Inhibition by capsazepine of resiniferatoxin- and capsaicin-induced contractions of guinea pig trachea.

The site of action of resiniferatoxin (RTX) and capsaicin and the pharmacological consequences of the resultant tachykinin release were examined in the guinea pig trachea. RTX and capsaicin were both potent and efficacious contractors of isolated tracheal smooth muscle. RTX was about 20-fold more potent than capsaicin, with -log (M) EC50 values of 8.88 +/- 0.09 (n = 14) and 7.55 +/- 0.07 (n = 14), respectively. The putative capsaicin receptor antagonist capsazepine (10 microM) effectively inhibited responses to both RTX and capsaicin in a competitive fashion. The -log (M) pKB values for capsazepine against resiniferatoxin and capsaicin were 6.28 +/- 0.25 and 6.04 +/- 0.13, respectively. Contractile responses to RTX and capsaicin were unaffected by the NK-1 antagonist CP 96345 (0.3 microM), partially inhibited by the NK-2 antagonist SR 48968 (0.3 microM) but nearly abolished by a combination of the antagonists. Capsaicin and RTX desensitized tissues to subsequent additions of either capsaicin (1 microM) or RTX (0.1 microM). Capsaicin showed maximal desensitization at 1 microM, and RTX at 0.1 microM. This study shows that RTX is a potent activator of capsaicin-sensitive tachykinin-containing nerves in the airways. The site of action of RTX and capsaicin appears to be a receptor sensitive to capsazepine. Moreover, RTX and capsaicin both release tachykinins that act on both NK-1 and NK-2 receptor subtypes.