1314

12(2): Medicinal and poisonous plants 2

Strophanthus DC.

Bull. Soc. Philom. 3: 122, t. 8 (1802).

APOCYNACEAE

x = 9; Strophanthus caudatus: 2n = 20, Strophanthus divaricatus, Strophanthus gratus, Strophanthus hispidus: 2n = 18

Major species Strophanthus gratus (Wallich & Hook.) Baill.

Strophanthus is an Old World genus comprising 38 species; represented by 30 species in continental Africa, 1 species on Madagascar, 2 species in India, and 6 species in South-East Asia. Strophanthus caudatus has the widest distribution in Malesia, whereas Strophanthus puberulus Pax is endemic to Sumbawa.

Strophanthus seeds and latex have been used in Africa and Asia for the preparation of dart and arrow poison since time immemorial. The poison acts quickly, and even very small quantities are effective; the usual practice is to crush a single seed and to smear the resulting pulp on the tip or barbs of an arrow, sometimes with an adhesive added. When the poison enters the bloodstream death results, in humans usually within a quarter of an hour, in large animals like elephant after a longer time. When the animal killed in this way is to be eaten, the meat directly around the wound is cut away.
Nearly a third of the African Strophanthus species have been used for dart and arrow poison. Medicinal use of many species includes treatment of a wide range of afflictions including rheumatism, venereal diseases, worms, fever, and snakebite. The most important ones, Strophanthus hispidus DC. and Strophanthus sarmentosus DC., are cultivated in West Africa and Strophanthus gratus and Strophanthus kombe Oliv. in Central and East Africa. The latter two species are used in modern medicine.

Recent information is lacking. Information from the 1970s is present for Africa only.

The seeds of Strophanthus gratus contain 4—8% of a mixture of cardiac glycosides (cardenolides), of which 90—95% is g-strophantin (= gratus-strophantin). This compound is better known under the name ouabain, and should not be confused with k-strophantin (= kombe-strophantin, from Strophanthus kombe) which is also a cardiac glycoside, but has a different chemical structure. Other glycosides include acolongifloriside K, strogosidea nd the sarmentosides (only very small amounts). The aglycone of both ouabain and acolongifloriside K is ouabagenin. This aglycone has 6 OH groups, with a correspondingly high water-solubility. Several cardenolides were isolated from Strophanthus divaricatus.
Strophanthus gratus is the ideal arrow poison plant. It fulfills all conditions for a perfect hunting poison: extremely high toxicity, fast and sure effect, unusually high concentrations of the active principle in the seeds, very easily water-soluble and thus easy and unproblematic extraction from the seed. The most important secondary glycoside, acolongifloriside K, is comparable in its toxic properties to ouabain; strogoside, however, present in about equal amounts, shows only low toxicity. The sarmentosides are also highly toxic but because of the very low concentration they play only a small role.
In medicine, ouabain is used as a remedy for congestive heart failure, like the digitalis glycosides. Congestive heart failure is a disease characterized by impaired blood circulation, due to a decrease in the force with which the heart muscle contracts. Cardiac glycosides such as ouabain have a direct cardiotonic action on the myocardium, resulting in an increase in the force of contraction. The increased contractility is caused by inhibition of the membrane bound enzyme Na⁺K⁺ATPase, leading to an increase in the intracellular stores of calcium. When given to a patient suffering from congestive heart failure, the stroke volume of the heart is increased (positive inotropic effect), causing a more effective emptying of the ventricles, and a lowering of the diastolic pressure. In higher doses, cardiac glycosides also have a direct inhibiting action on atrioventricular conduction (negative dromotropic effect) together with a decrease of the heart rate (negative chronotropic effect), and are employed in the treatment of atrial flutter, atrial fibrillation and paroxysmal atrial tachycardia. The effects of cardiac glycosides are particularly dramatic in patients suffering from a combination of congestive heart failure and atrial fibrillation.
When ouabain is applied, its actions are of rapid onset, but of short duration; furthermore, there is little risk of cumulation. It is mainly administered by injection, because it is poorly absorbed orally. Its major disadvantage is its narrow therapeutic range, which is the margin between the therapeutically effective and toxic doses. Toxic effects include vomiting and convulsions, and larger doses lead to cardiac arrest and death, which also explains their success as arrow poisons.
In addition, the reported interaction between ouabain and reserpine (from Rauvolfia spp.) is remarkable. Pretreatment with this alkaloid reduces toxicity of ouabain, simultaneous treatment increases it. This may well explain the success of these mixtures as hunting poison.
Aqueous extracts of the leaves of Strophanthus gratus and Strophanthus hispidus, a folk medicine against snakebite in Africa, show a dose-related ability to prolong the time taken to clot the blood treated with a standardized dose of the venom of the snake Echis carinatus. The venom of Echis carinatus causes rapid intra-arterial clotting of blood, resulting in the death of small animals; death occurs in larger animals and in man, due to depletion of fibrinogen reserves, which leads to internal haemorrhage.
Extracts of Strophanthus hispidus, a species used in African folk medicine for its antirheumatic activity, show significant anti-inflammatory activity against acute inflammation induced by phlogistic agents.
The extracts of both the roots and leaves of Strophanthus hispidus show in vitro inhibition of the bacteria Escherichia coli, Klebsiella pneumoniae, Neisseria gonorrhoeae, Proteus mirabilis, Pseudomonas aeruginosa, Staphylococcus aureus and Streptococcus pyogenes. Strains of the various pathogens with laboratory induced antibiotic resistance were also sensitive to the aqueous and ethanolic extracts of Strophanthus hispidus.

