Effect of an aqueous extract of Portulaca oleracea leaves on smooth muscle and rat blood pressure.

An aqueous extract of Portulaca oleracea leaves and stems produced a dose-dependent relaxation of guinea pig fundus, taenia coli and rabbit jejunum and a dose-dependent contraction of the rabbit aorta. On spontaneously-beating rabbit right atria and electrically-paced left atria, the extract produced a dose-dependent negative inotropic and chronotropic effects. On rat blood pressure, the extract produced dose-dependent pressor responses. Phentolamine reduced the relaxant effect of the extract on gut smooth muscle and abolished the contractile response on the aorta as well as the pressor response on blood pressure. Guanethidine and tetrodotoxin had no effect on extract-induced relaxant or contractile responses. On rat blood pressure atropine and cyproheptadine had no effect on extract-induced pressor response, whereas propranolol slightly reduced the pressor response. An increase in extracellular calcium reversed the inhibitory effect of the extract on the rabbit atria. The extract may, therefore, act in part on postsynaptic alpha-adrenoceptors and by interference with transmembrane calcium influx.