Antispasmodic effect of the essential oil of Plectranthus barbatus and some major constituents on the guinea-pig ileum.

The effects of the Plectranthus barbatus essential oil (PBEO) at concentrations ranging form 1 to 300 microg/mL and some major constituents, i. e., alpha-pinene, myrcene and caryophyllene in the ratio and amount found in the oil, were studied on the contractility of the guinea-pig ileum. PBEO decreased the basal tonus of the ileum with a maximal response (R (max); % of K (+)-contraction) of 62.7 +/- 3.8 % compared with 36.6 +/- 4.5 % inhibition achieved with alpha-pinene and 68 +/- 2.6 % with papaverine. The other constituents had only a slight effect. PBEO also blocked the phasic contractions evoked by acetylcholine with an R (max) of 85 +/- 3.6 % compared with 54.4 +/- 3.5 % inhibition induced by alpha-pinene and 12.4 +/- 5.6 % with caryophyllene. The contractions induced by histamine or barium chloride were also decreased in amplitude by PBEO with an R (max) of 94.3 +/- 5.7 % and 100 %, respectively. In addition, PBEO relaxed tissues pre-contracted with 60 mM K (+) with an R (max) of 89.7 +/- 2.7 % compared with 55.4 +/- 4.6 % relaxation induced by alpha-pinene. The other major constituents studied had no significant effect. Furthermore, at the higher concentration used, i. e., 300 microg/mL, PBEO decreased the maximal response of calcium chloride (10 ( - 7) to 10 ( - 3) M) induced contraction in depolarized tissues by 29.1 +/- 9.53 % (p < 0.05). In addition, the component of the contraction elicited by a single dose of carbachol (CCh; 100 microM) added on nifedipine (10 microM) treated tissues or at very low extracellular calcium concentration were blocked by PBEO (300 microg/mL) in 85.7 +/- 7.8 % (p < 0.05; n = 5) and 81.2 +/- 6.4 % (p < 0.05; n = 4), respectively. These data show that PBEO has intestinal relaxant and antispasmodic activity and suggest that this effect is not related to antagonism in extracellular receptors for neurotransmitters or autacoids and it seems to occur downstream of calcium entry or release from internal stores. The main active principle for its relaxant and spasmolytic activity seems to be alpha-pinene.