When the drug is administrated through various routes of administration of drugs, what is the effect of drug on our body! is studied under Pharmacodynamics.
Pharmacodynamics involves all the aspects of knowledge about drug Action and their mechanism of action on different parts of body and site of action. We also study the effect of drug after its modification.
Pharmacodynamics includes Principles of drug action, Mechanism of action of drug, Receptors, etc. Therapeutic effect of drug is only seen when the drug interact with the body but it may contain some adverse effect too.
What you get below?
- Principles of drug action
- Mechanism of drug action
- Receptors and its types in short.
- Frequently Asked Questions (FAQs)
Pharmacodynamics is derived from Greek words Pharmacon means drug and dynamis means power. It implies effect of drug on the site of action.
Action is defined as how and where the effect of drug is seen or produced. Effect is defined as the type and intensity of response produced by the administered drug.
Effects of drugs may be expected or unexpected. The expected effects are called as therapeutic effect or quantitative effect and unexpected effect are called as adverse effect or qualitative effect.
Effect of drug is always directly related to concentration of drug in the body.
We cannot determine the concentration of drug by the administered concentration, because after many metabolic processes the concentration that reaches the actual site of action is always less as that of administered concentration.
So, to check the concentration of drug at the site of action, we can determine the concentration in blood, plasma and many other fluids because the rule of Homogeneity implies.
Various site of action can be listed as:
- Within the cell
- In-between two or more cells.
- Outside the cell.
Principles of drug action
Principles of drug action (in Pharmacodynamics) include Stimulation, Depression, Irritation, Replacement, Cytotoxic action.
Principle of drug action states that any drug administered (except genetic based drugs) will never impart a new function to the site of action or any cell, tissue, organ or organ system. It just changes the pace or speed of ongoing activity.
When the drug is administered, the administered drug selectively enhances the activity of the specialized cell.
Example are, when Adrenaline is administered it selectively enhances the activity of Heart, similarly Pilocarpine enhances the activity of salivary glands.
This has a drawback that when excess stimulation occurs, stimulation turns to depression. Examples are High dose of picrotoxin produces convulsions, which is a CNS stimulant. Picrotoxin- CNS stimulant- Convulsion- coma- death.
It is completely opposite to Stimulation. Here when drug is administered it selectively decreases the activity of the specialized cell.
Examples are Quinidine depress heart, Barbiturates depress CNS and produce sedative and hypnotic effect.
The drugs which stimulate one type of cell but depress the other are also included in this category. Examples is Acetylcholine stimulates intestinal mucosa but depress SA node of heart.
Irritation is defined as the unwanted, noxious effect, which is undesirable to human body.
It in drug action says that it is non-selective, noxious effect seen in generally non-specialized cells. These non-specialized cells may be epithelial cells, connective tissue, etc.
Irritation may be mild or sometimes severe. Mild irritations produce some associated functions like bitterness increases salivary secretions along with gastric secretion, Counter-irritants produce increased blood flow to the site.
There are some adverse conditions where strong irritations have caused inflammation this has further changed to necrosis and then to corrosion this whole reaction results into loss of actual function and causes morphological functions.
Replacement simple means using other thing in place of actual. In Pharmacodynamics Same is applied over here use of natural metabolites, hormones or congeners in the deficiency state of actual drug.
5) Cytotoxic action:
Cytotoxic action means administering a drug which produces effect against only affected cell, it will not produce the effect against the other cells which are well.
This is seen during cancer therapy, where the drug shows its significant effect on the cancerous cell only and mostly other cells are unaffected or have minimal effect.
Example is Penicillins and Chloroquine, etc.
This principle is seen in case of cancer and neoplasm.
|Stimulation||Selectively enhances the activity of the specialized cell. If Excess stimulation occurs stimulation turns to depression||Adrenaline- Enhances the activity of Heart Pilocarpine-Enhances the activity of salivary glands|
|Depression||Decreases the activity of the specialized cell. Stimulate one type of cell but depress the other||Quinidine depresses heart. Barbiturates depress CNS.|
|Irritation||Unwanted, Noxious effect on non-specialized cells.||Bitterness increases salivary secretions and gastric secretion. Counterirritants increase blood flow to the site.|
|Replacement||Use of natural metabolites, hormones or congeners.||L-dopa in place of dopamine during Parkinsonism. Use of insulin in diabetes mellitus. Iron in iron deficiency anaemia.|
|Cytotoxic action||Cancer therapy, case of cancer and neoplasm.||Penicillins and Chloroquine.|
Mechanism of drug action
Mechanism of action of drug (in Pharmacodynamics) in unorganized manner can be explained as below but we will study it in detain and in organized manner in next blog. Below we will study mechanism of action of drug in short and have a overview, this will create a base to understand Mechanism in detail and in depth.
