Mechanism of Action
Because of its properties as a cardiac glycoside, oleandrin interferes in some essential processes within the cell, the most important of these being the inhibition of the Na-K ATPase. This protein enables the cell to exchange the cations Na+ and K+ between the intercellular and extracellular spaces by which, for instance, electronic signaling is made possible in nerve cells. Oleandrin binds to specific amino acids in the protein, causing it to lose its function. After depolarization of the cell in which Na+ flows into the cell, the Na+ cannot be transported back into the extracellular membrane, causing the sodium gradient to disappear. This gradient is the driving force for other transport proteins, such as the sodium-calcium exchanger, which plays an important role in cardiomyocytes.
To make muscle contraction possible, a calcium influx from the extracellular fluid into the cell is crucial. After the muscle contraction, the calcium is normally pumped out of the cell and exchanged for sodium. When the sodium gradient is depleted, calcium cannot be pumped back and, as a consequence, accumulates in the cardiomyocyte.
As a result of the high calcium concentration, actin and myosin filaments will bind stronger, unable to relax properly to make a new contraction possible. This may result in cardiac arrhythmias, in the worst case decreasing cardiac output and causing a shortage in oxygen supply in vital tissues.
Apart from being a potent toxic compound, it may also be used in therapeutic ways. Both oleandrin and oleandrigenin, as well as their relatives, may be able to inhibit proliferation of tumor cells and stimulate their apoptosis as a result of the high concentration of intracellular calcium. In addition, it inhibits excretion of fibroblast growth factor 2 through membrane interaction and through inhibition of the Na,K-ATPase pump.. However, there are no results from clinical testing on humans that support any use as a cancer treatment.
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