Most drug actions occur at the cellular level and occurs either on the cell surface or within the cell. Drug action is relatively selective where not all cells respond to a given drug and cell that do respond contain receptor sites for particular drugs. Receptors are chemical groups that participate in some aspect of cell metabolism. Drug molecules must then connect or interact with receptors for drug action to occur. To achieve maximum effect of the drug, all receptor sites should be occupied by the drug molecules.
Additional Elements of the Receptor Theory of Drug Action include the following
- Only drugs with an affinity for the affected tissues are able to interact with receptors and exert pharmacologic effects.
- When a drug molecule chemically binds with a cell receptor, two types of pharmacologic effects may occur:
- Agonism or agonist effect where it prevents stimulation of cell function.
- Antagonism or antagonist effect where it prevents stimulation of cell function by natural body substances (neurotransmitters and hormones) or molecules of other drugs. The antagonist drug will occupy all receptor sites and prevent their interaction with other substances.
- The number of receptor sites available to interact with drug molecules largely determines the extent of drug action.
Specific Mechanism of Drug Movement
In order to act or produce an effect, drugs must be able to reach tissue fluids surrounding responsive cells in adequate concentration.
- Passive diffusion is the most common mechanism. It is the movement of a drug from an area of higher concentration to lower concentration.
- Facilitated diffusion, where drug molecules combine with a carrier substances or enzymes of other protein.
- Active transport is when the drug molecules are moved from an area of lower concentration to one of higher concentration. It requires both a carrier substance and the release of cellular energy
Drug movement and drug action are affected by drug’s ability to cross cell membranes. Cell membranes are complex structures composed of lipid and protein.
- Lipid-soluble drugs are drugs that cross the cell membrane by dissolving in a lipid layer. Most drugs are lipid-soluble.
- Water-soluble drugs cross cell membranes through pores or openings. Most lipid-soluble drugs are able to cross cell membranes more easily than water-soluble ones.
