Distribution of peptide histidine isoleucine in the mammalian respiratory tract and some aspects of its pharmacology.

Peptide histidine isoleucine (PHI), a newly discovered neuropeptide, has been detected by RIA and immunocytochemistry in the upper respiratory tracts of the guinea pig, rat, and cat. HPLC of tracheal extracts showed a single peak of PHI immunoreactivity in each species. The immunoreactive PHI peak found in the guinea pig and rat trachea was eluted earlier than the corresponding peak from the cat, which was coeluted with the porcine PHI standard. Immunocytochemistry showed PHI immunoreactivity to be present within ganglion cells and nerve fibers in the respiratory tracts of all three species. The distribution of PHI was similar to that of vasoactive intestinal polypeptide and ganglion cells were found to contain both PHI and vasoactive intestinal polypeptide immunoreactivities. Pure natural porcine PHI induced a dose-dependent relaxation of isolated guinea pig tracheal muscle which was not blocked by antagonists to catecholamines, histamine, 5-hydroxy-tryptamine, and acetylcholine. PHI may thus be one of the local factors in respiratory control.