Pharmacodynamics

Pharmacodynamics is the actions of drugs on the body.

For drugs to act upon the body the must be able to exert some chemical influence upon a cell to result in a physiological response. They are capable of doing this by binding to a target molecule (usually proteins).

There are four main kinds of targets for the drugs to bind to:

• Receptors - these are protein molecules that are capable of responding to endogenous chemical signals. They are usually found on the cell membrane, in the cytoplasm or on the nucleus and other organelles.
• Enzymes - both intracellular and extracellular ones.
• Ion Channels
• Transport proteins

Agonists

An agonist can be defined as a drug that binds to a target molecule and results in activation of the receptor and thus a tissue response.

• An agonist forms a complex with the receptor. This complex is dynamic as the agonist will continously dissociate and associate with the receptor. The agonist will continue to do this and thus producing a response, until the concentration of the agonist is reduced to a level at which no tissue response occurs.

• The rate of complex fromation is dependent on two factors: agonist concentration and the number of free receptors.

• The affinty of a drug to a receptor varies and can be compared using the equilinbrum constant or K_A.

This can be defined as the concentration of a drug which results in 50% of receptors being bound in equilibrium or when K₁=K_-1.

Drug + Number of Free Receptors = Drug-Receptor Complexes
 
 Where K1 is the rate constant in a forward direction
 and K-1 is the rate constant in a backward direction

Therefore a drug that has a higher affinity to a receptor has a lower K_A value.