Motherwort has hypotensive, sedative and uterine tonic properties
Different investigators, at different times and with slightly different preparations, have reached conflicting conclusions regarding the efficacy of motherwort as a cardiac tonic and hypotensive substance.
The modern motherwort research began in 1930, when Manchurian researchers investigated the pharmacological properties of Leonurus sibiricus, an ancient Chinese species of motherwort closely related to L. cardiacus. They isolated an alkaloid which they called leonurin, and proceeded to determine its effects.
Leonurin caused central paralysis in frogs when injected subcutaneously. Injected into mice, it caused irritation, followed by cramps and finally, respiratory paralysis. Injected into the vein of a cat, it caused a temporary fall in blood pressure, sometimes followed by a slight rise; this seemed to be a peripheral action.
Certain amounts of leonurin were apparently not toxic to heart tissue. In excised frog heart, small doses were slightly stimulating, while large doses were paralysing. In perfusion experiments on toads and rabbits, contraction of the blood vessels was observed in several organs.
A small amount of leonurin injected intravenously into cats quickened the frequency and magnified the amplitude of respiration. An excessive dose however, induced temporary stimulation followed by paralysis and weaker, more irregular respiration. This effect was attributed to the alkaloid's action on the respiratory center.
Leonurin paralyzed the frog's motor nerve endings in a curare-like manner. A small amount of leonurin influenced the excised small intestine of a rabbit in such a way that tonus was regained and the amplitude of the movement magnified, while an excessive dose acted as an inhibitor; the effect was ascribed to the substance's action on the intestinal muscle itself. Tonus of excised rabbit uterus was increased and the amplitude of the movement magnified. Injected into rabbit veins, leonurin caused marked diuretic action.
In 1948, an Italian study investigated leonurus cardiacus and the closely related leonurus marrubiastrum, using the whole plant to prepare ethanol and aqueous extracts. Both plants exhibited mild sedative effects in frogs and rats. The effects were attributed to organic and inorganic components, which were practically insoluble in ethanol. The ethanol extracts had a fleeting depressor effect in the anesthetized dog and a less marked stimulating effect on respiration. L. cardiacus in large doses depressed the isolated frog heart, perhaps because of the K content. Intestine and uterus displayed a very slight stimulation effect. The investigator concluded the plants had no therapeutic value and could not substitute for valerian root.
Years later, utilizing more sophisticated extraction techniques, extracts of motherwort were obtained that exhibited powerful antispasmodic and hypotensive properties. These preparations had sedative effects three times stronger than those of valerian root. Clinical studies also showed antiepileptic activity.
In one study with rabbits, the minimum direct current required to cause bending of the hind leg was determined, both in untreated animals and in those receiving subcutaneous injections of a motherwort extract solution. Trials were made on the sedative action of infusions of motherwort and valerian at different doses. The sedative action of motherwort was one and a half times that of valerian. A mixture of motherwort and valerian at 0.5 ml/kg gave a persistent sedative action.
Using more conventional techniques, another study used liquid extracts of motherwort to obtain the paralysis of the central nervous system of frogs. Contraction of vessels in isolated organs was the reaction to fairly high doses of the concentrated extract. Low doses had no effect on isolated and in situ heart, but high doses decreased the amplitude of contraction. Intravenous injection of the extract sharply decreased blood pressure. Most importantly, lasting hypotension was observed in dogs with experimentally induced hypertension.
Recently, motherwort improved rat myocardial ischemia and mesenteric circulation caused by isoproterenol, reduced heart rate, increased coronary perfusion, and inhibited ADP-induced blood platelet aggregation. It acted more strongly against ADP-induced platelet aggregation than other known herbal inhibitors of platelet aggregation (such as red sage root and ophiopogon root).
In another study, both motherwort and its alcohol extract, K substance, exhibited direct inhibitory action on normal, beating myocardial cells in vitro. K substance was more potent than the whole herb and was able to lower the increased pulsation rate caused by either alpha (neo-synephrine) or beta (isoproterenol) receptor stimulants. This suggests a similar action between K substance and alpha or beta receptor-blocking agents. The action between K substance and calcium chloride varied according to the mode of drug administration. There was inhibitory action when K substance was given prior to CaCl2, but no change occurred when the order was reversed.