Field of Science

Sweet Hearts and Frogs

Last week I was at a workshop on teaching in British Columbia where I learned to make a string from a stick (if you're talking about teaching, it's really useful to be put in a situation where you have to learn something entirely new). We used obsidian points to scrape the outer layer of bark from dried stalks of dogs bane (Apocynum cannabinum), then pulled free the fibrous layer at the surface.  The instructor warned us about handling the plant before it was dried, noting that the sap could disrupt your cardiac rhythm. The ethnobotanist and two chemists in the group immediately murmured, "digoxin?" As you might imagine, an activity that features obsidian points, bone knives and an open fire doesn't lend itself to a quick search of the literature, so we were left to wonder for the evening.

The sap of apocynum plants, such as dogs bane, contains cymarine, which is a potent cardiac glycoside, like digoxin.  The term glycoside indicates these are structures that contain a sugar chemically bound to the rest of the molecule.  The sugary parts of each molecule are the hexagons with the O's in them on the left side.  Cymarine has one such hexose; digoxin has three.  At first glance the molecules might seem very different, but clip the sugars off and the remaining parts of the structures are very similar.

Cardiac glycosides are produced mostly by plants (foxglove, dogs bane, oleander), but toads also secrete them (check out this paper in Heart about someone who took a purported aphrodisiac that contained dried toad venom and died a few hours later from what looked like digoxin poisoning).  So don't kiss any toads, it's not sweet for your heart.

Update: Read the Naked Scientists on why people might lick toads: Tripping over psychogenic toads.