood) is potentially fatal and unfortunately, your body can not tolerate much margin for error in your blood serum potassium concentration. Therefore severe hyperkalaemia is rapidly and aggressively managed in hospital and sometimes salbutamol is used to “buy more time” while other agents work, because it “pushes” potassium back into cells for a short period.
Thanks for reading. Have a marvellous weekend, wherever you are!