ABSTRACT
The mechanisms whereby anticholinergics improve asthma outcomes, such as lung function, symptoms, and rate of exacerbation, can be numerous. The most obvious is by affecting the contraction of airway smooth muscle (ASM). The acetylcholine released from the cholinergic nerves is the most important bronchoconstrictor that sets the baseline degree of contractile activation of ASM in healthy individuals. Although the degree of ASM's contractile activation can also be fine-tuned by a plethora of other bronchoconstrictors and bronchodilators in asthma, blocking the ceaseless effect of acetylcholine on ASM by anticholinergics reduces, at any given moment, the overall degree of contractile activation. Because the relationships that exist between the degree of contractile activation, ASM force, ASM shortening, airway narrowing, airflow resistance, and respiratory resistance are not linear, small decreases in the contractile activation of ASM can be greatly amplified and thus translate into important benefits to a patient's well-being. Plus, many inflammatory and remodeling features that are often found in asthmatic lungs synergize with the contractile activation of ASM to increase respiratory resistance. This review recalls that the proven effectiveness of anticholinergics in the treatment of asthma could be merely attributed to a small reduction in the contractile activation of ASM.
