The mechanism by which leukotriene D4 (LTD4) induces airway narrowing in man is unclear. We have investigated this by examining the effect of the calcium channel blocker verapamil on the sensitivity of in vitro preparations of human bronchi to LTD4 and methacholine, and on the bronchoconstriction induced in normal subjects by these agonists in vivo. In vitro smooth muscle sensitivity was assessed by the concentration of LTD4 and methacholine causing a 50% of maximum contraction (EC50) and as the maximum tension generated. Verapamil did not alter baseline tension or the response to LTD4 but did inhibit contractile responses to methacholine. In vivo studies were performed in six normal subjects; they inhaled increasing concentrations of LTD4 (0.4-50 micrograms/ml) or methacholine (2-64 mg/ml). Airway responsiveness in vivo was expressed as the provocation concentration (PC) of agonist producing a 35% fall in specific airways conductance (PC35sGaw) and a 30% fall in flow at 30% of vital capacity (PC30 V30(p)). Verapamil did not alter baseline sGaw or V30(p). One subject did not respond to LTD4 on either day. In contrast to the in vitro results, verapamil produced a greater than 10 fold reduction in LTD4 induced bronchoconstriction, but had no effect on methacholine induced bronchoconstriction. These results suggest that in normal subjects bronchoconstriction induced by inhaled LTD4 is due to a combination of direct and indirect mechanisms.
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