Why does aspirin cause bronchospasm?

Answer 1

Aspirin is a non-selective and irreversible inhibitor of a class of enzymes known as COX. These enzymes are responsible for the production of prostaglandins and thromboxane. There are several subtypes of prostaglandins, some of which induce bronchospasm (PGE2) and others that cause bronchodilation (PGI2). But this is unimportant since aspirin will block the production of all prostaglandins.

If you look at the figure below, you can see that COX enzymes are only half of the story in the metabolism of Arachidonic acid. The other half is known as the Lipoxygenase (LOX) pathway

This LOX pathway is responsible for the production of pro-inflammatory chemicals known as Leukotrienes (LTs). By blockade of the COX pathways, all of the arachidonic acid is funneled into the LOX pathway, thereby increasing these pro-inflammatory chemicals. And since pro-inflammatory chemicals are the same chemicals that produce Allergies and anaphylaxis --> Bronchoconstriction occurs.

In other words, by blocking the synthesis of PGs (some of which can bronchodilate), the balance between bronchodilators and bronchoconstrictors is deregulated, leading to a predominance of bronchoconstrictors, hence the Asthma.