ouabain

Ouabain is a plant-derived cardiac glycoside, a steroidal compound historically obtained from species in the Apocynaceae family, most notably Strophanthus gratus. It is also known as ouabain or strophanthin G when isolated from plants. The substance has a long history as both an arrow poison and, at times, a medicina­l agent.

Mechanism of action is through inhibition of the Na+/K+-ATPase pump in cardiac myocytes. By blocking this pump, ouabain increases intracellular sodium, which reduces the activity of the sodium-calcium exchanger. The resulting rise in intracellular calcium enhances cardiac muscle contractility (positive inotropy) but can also slow conduction and predispose to arrhythmias. The effect is highly dose-dependent, with a narrow therapeutic window.

Clinical use and status: In the past, ouabain was used to treat heart failure and certain tachyarrhythmias. It has largely been superseded in contemporary medicine by safer cardiac glycosides such as digoxin and digitoxin due to a more favorable safety profile and pharmacokinetics. Today, ouabain is mainly of historical and research interest, and it remains a potent example of a cardiac glycoside with a narrow therapeutic index.

Pharmacokinetics and safety: Ouabain is relatively lipophilic, with good absorption and wide distribution. It is primarily excreted by the kidneys and has a long half-life. Toxicity presents with nausea, vomiting, dizziness, visual disturbances, and life-threatening arrhythmias; electrolyte disturbances, particularly hyperkalemia, are common in poisoning. Management is supportive and focuses on correcting electrolyte imbalances; antidotal therapies may be employed in certain contexts.