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 mug/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 mum tetrodotoxin, 1 mum indomethacin, and 100 mum L-nitroarginine methyl ester (L-NAME). Epinephrine- (100 mum) and EOCN-induced maximal relaxation of ileum pre-contracted with 60 mm KC1 represented 16.8 +/- 2.3% (n = 10) and 95.0 +/- 6.4% (n = 6) of KC1-induced contraction, respectively. EOCN (200 mug/mL) had no effect on the transmembrane resting potential (E..) of ileum in nutrient solutions with normal (5 mm) and high (80 mm) [K+]. EOCN similarly inhibited the contractions induced by KC1, acetylcholine (ACh) and histamine (IC50 values of approximately 18, 28 and 21 mug/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 KC1-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 E-m and Ca2+ influx, possibly at the level of the contractile apparatus.