RESINIFERATOXIN BINDING TO VANILLOID RECEPTORS IN GUINEA-PIG AND HUMAN AIRWAYS

We have used the [H-3]resiniferatoxin (RTX) binding assay to character ize for the first time a vanilloid (capsaicin) receptor in tracheobron chial tissues of the guinea pig. Membranes obtained from the trachea a nd the main bronchi bound RTX with an affinity of 1 nM; the cooperativ ity index was close to unity, indicating noncooperative binding. Speci fic [H-3]RTX binding was fully inhibited by capsaicin (K-i = 500 nM) a nd capsazepine (K-i = 100 nM), but it was not inhibited at all by the inactive RTX structural analog resiniferonol 9, 13, 1 4-orthophenylace tate (10 mu M), confirming the specificity of the binding. Neither was RTX binding inhibited by the functional vanilloid antagonist rutheniu m red (100 mu M). The density of specific RTX binding sites was simila r in the trachea (Bmax = 150 fmol/mg protein) and the bronchi (Bmax = 170 fmol/mg protein). In keeping with the marked resistance of hamster s to capsaicin actions, no specific RTX binding could be detected in t he airways of this species. By contrast, we have been able to demonstr ate specific RTX binding sites in human bronchi: the estimated affinit y for RTX, 2 nM, was similar to that (7 nM) determined in guinea pig b ronchi. We conclude that (1) the [H-3]RTX binding assay affords a nove l biochemical marker for vanilloid-sensitive nerves in the airways, an d (2) this binding assay may be a useful tool to explore species-relat ed differences in the expression and pharmacologic profile of vanilloi d receptors in the airways.