Capsaicin-sensitive structures as potential target sites for neurotensin and bradykinin in guinea pig atria.

We tested the influence of capsaicin (CAP) desensitization on the positive chronotropic and inotropic effects of neurotensin (NT), bradykinin (BK), calcitonin gene-related peptide (CGRP) and noradrenaline (NA) in guinea pig isolated atria. The positive chronotropic and inotropic effects of NT and BK were completely inhibited, whereas those elicited by CGRP and NA were either slightly reduced (CGRP) or unaffected (NA), in CAP-desensitized compared to control atria. Cross-desensitization studies using CAP, NT and BK showed that the positive chronotropic and inotropic effects of CAP are slightly affected, whereas those evoked by BK are markedly reduced in NT-desensitized atria. On the other hand, the positive chronotropic and inotropic effects of CAP and NT were similar in BK-desensitized and control atria. The results were interpreted as an indication that NT, BK and CAP produce their excitatory effects in guinea pig atria by interacting with a common population of CAP-sensitive sensory nerve fibers (presumably substance P (SP)- and CGRP-containing nerve fibers). The absence of cross-desensitization between NT or BK and CAP, or between NT and BK, suggests that the activation and desensitization of atrial, CAP-sensitive sensory nerve fibers by the latter agents involve different receptors and/or mechanisms.