Nitric oxide synthase in neurons and nerve fibers of lower airways and in vagal sensory ganglia of man. Correlation with neuropeptides.

The mediator accounting for the major relaxant responses to electrical field stimulation of human airways was previously identified as nitric oxide (NO). In the present study, we examined the distribution of the neuronal isoform of the NO-generating enzyme, nitric oxide synthase (bNOS, type I NOS) in nerve fibers of the human airways (trachea, large and small bronchi, bronchioli) as well as in human intrinsic and sensory ganglia of airway innervation by means of quantitative histochemistry (NADPH-diaphorase technique) and immunohistochemistry. Correlation with substance P (SP) and vasoactive intestinal peptide (VIP) was performed by double-labeling immunohistochemistry. NOS-containing nerve fibers were found to be present in the airway smooth muscle, around submucosal glands, around blood vessels and, very rarely, in the lamina propria. The innervation density of airway smooth muscle by NOS-containing nerve fibers decreased significantly from trachea to large-diameter bronchi to small-diameter bronchi, whereas NOS-containing nerve fibers were completely absent from bronchioli. Colocalization of NOS with VIP but not with SP was frequent in these nerve fibers. In airway intrinsic ganglia, the number of NOS-containing neuronal cell bodies increased from 57% in the trachea up to 83% in small bronchi. Around these perikarya, many nerve fibers displaying VIP-immunoreactive (VIP-IR) or SP-IR were found. In the superior vagal sensory (i.e., jugular) ganglion most of the neuronal cell bodies contained either NOS-IR or SP-IR; a colocalization of both was not as frequent. These data contribute to the understanding of the morphologic basis underlying the functional differences of the neural relaxant responses mediated by NO at different levels of the airway tree.