Changes in airway sensitivity to histamine are not necessarily paralleled by changed sensitivity to acetylcholine.

In allergic asthmatics, it has been shown that airway sensitivity to histamine closely parallels sensitivity to methacholine, a finding which underlies the widely-held view that airway hyperreactivity of asthma is non-selective. We have been interested to evaluate the capacity of various agents to influence reactivity to intravenous injections of both histamine and acetylcholine in the anaesthetized guinea-pig. Intravenous infusion of PAF or (+/-)isoprenaline are procedures which increase airway reactivity to histamine, but leave reactivity to acetylcholine unaffected; by way of contrast, there is comparable enhancement of hyperreactivity to both histamine and acetylcholine following endotoxin infusion. These observations do not accord with the concept that obstruction of the airways per se accounts for changed reactivity of the airways and question the presumption that PAF might be a pivotal mediator of airway hyperreactivity in allergic asthma.