In vitro antispasmodic activity of peracetylated penstemonoside, aucubin and catalpol.

In vitro, the peracetates of penstemonoside, aucubin and catalpol, iridoids isolated from Parentucellia latifolia, antagonize the uterine muscular contractions induced by acetylcholine and calcium, in a similar way to papaverine. The antagonism is non-competitive against acetylcholine (pD2' values: 5.60 for the peracetates of penstemonoside, 5.74 for aucubin, 5.59 for catalpol, and 5.32 for papaverine) and competitive against calcium (pA2 values: 6.60 for the peracetates of penstemonoside, 6.34 for aucubin, 6.48 for catalpol, and 6.23 for papaverine). The two components, phasic and tonic, of the response of the vas deferens to potassium are reduced by the three iridoids. The reduction is similar for both phases. The antispasmodic activity of the three iridoids, similar to papaverine, is related to an inhibiting effect of extracellular calcium, intracellular or both.