Glycyrrhizae Radix is used to treat abdominal pain as a component of Shakuyaku-kanzo-to, a traditional Chinese medicine formulation. We aim at clarifying the antispasmodic principles of Glycyrrhizae Radix, and consequently isolated glycycoumarin as a potent relaxant on the carbamylcholine (CCh)-induced contraction of mouse jejunum. In this paper we investigated the effects and the action mechanism of glycycoumarin on the contraction of mouse jejunum. Glycycoumarin inhibited the contraction induced by various types of stimulants, such as CCh, KCl, BaCl(2), and A23187 (calcium ionophore III) with IC(50) values of 2.93+/-0.94 micromol/l (1.08+/-0.35 microg/ml), 2.59+/-0.58 micromol/l (0.95+/-0.29 microg/ml), 4.09+/-1.82 micromol/l (1.51+/-0.67 microg/ml) and 7.39+/-5.19 micromol/l (2.72+/-1.91 microg/ml), respectively, with a potency similar to that of papaverine (a representative antispasmodic for smooth muscle). Furthermore, pretreatment with glycycoumarin enhanced the relaxation induced by forskolin on CCh-evoked contraction, similar to that by pretreatment with IBMX, a non-specific inhibitor of phosphodiesterases (PDEs). Pretreatment with glycycoumarin also enhanced the relaxation effect of rolipram, a specific inhibitor of PDE isozyme 4, as pretreatment with milrinone, a specific inhibitor of isozyme 3, did. Moreover, the effect of glycycoumarin was associated with dose-dependent accumulation of cAMP, but not cGMP, in mouse jejunum. These results indicate that glycycoumarin has an inhibitory effect on smooth muscle contraction induced by various types of stimulants through the inhibition of PDEs, especially isozyme 3, followed by the accumulation of intracellular cAMP.