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

Abstract

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 μM L-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 (IC 50 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.