Key Tasks and Topics
Pharmacodynamics provides a systematic view of the interaction between the body and the drug. A basic understanding of this field is built upon finding answers to three key questions:
- What happens? This refers to the recording of all emerging pharmacological effects.
- Where does it happen? This is the determination of the precise localization of the drug's action.
- How does it happen? Decoding the internal mechanisms of action.
As an additional yet equally significant aspect, pharmacodynamics involves a detailed study of various types of drug action.
Pharmacological Effects (Effectus pharmacologici)
A pharmacological effect is defined as any functional change in the work of organs and systems initiated by the administered drug substance.
The clinical manifestations of such effects are extremely diverse. Notable examples include:
- Hemodynamic shifts: an increase in heart rate or a decrease in blood pressure.
- Sensitivity modulation: an increase in the pain threshold.
- Effects on the central nervous system (CNS): relief of delusions and hallucinations, as well as therapeutic sleep prolongation.
- Thermoregulation: effective reduction of elevated body temperature.
The body's complex of reactions to the same drug substance is traditionally classified into two categories:
- Main (therapeutic) effects. These are beneficial, targeted changes in the functional state for the achievement of which the drug is prescribed in medical practice.
- Side effects. Concomitant changes in the body that are not used for the specific treatment or are outright undesirable and harmful.
Localization of Action (Localisatio actionis)
Localization of action describes the site of predominant influence of the drug substance in the body. In other words, this is the specific target structure with which the drug interacts.
Clear examples of pronounced organotropism (selectivity) include:
- Brain structures, which serve as the primary target for antipsychotics (neuroleptics) and antiparkinsonian drugs.
- The heart, which is the classical site of action for cardiac glycosides (Glycosida cardiaca).
Hierarchy of Drug Action Levels
Modern pharmacology views the localization of action not only anatomically but also systematically, deepening the analysis from the macroscopic to the microscopic level. A classic example of such a hierarchy is the mechanism of action of cardiac glycosides:
- Systemic and organ level. The global effect on the organ as a whole is assessed. In this example, the target organ is the heart.
- Cellular level. The drug's effect on a specific cell pool is studied. For cardiac glycosides, the targets are cardiomyocytes.
- Molecular level. The direct interaction of the drug molecule with a specific biochemical structure (inside or on the surface of the cell) is analyzed. In the case of glycosides, this target is the Na⁺,K⁺-ATPase enzyme located on the membranes of cardiomyocytes.
Briefly: answers to topical questions
What types of drug action exist?
Pharmacology distinguishes paired categories of types of drug action:
- local and resorptive;
- reflex;
- direct and indirect;
- main and side.
What are the mechanisms of drug action?
The interaction of a drug substance with a receptor occurs through intermolecular bonds:
- covalent bonds provide irreversible binding, but are not characteristic of the interaction of drug substances with receptors;
- ionic bonds represent electrostatic attraction between oppositely charged groups;
- ion-dipole and dipole-dipole bonds are characteristic of the interaction of drug substances with receptors;
- hydrogen bonds occur between a positively charged hydrogen atom and electronegative atoms.
What types of drug side effects are distinguished?
Adverse and toxic effects of drug substances include:
- embryotoxic effect — occurs in the first 2–3 weeks after fertilization;
- teratogenic effect — disruption of organ and system formation leading to congenital malformations and developmental anomalies;
- fetotoxic effect — negative impact on the fetus in late pregnancy, including impaired function or maturation of already formed organs;
- mutagenic effect — damage to the genetic apparatus and the occurrence of mutations;
- carcinogenic effect — the ability to provoke the development of malignant neoplasms.
Toxic effects also include organotoxicity, such as damage to the CNS, cardiovascular system, and liver.
What types of receptors are targets for drugs?
According to the method of signal transmission from the ligand to the cell, four types of receptors are distinguished:
- enzyme-linked receptors;
- ligand-gated ion channels;
- G protein-coupled receptors;
- intracellular receptors.
Enzyme-linked receptors include a catalytic domain and provide direct substrate phosphorylation. Ligand-gated ion channels provide direct opening of the ion channel upon ligand binding. G protein-coupled receptors alter the concentration of intracellular second messengers. Intracellular receptors form a complex with the ligand that interacts with DNA in the nucleus, thereby regulating gene transcription.
To quickly recall the three main questions of pharmacodynamics, use the acronym WWH: What happens (effects)? Where does it happen (localization)? How does it happen (mechanisms)?
Class and exam questions
What is a pharmacological effect?
These are specific changes in the functioning of the body's organs and systems initiated exclusively by taking the drug substance.
What is the fundamental difference between a primary effect and a side effect?
The primary effect is the beneficial alteration for which a physician prescribes therapy. The side effect is a concomitant alteration that is not needed for treatment and is frequently undesirable or harmful.
What is meant by a target structure in pharmacology?
This is the site of the drug's predominant action (localization of action). A target can be considered at the level of an entire organ, a cell pool, or specific molecules (e.g., enzymes).
What else to study on this topic
- Types of drug action
- Mechanisms of receptor interaction
- Molecular targets: enzymes, ion channels, and transport systems
- Principles of drug side effect classification
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