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Principles and Mechanisms of Drug Action

Drug action is the pharmaco-dynamic parameter in which the study of drug effects and their action is described.
Principles of Drug Action
Drug action is the pharmaco-dynamic parameter in which the study of drug effects and their action is described. One drug alters or inhibits the action of another drug. Drugs don’t produce new functions to any cells/ tissues/organ they just alter the effect of the specific activity.

The basic type of drug action is classified as-
1. Stimulation
Stimulation is defined as the enhancement of the level of activity of specialized cells.
Example:
a. Adrenaline stimulates the heart.
b. Pilocarpine stimulates salivary gland.
c. CNS stimulant action is produced by a high dose of picrotoxin.

2. Depression
Depression is defined as the selective reduction of the activity of specialized cells.
Example:
a. CNS depressant action is produced by barbiturates.
b. Quinidine depresses the heart.
c. Acetylcholine stimulates intestinal smooth muscles but depresses SA node in the heart.

3. Irritation
This predicate a non-selective, often toxic effect on the specialized cells(epithelial cells, connective tissue). Strong irritation can also lead to inflammation, corrosion, necrosis etc.

4. Replacement
Replacement is defined as the use of natural metabolites, hormones, or their derivatives in the state of deficiency.
Example: 
a. Levodopa is used in parkinsonism.
b. Insulin is used in diabetes mellitus.
c. Iron is used in anemia.

5. Cytotoxic Action
Cytotoxic action is the toxic action on cancer cells or on the growing parasites without significantly affecting the host cells that are used for the diagnosis of infection and neoplasms.
Example: 
a. Penicillin
b. Zidovudine
c. Cyclophophamide
d. Chloroquine

Mechanism of Drug Action
- The steps and path followed by the drug to produce its pharmacological action are called the mechanism of action.
- It involves- “HOW, WHERE & WHEN”.
- Most of the drugs produce their effects by interacting with the target molecules and shows the action.
- Drug gives its action through four main functional proteins-
1. Enzymes- Enzymes are complex structure proteins that are produced by cells and acts as a catalyst in specific biochemical reactions.
- Enzymatic activity can be increased by enzyme induction whereas enzymatic activity decreases when enzyme inhibition occurs.
- Drugs can either increase or decrease the rate of reactions mediated by enzymes.
Example: adrenaline stimulates hepatic glycogen phosphorylases and physostigmine competes with acetylcholine for cholinesterase.

2. Ion-Channels
- Ion-channels are present on the membranes of all cells
- They allow ionic movement across the cells from and into the extracellular fluid.
- Ion-channels respond to their specific ion.
- Each drug modulates the opening and closing of the ion channel.
- Drugs acts on ion channels by directly binding to them as the movement of ions occurs.
Types of ion-channel are-
A. Ligand- gated ion channel
B. Voltage- gated ion channel.
Example:
a. Quinidine blocks the myocardial NA+ ion channel.
b. Ethosuccimide inhibits T- type Ca2+ion-channel.
c. Phenytoin modulates Na+ ion channel.
d. Nicorandil opens ATP-sensitive K+ channel.

3. Transporters
- Transporters are the protein that is membrane-bound and transports the drugs inside and outside of the cell.
- Many drugs interact with transporters and bind with them & show their specific actions.
- They are also known as carriers.
Examples:
a. Reserpine blocks the vesicular uptake of nor-epinephrine.
b. Furosemide inhibits Na+Ka+2Cl- co-transporter in ascending loop of Henle.
c. Probenecid restricts the active transport of uric acid, penicillins in renal tubules.
d. Hydrochlorothiazide restricts the Na+Cl- symporter in distal tubule.

4. Receptors
- Receptors are the cellular structures composed of proteins that are postulated to exists in order to mediate between a chemical agent that acts on cells and gives the physiological response.
- These are the binding site located on the surface or inside the effector cell that initiates a response.
- Receptor does not have their own function.
- Receptors are macromolecular having a large complex structure.
- Most of the drugs do not bind directly to the effectors.





Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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