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Control of the bronchial tone.

The tone of airway smooth muscle is the functional expression of a dynamic equilibrium between various excitatory and inhibitory mechanisms. On the excitatory side, the vagal reflex mechanism, mainly activated by central airway irritant receptors, is the most important. Bronchial epithelium plays a pivotal role in the regulation of the responsiveness of these receptors. Cholinergic responses of the bronchial muscle are mediated by muscarinic (M2) receptors. On the inhibitory side, the main mechanism is the adrenergic one: beta 2-receptors are mainly stimulated by circulating adrenaline. A dense population of beta 2-receptors has recently been observed in bronchial epithelium. Nonadrenergic-noncholinergic mechanisms--both excitatory and inhibitory--participate in the regulation of bronchial motor tone, but their role in man is still to be defined. Nonsteroidal anti-inflammatory drugs do not modify bronchial motor tone; thus, in this respect, at least in physiological conditions, a modulatory function of the prostaglandin system can be excluded. Probably the same applies to leukotrienes, but for a definite answer, specific antagonists are needed.