Pharmacodynamics

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Chapter: Essential pharmacology : Pharmacodynamics Mechanism Of Drug Action; Receptor Pharmacology

Pharmacodynamics is the study of drug effects. It attempts to elucidate the complete actioneffect sequence and the doseeffect relationship. Modification of the action of one drug by another drug is also an aspect of pharmacodynamics.


PHARMACODYNAMICS

 

Pharmacodynamics is the study of drug effects. It attempts to elucidate the complete actioneffect sequence and the doseeffect relationship. Modification of the action of one drug by another drug is also an aspect of pharmacodynamics.

 

PRINCIPLES OF DRUG ACTION

 

Drugs (except those gene based) do not impart new functions to any system, organ or cell; they only alter the pace of ongoing activity. The basic types of drug action can be broadly classed as:

 

       Stimulation

 

It refers to selective enhancement of the level of activity of specialized cells, e.g. adrenaline stimulates heart, pilocarpine stimulates salivary glands. However, excessive stimulation is often followed by depression of that function, e.g. high dose of picrotoxin, a central nervous system (CNS) stimulant, produces convulsions followed by coma and respiratory depression.

 

      Depression

 

It means selective diminution of activity of specialized cells, e.g. barbiturates depress CNS, quinidine depresses heart.

 

Certain drugs stimulate one type of cells but depress the other, e.g. acetylcholine stimulates intestinal smooth muscle but depresses SA node in heart. Thus, most drugs cannot be simply classed as stimulants or depressants.

 

          Irritation

 

This connotes a nonselective, often noxious effect and is particularly applied to less specialized cells (epithelium, connective tissue). Mild irritation may stimulate associated function, e.g. bitters increase salivary and gastric secretion, counterirritants increase blood flow to the site. But strong irritation results in inflammation, corrosion, necrosis and morphological damage. This may result in diminution or loss of function.

 

          Replacement

 

This refers to the use of natural metabolites, hormones or their congeners in deficiency states, e.g. levodopa in parkinsonism, insulin in diabetes mellitus, iron in anaemia.

 

          Cytotoxic Action

 

Selective cytotoxic action for invading parasites or cancer cells, attenuating them without significantly affecting the host cells is utilized for cure/palliation of infections and neoplasms, e.g. penicillin, chloroquine, zidovudine, cyclophosphamide, etc.

 

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