Substance P and neurokinin A in the cat carotid body: localization, exogenous effects and changes in content in response to arterial pO2

Abstract In the present work we studied the occurrence of substance P (SP) and neurokinin A (NKA) in the carotid bodies of cats by means of immunocytochemistry and radioimmunoassay (RIA). We also compared the exogenous effects of SP and NKA on carotid body sensory discharge. SP- and NKA-like immunoreactivities (SP-LI; NKA-LI) were seen in many glomus cells and in a sparse plexus of fine fibers. The SP-LI containing glomus cells and fibers also exhibited NKA-LI, suggesting that both these tachykinins coexist in the carotid body. Chemoreceptor discharge increased both by SP and NKA in a dose-dependent manner. The peak excitation produced by SP and NKA was the same when the effects were compared on an equimolar basis. The tachykinin content of the carotid bodies varied with changes in arterial p O 2 . During normoxia, SP and NKA levels were 57 ± 8 and 85 ± 14 fmol/mg, respectively. When the animals were exposed for 1 h to 100% O 2 , SP content was unchanged (51 ± 4 fmol/mg), whereas NKA levels were significantly lower than during normoxia (29 ± 3 fmol/mg, P