Mechanism of action of drug is divided into five main classes as:
- Physical action.
- Chemical action.
- Enzymatic action.
- Transport mechanism.
- Specific action.
Physical action can be explained as bulk laxative absorbs water and it swells, they form a mass in large intestine and this facilitates passage of stools.
2. Chemical action:
Chemical action can be explained in Pharmacodynamics as antacids neutralize acids produced in the stomach and reduces the acidity. This is due to neutralization reaction and a type of chemical reaction.
3. Enzymatic action:
Enzymatic reaction is the most common type of reaction. Enzymes are proteins which do not take part in the chemical reaction but it enhance the rate of reaction.
After the reaction is completed the enzyme can be recovered. Enzymes have the potential to stimulate the reaction. There are two type of enzymatic action and they are Enzymes induction, Enzymes inhibition.
4. Transport mechanism:
It works as; the drugs can alter the movement of ions inside or outside the cell.
In Pharmacodynamics, to explain examples are, Verapamil and Nifedipine prevents the entry of calcium inside the cell and this prevents the contraction of muscle, relaxation of vascular smooth muscle by Hydralazine which opens the potassium channels and promote potassium ion inside the cells.
5. Specific action:
Drugs such as general anesthetics alter the lipids, proteins and water in nerve cell membrane and produce anesthetic action; this explains drugs alter composition of cell membrane.
Drugs can also alter the specific metabolic activity of human cell and also affect the microorganism. Example will explain it better- Penicillin is a antibiotic which inhibit the cell wall synthesis of microorganism but do not have any effect on human cell.
Now a day’s most of the drugs produce their effect by interacting with receptors.
Receptor is a macro-molecule which are present on cell surface or inside the cell that recognises and binds the specific molecules producing specific reaction in the cell. Pharmacodynamics is nothing without Receptor.
In short, it contains a site on which drugs gets attached and desired effect is produced, this effect regulates functioning of cell.
Types of receptors are:
- Ligand gated ion channels (Ionotropic receptors).
- G-protein coupled receptor (Metabotropic receptors).
- Enzymatic receptor (Tyrosinekinase)
- Receptor regulating gene expulsion (Transcription factor or steroid).
Functions of receptors are:
- Receptors propagates regulatory signals.
- It amplifies the signals.
- Receptors integrates with many regulatory signals.
- It maintains homeostasis by many regulatory signals.
Many drugs produce variety of effect on the site of action, this is studied under Pharmacodynamics.
Drugs produce action by their specific mechanism of action that is studied under Pharmacodynamics. Majority of drugs produce their effect by action on receptors.
Receptors provide site for natural substance and for drugs.
Drugs may be agonist or antagonist; this will be studied in next blog in detail.
Agonist has got Affinity as well as intrinsic activity but Antagonist has got Affinity but no intrinsic activity.
The drugs may have adverse effect too, but it is of minor consideration. The therapeutic effect is considered more desirable.
Frequently Asked Questions (FAQs)
Pregnancy may not directly alter Pharmacodynamics but it alter the pharmacokinetic aspect of drug like ADME (Absorption, distribution, metabolism, and Excretion), and this pharmacokinetics and Pharmacodynamics are inter-related with each other.
Drugs act on body through many mechanism like physical, chemical, enzymes and receptors.
Paracetamol or acetaminophen acts as analgesics and antipyretic agent.
Receptors provide the site where the drug gets attached and produce desired effect. There are many types of receptors for many types of drugs.
We cannot say what the chances of adverse effect are because as the name indicates they are undesirable effects. As the effects differ from patient to patient but as the researches have carried out very advance techniques about the desired and undesired effects, we can definitely say that chances have drastically reduced.
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