Antispasmodic effects of the essential oil of Croton nepetaefolius on guinea-pig ileum: a myogenic activity

The effects of essential oil of Croton nepetaefolius (EOCN) on guinea‐pig‐isolated ileum were studied. We previously demonstrated that EOCN induced reversible relaxation of ileal tone artificially increased by high [K+] nutrient solution. This study aimed to elucidate whether these effects were caused by indirect, neural, or primarily by myogenic effect. EOCN (40 μg/mL) induced a relaxation of basal tone corresponding to approximately 38% of the reference contraction (K+ 60 mm), and was unaltered by 0.5 mm hexamethonium, 0.5 μm tetrodotoxin, 1 μm indomethacin, and 100 μml‐nitroarginine methyl ester (l‐NAME). Epinephrine‐ (100 μm) and EOCN‐induced maximal relaxation of ileum pre‐contracted with 60 mm KCl represented 16.8 ± 2.3% (n = 10) and 95.0 ± 6.4% (n = 6) of KCl‐induced contraction, respectively. EOCN (200 μg/mL) had no effect on the transmembrane resting potential (Em) of ileum in nutrient solutions with normal (5 mm) and high (80 mm) [K+]. EOCN similarly inhibited the contractions induced by KCl, acetylcholine (ACh) and histamine (IC50 values of approximately 18, 28 and 21 μg/mL, respectively). EOCN also inhibited both the nifedipine‐insensitive component of ACh‐induced contraction and the contraction induced by ACh in Ca2+‐free solution. EOCN accelerated the reversal of a KCl‐induced tonic contraction upon withdrawal of Ca2+ from the extracellular medium. Our results suggest that EOCN induces relaxation of guinea‐pig ileum by a direct action on smooth muscle via a mechanism largely independent of alterations of Em and Ca2+ influx, possibly at the level of the contractile apparatus.