Phenyldiguanide and capsaicin stimulate functionally different populations of afferent C-fibers.

Effects of phenyldiguanide (PDG) and capsaicin (CAP) were compared. (1) The reflex fall in blood pressure following PDG i.v. was only partially but that following CAP i.v. completely inhibited in capsaicin-desensitized rats. (2) Bilateral vagotomy in rats abolished cardiovascular reflexes by PDG i.v., but not those by CAP i.v. (3) Instillation of PDG into the rat trachea did not cause neurogenic plasma extravasation whereas CAP does. (4) Bronchial tone of guinea-pigs remained unchanged after PDG i.v. whereas it was increased by CAP i.v. It is concluded that PDG activates vagal afferent fibers which are distinct from the substance P-containing afferent fibers stimulated by CAP.