Pharmacodynamics - Pharmacology

Pharmacodynamics is a drug’s effect on the physiology of the cell and the mechanism that causes the pharmaceutical response. There are two types of effects that a drug delivers. These are the primary effect and the secondary effect. The primary effect is the reason for which the drug is administered. The secondary effect is a side effect that may or may not be desirable.

For example, diphenhydramine (Benadryl) is an antihistamine. Its primary effect is to treat symptoms of allergies. Its secondary effect is to depress the central nervous system causing drowsiness. The secondary effect is desirable if the patient needs bed rest, but undesirable if the patient is driving a car.

A period of time passes after a drug is administered until the pharmaceutical response is realized. This is referred to as the drug’s time response. There are three types of time responses: onset, peak, and duration.

The onset time response is the time for the minimum concentration of drug to cause the initial pharmaceutical response. Some drugs reach the onset time in minutes while other drugs take days. The peak time response is when the drug reaches its highest blood or plasma concentration. Duration is the length of time that the drug maintains the pharmaceutical response.

The response time is plotted on a time–response curve that shows the onset time response, the peak time response, and the duration. All three parameters are used when administering the drug in order to determine the therapeutic range when the drug will become effective, when it will be most effective, and when the drug is no longer effective. It is also used to determine when a drug is expected to reach a toxic level.

For example, the time–response curve of an analgesic is used for pain management.

Once the peak response time is reached, the effectiveness of the drug to block pain diminishes. The time–response curve indicates when the pharmaceutical response is no longer present requiring that an additional dose be administered to the patient.

RECEPTOR THEORY

The pharmaceutical response is realized when a drug binds to a receptor on the cell membrane. These are referred to as reactive cellular sites. The activity of the drug is determined by the drug’s ability to bind to a specific receptor. The better the fit, the more biologically active the drug. Receptors are proteins, glycoproteins, proteolipids, or enzymes. Depending on the drug, binding either initiates a physiological response by the cell or blocks a cell’s physiological response.

Receptors are classified into four families.

  1. Rapid-Cell Membrane-Embedded Enzymes: A drug binds to the surface of the cell causing an enzyme inside the cell to initiate a physiological response.
  2. Rapid-Ligand-Gated Ion Channels: The drug spans the cell membrane causing ion channels within the membrane to open resulting in the flow of primarily sodium and calcium ions into and out of the cell.
  3. Rapid-G Protein-Couple Receptor Systems: The drug binds with the receptor causing the G protein to bind with guanosine triphosphate (GTP). This in turn causes an enzyme inside the cell to initiate a physiological response or causes the opening of the ion channel.
  4. Prolonged-Transcription Factors: The drug binds to the transcription factors on the DNA within the nucleus of the cell and causes the transcript factor to undergo a physiological change.

A drug that causes a physiological response is called an agonist and a drug that blocks a physiological response is referred to as an antagonist. The effect of an antagonist is determined by the inhibitory (I) action of the drug concentration on the receptor site. An inhibitory action of 50 (I50) indicates that the drug effectively inhibits the receptor response in 50% of the population.

Agonists and antagonists lack specific and selective effects. They are called nonspecific and have non specificity properties. Each receptor can produce a variety of physiologic responses. Cholinergic receptors are located in the bladder, heart, blood vessels, lungs, and eyes. A cholinergic stimulator or blocker will affect all of these sites. These drugs are called nonspecific or are said to havenonspecificity properties. A drug that is given to stimulate the cholinergic receptors will decrease the heart rate and blood pressure, increase gastric acid secretion, constrict bronchioles, increase urinary bladder contraction, and constrict the pupils. The effects may be beneficial or harmful.


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