Capsaicin-induced, capsazepine-insensitive relaxation of the guinea-pig ileum

The mechanisms underlying transient receptor potential vanilloid receptor type 1 (TRPV1)-independent relaxation elicited by capsaicin were studied by measuring isometric force and phosphorylation of 20-kDa regulatory light chain subunit of myosin (MLC20) in ileum longitudinal smooth muscles of guinea-pigs. In acetylcholine-stimulated tissues, capsaicin (1–100 μM) and resiniferatoxin (10 nM–1 μM) produced a concentration-dependent relaxation. The relaxant response was attenuated by 4-aminopyridine and high-KCl solution, but not by capsazepine, tetraethylammonium, Ba2+, glibenclamide, charybdotoxin plus apamin nor antagonists of cannabinoid receptor type 1 and calcitonin-gene related peptide. A RhoA kinase inhibitor reduced the relaxant effect of capsaicin at 30 μM. Capsaicin and resiniferatoxin reduced acetylcholine- and caffeine-induced transient contractions in a Ca2+-free, EGTA solution. Capsaicin at 30 μM for 20 min did not alter basal levels of MLC20 phosphorylation, but abolished an increase by acetylcholine in MLC20 phosphorylation. It is suggested that the relaxant effect of capsaicin at concentrations used is not mediated by TRPV1, but by 4-aminopyridine-sensitive K+ channels, and that capsaicin inhibits contractile mechanisms involving Ca2+ release from intracellular storage sites. The relaxation could be explained by a decrease in phosphorylation of MLC20.