The labdane diterpene 3alpha-angeloyloxy-2alpha-hydroxy-13,14Z-dehydrocativic acid (AAHDD) isolated from Brickellia paniculata leaves, produces relaxation of the guinea-pig ileum in vitro by Ca2+ antagonistic effects. In the longitudinal ileal muscle the compound inhibited the tonic contractions induced with 60 mM K+(IC50 = 15.52 +/- 1.93 microM) with half the potency than that of papaverine (IC50 = 6.70 +/- 1.31 MM). Phasic response to high K+ was also blocked by AAHDD, with the ratio IC50 phasic/IC50 to-nic = 7.11. In Ca2+-free, high K+ (60 mM) solution, AAHDD inhibited the contractions induced with 2 mM Ca2+ and pretreatment of the tissues with 10 uM cyclopiazonic acid or 30 pM ryanodine failed to influence the blockade of AAHDD on Ca2+ induced contractions. In muscles loaded with 45Ca, after washing the preparations with AAHDD in high K+ (60 mM) solution, determined 45Ca efflux was similar to that of the control without the diterpene. As the relaxant effect of AAHDD was not influenced by cyclopiazonic acid and by ryanodine, two agents which have been described as modulators of Ca2+ uptake and Ca2+ release from intracellular stores, the obtained results suggest that tension inhibitory effects produced by AAHDD do not involve the mobilization of Ca2+ at intracellular deposits.