[Basic study of peptide histidine isoleucine (PHI) immunoreactivity in the rat respiratory tract]

Peptide histidine isoleucine (PHI) as well as vasoactive intestinal peptide (VIP) is considered to be a neurotransmitter of non-adrenergic and non-cholinergic inhibitory nerves in the respiratory tract. We tried to measure PHI immunoreactivity in rat respiratory tracts to evaluate the movement of PHI. A high titer of PHI antiserum which did not react with VIP was obtained from a rabbit immunized with rat PHI, and rat PHI radioimmunoassay was established with this antiserum. Tracheas, extrapulmonary bronchi and lungs were dissected from rats sacrificed by microwave irradiation. They were homogenized with 0.5 N acetic acid and centrifuged at 35,000 X g. Lyophilized supernates were used as samples for the radioimmunoassay. PHI immunoreactivity per g wet weight tissue was 5.12 +/- 1.35 (mean +/- SD) pmol in the tracheas, 1.66 +/- 1.15 pmol in the extrapulmonary bronchi and 0.12 +/- 0.06 pmol in the lungs. PHI immunoreactivity occurred more in the central airways than in the peripheral ones. VIP immunoreactivity in rat respiratory tracts, assayed simultaneously by radioimmunoassay using commercialized VIP antiserum, was as strong as the of PHI, suggesting that PHI and VIP may be produced from the same precursor at the ratio of 1:1 in rat respiratory tracts.