Famacologia de la flora

CURVAS DOSIS vs RESPUESTA
|Introducing dose-response curves |
|What is a dose-response curve? |
|Dose-response curves can be used to plot the results of many kinds of experiments. The X-axis plotsconcentration of a drug or |
|hormone. The Y-axis plots response, which could be almost anything. For example, the response might be enzyme activity, |
|accumulation of an intracellular second messenger, membrane potential, secretion of a hormone, heart rate or contraction of a |
|muscle.|
|The term "dose" is often used loosely. The term "dose" strictly only applies to experiments performed with animals or people, |
|where you administer various doses of drug. You don't know the actual concentration of drug -- you know the dose you administered.|
|However, the term "dose-response curve" is also used more loosely to describe in vitro experimentswhere you apply known |
|concentrations of drugs. The term "concentration-response curve" is a more precise label for the results of these experiments. The|
|term "dose-response curve" is occasionally used even more loosely to refer to experiments where you vary levels of some other |
|variable, such as temperature or voltage.|
|An agonist is a drug that causes a response. If you administer various concentrations of an agonist, the dose-response curve will |
|go uphill as you go from left (low concentration) to right (high concentration). A full agonist is a drug that appears able to |
|produce the full tissue response.  A partial agonist is a drug that provokes a response, butthe maximum response is less than the|
|maximum response to a full agonist. An antagonist is a drug that does not provoke a response itself, but blocks agonist-mediated |
|responses. If you vary the concentration of antagonist (in the presence of a fixed concentration of agonist), the dose-response |
|curve will run downhill.|
|The shape of dose-response curves |
|Many steps can occur between the binding of the agonist to a receptor and the production of the response. So depending on which |
|drug you use and which response you measure, dose-response curves can have almost anyshape. However, in very many systems |
|dose-response curves follow a standard shape, shown below. [pic] |
|Dose-response experiments typically use 10-20 doses of agonist, approximately equally spaced on a logarithmic scale. For example |
|doses might be 1, 3, 10, 30, 100, 300, 1000, 3000, and 10000 nM. When converted tologarithms, these values are equally spaced: |
|0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0. |
|Note: The logarithm of 3 is actually 0.4771, not 0.50. The antilog of 0.5 is 3.1623. So to make the doses truly equally spaced on |
|a log scale, the concentrations ought to be 1.0, 3.1623, 10.0, 31.623 etc.|
|Since the linkage between agonist binding and response can be very complex, any shape is possible. It seems surprising, therefore,|
|that so many dose-response curves have shapes identical to receptor binding curves. The simplest explanation is that the link |
|between receptor binding and response is direct, so response is proportional to...