In the previous tutorial, we introduced bronchodilators as a class of drugs which
treat a collection of pulmonary diseases, by reducing resistance to air flow in the
vessels of the lungs. Let’s now get more specific in terms of the mechanisms by
which this pharmacological effect takes place. Salbutamol, sometimes referred to as albuterol,
is the most commonly used bronchodilator. The most common method of drug administration is inhalation
through a small device called an inhaler. This offers spectacular tissue selectivity
and fast pharmacokinetics, administering the drug almost exclusively to where it is needed,
the vessels where air flow is impeded, and very quickly, since it does not have to travel
through the bloodstream like many other drugs. After inhalation of a beta-2 adrenoceptor agonist,
the drug will travel down the airways and pass through the cells lining the bronchus. It will
then bind to the receptor located on the surface of smooth muscle cells. The beta-2
adrenergic receptor is a G protein coupled receptor and signals though the Gαs pathway.
Following receptor activation, the heterotrimer consisting of Gαs, Gβ, and Gγ dissociate into Gαs
monomers and Gβ/Gγ heterodimers. Gαs moves across the cell membrane to bind to adenylate cyclase.
Once bound, it increases its enzymatic conversion of ATP into cAMP. With increased intracellular
cAMP, more cAMP is able to bind to protein kinase A, activating its phosphorylation capabilities.
One of the important targets protein kinase A activates is myosin phosphatase. After myosin
phosphatase is activated, it can dephosphorylate the myosin light chain. When myosin light chain
is in its dephosphorylated state, it is unable to bind to actin and produce contractility cycles.
Lower degrees of contractility leads to more smooth muscle cell relaxation.
This goes some distance in explaining how beta-2 receptor stimulation results in smooth muscle cell
relaxation. However, it is worth considering the role of the Gβ/Gγ dimer-related signaling too.
Once this dimer has dissociated from the G protein heterotrimer,
it is able to activate potassium cation channels. This leads to an efflux of K+ ions
down their concentration gradient, hyperpolarizing the smooth muscle cell.
Depolarization of the smooth muscle cell is required to release calcium
ions from intracellular stores, so this activity results in lower mobilization of calcium ions.
As we recall from the Anatomy & Physiology series when we learned about the mechanism of muscle
contraction, calcium ions are a critical positive regulator of smooth muscle cell contraction,
which act by binding to calmodulin. Then once calcium ions have activated calmodulin,
this complex stimulates myosin light chain kinase, which phosphorylates myosin light chain, leading
to increased myosin-actin contraction cycling. In short, inhibition of calcium mobilization
by beta-2 adrenoceptor activation further inhibits contractility of smooth muscle cells.
In summary, beta-2 adrenoceptor stimulation leads to a relaxation of the smooth muscle
cells in the airways by reducing intracellular calcium ion concentrations and enhancing protein
kinase A activity. Both of these changes in the intracellular signaling dynamic
leads to the equilibrium between myosin light chain phosphatase and myosin light
chain kinase, shifting towards more kinase activity and less phosphatase activity.
This leads myosin light chain to be relatively more dephosphorylated, and as a consequence,
less active. This lower activity leads to less myosin-actin interaction cycling, which causes
greater relaxation of airway smooth muscle cells. The other class of bronchodilators we must discuss
are the muscarinic receptor antagonists, such as tiotropium bromide. Due to the overlapping aspects
of intracellular signaling, a good mechanistic understanding of beta-2 adrenoceptor agonists
makes an understanding of muscarinic receptor antagonists relatively straightforward.
There are five subtypes of muscarinic receptors, which we refer to as M1, M2, M3, M4 and M5,
and these are all activated endogenously by acetylcholine. Interestingly, they are named
“muscarinic” because of the discovery of the agonist muscarine, a powerful toxin produced
by many species of mushrooms, which we have discussed at great length in the mycology series.
These five subtypes have diverse roles all over human physiology, but in the context of
bronchodilators, M3 muscarinic receptors are the most prevalent on bronchial smooth muscle cells,
so that is what we will focus on here, and we will return to the other receptors in later tutorials.
In a drug naïve context, tonic acetylcholine tone activates the M3 muscarinic receptor signaling
cascade. This leads to Gαq dissociation from the Gβ, Gγ heterodimer. Gαq then activates the
membrane-bound enzyme phospholipase C, or PLC. PLC hydrolyzes phosphatidylinositol 4,5-bisphosphate,
referred to as PIP2, into diacyl glycerol, or DAG, and inositol 1,4,5-trisphosphate,
referred to as IP3. DAG remains membrane-bound and activates protein kinase C,
or PKC. Protein kinase C negatively regulates myosin light chain phosphatase,
leading to a shift favouring myosin light chain phosphorylation. As we already discussed,
phosphorylation of myosin light chain leads to greater smooth muscle contractility.
IP3 binds to the IP3 receptor on the endoplasmic reticulum, and activation of this intracellular
receptor liberates calcium ions from the stores possessed by the endoplasmic reticulum.
Again, elevation of intracellular calcium ion concentration leads to an increase in smooth
muscle cell contractility, through calmodulin increasing myosin light chain kinase activity.
With this signaling mechanism in mind, it can be appreciated that blocking acetylcholine from
activating the M3 muscarinic receptor with muscarinic receptor antagonists
reduces contractility and enhances the relaxation of airway smooth muscle cells.
With that, we should now have an idea of the pharmacological interventions
used to achieve bronchodilation as well as a brief introduction into some G protein coupled
receptor signaling cascades which serve as mechanisms of action for these important drugs.
Let’s move forward and look at some other drugs in the pulmonary class.