Enhanced Ca(2+) mobilization in airway smooth muscle contributes to airway hyperresponsiveness in an inbred strain of rat.

The mechanisms underlying airway hyperresponsiveness are still unknown but increased contractility of airway smooth muscle may play a role. This study sought to demonstrate a relationship between in vivo airway responsiveness and a number of measures of airway smooth muscle responsiveness ex vivo, including intracellular Ca(2+) signaling, by comparing three inbred strains of rat with different degrees of airways responsiveness to methacholine. Lewis, ACI, and Fisher strains of rat were characterized for their pulmonary responses to 5-hydroxytryptamine (5HT) in vivo and Fisher rats were found to be hyperresponsive to 5HT compared with ACI and Lewis rats. The responsiveness of the airways from these strains of rat ex vivo revealed that intraparenchymal airways from Fisher rats significantly narrowed to a greater degree and at a faster rate to 5HT than Lewis rat airways, consistent with their differences in vivo. Intraparenchymal ACI airways, however, narrowed to the same degree as Fisher airways but took longer to do so at a high concentration of 5HT. 5HT caused concentration-dependent increases in intracellular Ca(2+) in airway smooth muscle cells from all three strains of rat, but Fisher and ACI displayed higher responses than Lewis airway smooth muscle. Our results demonstrate that the degree of intracellular Ca(2+) mobilization by 5HT in airway smooth muscle parallels the rate and degree of intraparenchymal airway narrowing and suggest that the degree of intracellular Ca(2+) mobilization plays a role in determining airway smooth muscle contractility.