Another cardiac glycoside-yielding plant often used as ingredient of arrow poison is Antiaris toxicaria Lesch.
g-Strophantin (ouabain) (obtained from the seeds of Strophanthus gratus) is also found in the wood and bark of Acokanthera schimperi (A.DC.) Schweinf., another famous arrow poison from Apocynaceae in East Africa. Several other members of the Apocynaceae also contain cardiac glycosides, e.g. Cerbera spp. and Thevetia spp. (peruvoside).

Lianas or sarmentose shrubs; branches lenticellate, usually glabrous. Leaves opposite or ternate, simple, glabrous, with glands in the axils, resembling small stipules; stipules absent. Inflorescence a terminal cyme; bracts mostly caducous. Flowers 5-merous, actinomorphic; calyx mostly caducous, sepals with colleters inside; corolla lobes overlapping to the right in bud, often forming a long twisted acumen, tube widening around the middle, corona 10-lobed, inserted at the base of the lobes, lobes ovate or elongated into narrow tails; stamens included or partly exserted; ovary consisting of two free carpels, often partly connate at base, ovules numerous, style glabrous. Fruit consisting of 2 divergent follicles, connate at base. Seed flattened, with a caducous basal coma and a beaked apical coma. Seedling sometimes with a swollen primary root, cotyledons elliptical or ovate, obtuse at the apex, primary and first leaves resembling mature leaves, but often more narrow.

In areas with distinct dry and rainy seasons, Strophanthus flowers towards the end of the dry season and the beginning of the rainy season; fruits are mature in the dry season. If no distinct dry period occurs, some species flower throughout the year with a peak in the relatively driest period. Strophanthus gratus is a compulsory cross pollinator. Fruit maturation takes about 1 year.

Traditionally the genus Strophanthus is placed in the subfamily Apocynoideae, tribe Wrightieae and subtribe Wrightiinae along with Wrightia, Vallaris, Spirolobium and Beaumontia amongst other Asian representatives of this subtribe. Recently, however, some of these genera have been removed from this tribe.

The Strophanthus species of medicinal importance can be found in primary and secondary forest, often in forest margins and thickets from sea-level to about 1000 m altitude.

