Hi there
What is the difference between IC50 and EC50?
Are they the same? If not, in which cases we should use IC50, and in which cases we should usd EC50?
Thank you in advance.

EC50 describes the effective dose of an agonist

IC50 describes the dose that causes half-maximal inhibition by an antagonist.

Thanks for your reply.
IC50 for the case that a complete inhibition was observed.
EC50 for the case that only parcial inhibition was observed.
It this correct?
Thanks.

No that is not correct

EC50 is the effective dose of AGONIST that half maximally activates the channel/enzyme. You establish an EC50 by applying increasing amounts of AGONIST to your system and you measure the responses for each dose until you reach saturation - ie the point at which increasing the concentration of agonist dose not increase the magnitude of the response. If you normalize all your responses to the maximum and plot them against AGONIST concentration (plot the concentration on Log scale) you will create a dose-response plot, from which you can determine the dose that gives 50% activation - the EC50

Click here for an example of EC50 calculation Agonist-response

IC50 is the dose of ANTAGONIST that half maximally inhibits the channel -

example - if you establish a dose of AGONIST which gives you maximum activated response and then you coapply a dose of ANTAGONIST with that dose of AGONIST which previously gave the maximum activated response when applied alone and you now see only half the maximum activated response, the dose of ANTAGONIST that you applied is the IC50 dose.


Click here for an example of IC50 calculation
innhibitor-response curve

I strongly recommend you go to your library or a bookstore and look at this book

Rang & Dale's Pharmacology: With STUDENT CONSULT Online Access (Paperback)
by Humphrey P. Rang (Author), Maureen M. Dale (Author), James M. Ritter (Author), Rod Flower (Author)

Publisher: Churchill Livingstone; 6 edition (January 25, 2007)
# ISBN-10: 0443069115
# ISBN-13: 978-0443069116

Here is the link to the book on amazon.com

http://www.amazon.com/Rang-Dales-Pharmacology-STUDENT-CONSULT/dp/0443069115/ref=sr_1_1?ie=UTF8&s=books&qid=1205861346&sr=1-1

For a very quick reference, here is the Wikipedia entry for EC50
http://en.wikipedia.org/wiki/Half_maximal_effective_concentration#References

it's ok but only for a staring point of reference - I still recommend you read the text book I told you about in my last post