INHIBITORY NERVE DISTRIBUTION AND MEDIATION OF NANC RELAXATION BY NITRIC-OXIDE IN HORSE AIRWAYS

The distribution of inhibitory nerves and the mediator of the inhibitory nonadrenergic noncholinergic (iNANC) nervous system were investigated in smooth muscle preparations from seven regions of equine airways. In tissues incubated with atropine and precontracted with histamine, electrical field stimulation produced frequency-dependent relaxation, and the magnitude of the relaxation decreased from trachea to central bronchi and was absent in peripheral airways. The degree of relaxation in bronchi was not simply a function of bronchial size or generation. Propranolol inhibited part of the relaxation only in the cranial trachealis. After propranolol, N-G-nitro-L-arginine, a nitric oxide (NO) synthase inhibitor, eliminated the remaining relaxation in all preparations. This effect was reversed by L-arginine, the NO precursor, but not by D-arginine. Exogenous NO concentration dependently relaxed trachealis. These results indicate that: 1) adrenergic innervation is limited to cranial trachealis, 2) iNANC nerves supply the trachea and central bronchi, and 3) NO mediates iNANC function.