Smooth muscle in abnormal airways

The degree of airway responsiveness is generally measured by directly activating the airway smooth muscle (ASM) with incremental doses of inhaled methacholine. In this context, airway hyperresponsiveness (AHR) is defined as an excessive decline in lung function in response to methacholine. Innate or acquired defects in ASM size and/or contractile capacity are often thought to account for AHR. However, many factors lying between inhaled methacholine and the resulting decrease in lung function alter the degree of airway responsiveness. Herein, I review multiple mechanisms whereby an ASM with a normal size and a normal contractile capacity can trigger AHR when it operates in abnormal airways. Cited examples are restricted to studies published from 2018 to 2021.