Nerve fibres containing peptide histidine isoleucine (PHI) in the respiratory tract

Recent studies have indicated that vasoactive intestinal peptide (VIP) and peptide histidine isoleucine (PHI) are formed by cleavage of a common precursor protein. In the present study we have examined the distribution of nerve fibres displaying PHI-like immunoreactivity in the upper respiratory tract of several mammalian species including man. PHI fibres were found to have the same general distribution as previously described for VIP fibres. These fibres were distributed mainly in the subepithelial connective tissue around both seromucous glands and blood vessels. In the tracheal wall, PHI fibres were also seen in the non-vascular smooth muscle. Sequential immunostaining for PHI and VIP revealed co-existence of the two peptides in the same nerve fibres.