Strophanthus is usually propagated by seed but can also be propagated by ripewood cuttings. Under glasshouse conditions in temperate climates ripewood cuttings in early spring are rooted in moist sand in a closed case with bottom heat. Strophanthus should preferably be grown in full light in a fertile moist but well-drained fibrous loam with additional leaf mould.

Cardenolide production in Strophanthus is strictly correlated to morphological differentiation of the tissue or callus culture.

Cultivation of Strophanthus gratus for seed production in South-East Asia is feasible, using clonal material from at least 2 different parental plants. Plants should preferably be grown on trellises to facilitate the work involved for artificial cross pollination. For successful fertilization pollen should be applied to the base of the pistil head.

In Vietnam, ripe fruits of Strophanthus divaricatus are collected in November—December. Leaves can be harvested throughout the year.

Seed weight of naturally grown Strophanthus gratus ranges from 25—30 mg containing 3.6—7.7% g-strophantin. Seed weight of cultivated, artificially cross-pollinated Strophanthus gratus from Java is about 20 mg, containing 3.6—7.2% g-strophantin.

Ripe fruits of Strophanthus divaricatus are opened, the seeds removed, their feathery awn (coma) detached and seeds are dried in the sun or artificially. The seeds serve as a basic source for the extraction of d-strophantin.

Several Strophanthus species, in particular those of ornamental value, are represented in botanical gardens or university collections, in both tropical and temperate regions. However, no concerted efforts to conserve genetic resources or breeding programmes are known.

At present in medicine, cardiac glycosides are only applied in distinct, special cases e.g. the combination of congestive heart failure and atrial fibrillation. In the western world, the drug of choice is in general digoxin (from Digitalis lanatae Ehrh.), but in acute situations ouabain is often preferred and used. g-Strophantin or ouabain is exclusively extracted from the seeds of Strophanthus gratus, which is cultivated for that purpose. In rural communities, however, cardiac glycosides may not be readily available at all times, and therefore local sources may play a role of interest. This is the case with d-strophantin from Strophanthus divaricatus in Vietnam, which is used as an alternative for ouabain, as the latter has to be imported.

Akah, P.A. & Nwambie, A.I., 1994. Evaluation of Nigerian traditional medicines: 1. Plants used for rheumatic (inflammatory) disorders. Journal of Ethnopharmacology 42(3): 179—182.
Beentje, H.J., 1982. A monograph on Strophanthus DC. (Apocynaceae). Mededelingen Landbouwhogeschool Wageningen 82-4. 192 pp.
Ebana, R.U.B., Madunagu, V.E. & Etok, C.A., 1993. Antimicrobial effect of Strophanthus hispidus and Secamone afzeli on some pathogenic bacteria and their drug resistant strains. Nigerian Journal of Botany 6: 27—31.
Houghton, P.J. & Skari, K.P., 1994. The effect on blood clotting of some West African plants used against snakebite. Journal of Ethnopharmacology 44(2): 99—108.
Neuwinger, H.D., 1996. African ethnobotany. Poisons and drugs. Chemistry, pharmacology, toxicology. Chapmann & Hall, London, United Kingdom and Weinheim, Germany. pp. 145—196.
Samuelsson, G. (Editor), 1992. Drugs of natural origin, a textbook of pharmacognosy. Swedish Pharmaceutical Press, Stockholm, Sweden. pp. 192—193.

R. Hendrian

Strophanthus caudatus
Strophanthus divaricatus
Strophanthus gratus

Hendrian, R., 2001. Strophanthus DC.. In: van Valkenburg, J.L.C.H. and Bunyapraphatsara, N. (Editors): Plant Resources of South-East Asia No 12(2): Medicinal and poisonous plants 2. PROSEA Foundation, Bogor, Indonesia. Database record: prota4u.org/prosea

The following species in this genus are important in this commodity group and are treated separatedly in this database:
Strophanthus caudatus
Strophanthus divaricatus
Strophanthus gratus