PHARMACOLOGY - DRUG RECEPTORS - 1

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For a drug to give effect, it needs to interact with certain objects that will cause a sequence of reactions that will give the desired effect. 

These things that they react with are called as RECEPTORS.
The drug can have either of the actions - 
1. Full agonist
2. Full antagonist
3. Partial agonists
4. Inverse agonists

Full agonists have maximum effect. 
Antagonists have zero effect.
Partial agonists give sub-maximal or moderate effect.
Inverse agonists give the opposite effect.

If the drugs having a Full agonistic activity is given with a drug of partial agonistic activity, the partial agonist behaves as a antagonists decreasing the net effect.

Types of antagonisms - 
1. Pharmacokinetic - one drug inhibiting the effect of other one. Example Rifampicin and Oral Contraceptive pills.

2. Chemical - Chemical reactions of drugs of opposite charges that neutralise the effects. Example; Heparin (-ve) and protamine sulphate (+ve).

3. Physiological - 2 different drugs acting on receptors, producing 2 different effects, that may or may not be beneficial. Example - Histamine on H1 receptors causing anaphylaxis and bronchoconstriction; Adrenaline on beta2 receptors causing bronchodilation and relief of those anaphylactic symptoms.

4. Pharmacological - it consists of Competitive and Non - competitive Inhibitors. Competitive inhibition is further described under 2 headings - 
a. Reversible
b. Irreversible

Reversible competitive antagonism is the most common type of antagonism seen. Here there is no change in efficacy, but the potency of the drug is reduced. This is best described as reduction of activity of one drug in presence of other drug which act on same receptor.
Irreversible competitive antagonism is seen with drugs like Aspirin. If low dose aspirin is given (75 - 150 mg), it acts on the COX enzyme (Cyclooxygenase enzyme) causing reduction in platelet formation. This process lasts for 7 days. Hence multiple doses are usually not advised for low dose of aspirin as platelet regeneration is delayed.

Non competitive antagonism is similar to Irreversible competitive antagonism but, the only difference is the location of receptor. The receptor of the irreversible competitive antagonism is the same as site as where the drug acts, but in non competitive it acts on a different site called as ALLOSTERIC site.
Here, there is decrease in efficacy but not in drug potency, which means that, the drug is potent, but the overall action and power is reduced.




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