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 microg/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 microM tetrodotoxin, 1 microM indomethacin, and 100 microM L-nitroarginine methyl ester (L-NAME). Epinephrine- (100 microM) 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 micro/mL) had no effect on the transmembrane resting potential (E(m)) 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 microg/mL, respectively). EOCN also inhibited both the nifedipine-insensitive component of ACh-induced contraction and the contraction induced by ACh in Ca(2+)-free solution. EOCN accelerated the reversal of a KCl-induced tonic contraction upon withdrawal of Ca(2+) 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 Ca(2+) influx, possibly at the level of the contractile apparatus.