CARDIOVASCULAR JOURNAL OF AFRICA • Vol 23, No 9, October 2012
AFRICA
477
into small pieces, making botanical identification difficult or
impossible.
Plants known to be toxic contain chemical constituents that can
affect a wide range of organ systems; these have been documented
in a number of publications. As far as the cardiovascular system
is concerned, cardiac glycoside-containing plants are important
in both lethal livestock and human poisonings.
3,7
The cardiac
glycosides, which are highly toxic and found in a number of
plants, are usually phytochemicals consisting of an aglycone
(
structurally related to steroid hormones) linked to one or
more sugar molecules. The aglycones of cardiac glycosides
can be divided into two chemical groups, the cardenolides and
bufadienolides.
The primary pharmacological effect of these cardiac glycosides
is to inhibit the Na
+
/
K
+
ATPase exchanger, which increases
intracellular Na
+
concentration, thus reducing the Na
+
gradient
across the membrane and decreasing the amount of Ca
2
+
pumped
out of the cell by the Na
+
/
Ca
2
+
exchanger during diastole.
Consequently, the intracellular Ca
2
+
concentration rises, thereby
occasioning positive inotropy.
Cardiac glycosides have very narrow therapeutic indices and
acute toxicity is most commonly associated with ingestion of
plant material, although chronic toxicity may also sometimes be
seen. In cases of acute intoxication, nausea, emesis and abdominal
pain typically occur, as well as central nervous system effects
including lethargy and weakness. Cardiac effects may manifest
as nearly any type of dysrhythmia, and sudden death with few
premonitory signs may occur.
Prominent among the garden-variety cardiac glycoside-
containing plants is
Nerium oleander
(
oleander,
selonsroos
),
which contains two cardenolides (oleandroside and neriin)
characterised as potent cardiotoxins.
Thevetia peruviana
and
Thevetia thevetioides
(
yellow oleander) contain thevetin, a potent
toxic cardenolide that is widespread throughout the plant, but
particularly concentrated in the fruits. Toxicity is retained when
oleander is dried, and the plant material is very poisonous for both
animals and humans. Ingestion of a single leaf by a child can be
lethal. It is significant that the high toxicity of the plant is reflected
in one of its colloquial names, the ‘Be-still tree’, and its fruit is
referred to as the ‘Be-still nut’.
Other well-known cardiac glycoside-containing plants, such
as
Digitalis purpurea
(
foxglove), contain the cardenolide digoxin
that is used therapeutically in the treatment of cardiac failure
and supraventricular tachycardias. Its use in modern cardiology
is currently limited, primarily because of its toxicity, narrow
therapeutic index and the availability of superior treatment
modalities.
Acokanthera oppositifolia
(
bushman poison bush,
boesmansgif
)
sap contains cardenolides and has been used by the San people
for applying to the tips of their hunting arrows. It is also used
for treating headaches, snakebite, toothache, colds, anthrax and
tapeworm infestation.
2
The cardiotoxic bufadienolides present in
Drimia sanguinea
(
sekanama,
slangkop
)
and
Bowiea volubilis
(
climbing potato,
knolklimop
)
species have also been implicated in human
poisoning.
2
These plants are used by traditional healers as
‘
blood purifiers’ and for treating a variety of conditions, such as
headaches, oedema, infertility and bladder complaints.
It may be challenging to establish the diagnosis of plant
poisoning in patients. It primarily relies on a history of ingestion
of cardiotoxic plant material and/or suspicion generated by
cardiac dysrhythmias. However, obtaining a history may be
difficult as the constituents of many traditional medicines are
often tightly guarded secrets not shared with patients or third
parties. Laboratory analyses for cardiac glycosides are available,
and an immunoassay developed for the detection of digoxin also
potentially cross-reacts with other cardiac glycosides, such as
oleandrin. However, more specific tissue and biological fluid
assays for oleandrin have been developed.
8
Of further interest has been the roles of other African plant
materials which have been claimed to produce antihypertensive as
well as negative inotropic and chronotropic effects.
9-11
While some
of the claims made for these plant materials can be supported by
animal studies, more rigorous preclinical, clinical and toxicity
(
including cardiotoxicity) studies will have to be undertaken.
Management of plant-intoxicated patients includes immediate
discontinuation of further exposure to the toxic plant materials,
administration of activated charcoal and gastric lavage (caveat:
within one hour of ingestion) and monitoring for dysrhythmias.
Digoxin-specific antibody fragments appear to cross-react with at
least some other cardiac glycosides, and therefore have a potential
application in the treatment of poisoning in humans with the
latter phytochemicals.
8
If dysrhythmias are present, appropriate
intervention and general supportive measures should be instituted.
However, the mainstays of protection against the potentially toxic
effects of plant materials remain in the educational realm and the
prevention of ingestion.
PIETER VAN DER BIJL (jun),
Division of Cardiology, Department of Medicine, Faculty of
Health Sciences, Stellenbosch University and Tygerberg
Academic Hospital
PIETER VAN DER BIJL (sen),
Emeritas professor, Department of Pharmacology, Faculty of
Health Sciences, Stellenbosch University
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