Venom Uses and Pharmacology
Venoms may have multiple uses in the same organism. Venom may be offensive or defensive. Offensive uses of venom are usually used for capturing and subduing prey and are typically injected orally by the envenomating animal. Defensive venoms are usually epidermal such as with poison dart frogs and stingrays. Some animals, such as wasps, may use venom for both offensive and defensive purposes.
Poison Dart Frogs have epidermal toxins that are used primarially for defense. Image taken in Costa Rica by Hays Cummins.
Snakes use venom predominately to immobilize prey, although there is also a digestive function to venom as well. De-fanged snakes still live functionally, so the digestive element of venom function cannot be essential, but it has been observed that venom causes more rapid digestion and hair loosening in prey.
The chemistry and pharmacology of serpent venom is very complex. Venoms are very stable compounds, retaining potency even after 50 years of storage (Scholium, 1983). The rout of venom entry in the envenomated animal is very important in determining the lethality of the dose. The LD50 is a common term in pharmacology that represents the dose that is lethal to 50% of test animals. The LD50 of intravenously injected venom was almost always the lowest in tests with different venoms and different test animal species. This is largely due to membrane impermeability issues with subcutaneous and intraperitoneal administration of venom. Studies seem to indicate that membrane transport of venom is mostly through passive diffusion and some facilitated diffusion. Venom distribution (after blood circulation) is usually unequal and is affected by protein and glycoprotien binding on membrane surfaces on certain types of cells as well as membrane permeability. Therefore, certain venoms tend to build up in certain tissue types. Venom is metabolized, usually at a slow rate (in the liver), and also elicits and immune response. Venom, once metabolized, and sometimes in its original form, is excreted through the kidneys. This can be a severe problem with nephrotoxic venoms because the kidney itself is the site of venom action, thus inhibiting the proper filtration and removal of venom from the body.
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