FARMACODINAMIA GOODMAN Y GILMAN | GuiaMed

Introduction to Pharmacodynamics

Overview of the Topic

• The presentation introduces pharmacodynamics, led by Cristian Pusar Yalcón as part of the GuíaMed project.

• Key topics include definitions, cellular communication, types of pharmacological receptors, mechanisms of action, drug interactions, and characteristics of drugs with examples.

Definition of Pharmacodynamics

• Pharmacodynamics is defined as the study of biochemical and physiological effects and mechanisms of action of drugs on the body.

• It emphasizes understanding how drugs communicate with the body through cellular communication.

Cellular Communication

Mechanism of Cellular Interaction

• Cells communicate by synthesizing substrates that convert into enzymes leading to messengers that exit cells to interact with other cells.

• These messengers seek out specific receptors on target cells to elicit various physiological effects.

Effects Induced by Messengers

• The effect induced by a messenger depends on the type of cell affected; for example:

• Stomach: Increases or decreases hydrochloric acid secretion.

• Heart: Alters heart rate (increase or decrease).

• Muscles: Causes contraction or relaxation.

• Blood Vessels: Triggers vasodilation or vasoconstriction.

• Brain: Can excite or relax neuronal activity.

Receptors in Pharmacodynamics

Understanding Receptors

• A receptor is defined as a binding site for a drug where it exerts its selective action.

• Drugs directly bind to these receptors within cells to produce their intended effects.

Types of Receptors

1. Ionotropic Receptors

• These are ligand-gated ion channels that act within milliseconds. They allow ions like sodium and chloride to enter or exit cells, influencing excitatory or inhibitory responses.

2. Metabotropic Receptors

• Coupled with G-proteins, these receptors act over seconds. Upon activation by a drug, they initiate signaling cascades that can open ion channels indirectly.

3. Enzymatic Receptors

Mechanisms of Drug Action and Receptor Interaction

Intracellular Receptors and Drug Interaction

• Intracellular receptors are located in the cytoplasm or nucleus, where drugs can bind to proteins or receptors, leading to changes at the DNA level.

• An example is dexamethasone, which enters cells and stimulates protein synthesis that alleviates pain.

Mechanism of Action: Agonism vs. Antagonism

• Drugs interact with receptors to either stimulate (agonism) or inhibit (antagonism) physiological processes; they do not create new functions within the body.

• For instance, a muscle can only contract or relax; drugs may enhance these actions but cannot introduce new capabilities.

Understanding Agonists

• Agonists stimulate physiological processes; for example, they can induce vasodilation in blood vessels.

• In cases of hypertension (high blood pressure), agonists can be used to promote vasodilation and normalize blood vessel function.

Example of Agonist Action

• In hypertension, increased vasoconstriction reduces blood flow, causing symptoms like headaches.

• Administering vasodilators as agonists helps restore normal vascular function by promoting dilation and reducing high blood pressure.

Understanding Antagonists

• Antagonists bind to receptors without activating them, effectively blocking their function.

• In cases of excessive vasodilation (e.g., septic shock), antagonists can prevent further dilation by occupying receptor sites without triggering a response.

Example of Antagonist Action

• If a patient experiences extreme vasodilation due to shock, using antagonists prevents additional dilation by blocking receptor activation.

• This indirect effect helps stabilize vascular tone by preventing further drops in blood pressure through receptor blockade.

Conclusion on Drug Interactions

Understanding Pharmacological Interactions

Synergism in Pharmacology

• Definition of Synergism: Refers to the increased pharmacological action of a drug when administered alongside another drug.

• Types of Synergism: There are two types:

• Synergism of Sum: The combined effect is equal to the sum of individual effects.

• Synergism of Potentiation: One drug enhances the effect of another beyond mere summation.

• Example of Synergism of Sum: In a case where a patient has bradycardia (40 beats per minute), administering Drug X increases heart rate by 20 bpm, and Drug Y increases it by 30 bpm. Together, they can raise the heart rate to 90 bpm.

• Example of Synergism of Potentiation: When treating a severe bacterial infection, using two antibiotics together (e.g., trimethoprim and sulfamethoxazole) can effectively eliminate resistant bacteria more than either antibiotic alone.

Antagonism in Pharmacology

• Definition of Antagonism: The reduction or nullification of one drug's pharmacological action due to another drug's presence.

• Types of Antagonism:

• Competitive Antagonism: One drug competes with another for binding at the same receptor site, blocking its action.

• Example of Competitive Antagonism: Acetylcholine promotes muscle contraction while atropine blocks this effect by occupying the same receptor site.

Bronchodilator Function and Competitive Antagonism

Understanding Non-Competitive Antagonism

• The function of bronchodilators can be lost due to non-competitive antagonism, where both substances reach their receptors but diminish or nullify each other's actions.

Competitive Antagonism Explained

• Competitive antagonism occurs when two substances compete for the same receptor. An example is histamine (causing bronchoconstriction) versus adrenaline (causing bronchodilation).

Key Concepts in Pharmacodynamics

Accumulation

• Accumulation refers to administering a drug at intervals that do not allow the body to eliminate the previous dose, potentially leading to toxicity over time.

Tolerance

• Tolerance is an exaggerated resistance to ordinary doses of a drug. For instance, caffeine acts as an antagonist of adenosine receptors, which signal fatigue; chronic consumption leads to increased receptor production and diminished effects.

Intolerance

• Intolerance results in exaggerated responses to standard doses of medication. For example, anesthetics may cause excessive muscle relaxation, including respiratory muscles, leading to serious complications.

Tachyphylaxis

• Tachyphylaxis describes a rapid decrease in response intensity after repeated drug administration, contrasting with tolerance which develops chronically.

Characteristics of Drugs

Affinity

• Affinity measures how likely a drug molecule will interact with its receptor. Acetylcholine has higher affinity than propionyl choline, resulting in greater pharmacological effect at lower doses.

Potency

Pharmacodynamics: Key Concepts and Insights

Analgesic Potency Comparison

• Naproxen is identified as a more potent analgesic compared to Aspirin, requiring larger doses of Aspirin to achieve similar effects.

• While both Aspirin and Diclofenac serve pain relief functions, Aspirin exhibits superior antiplatelet activity, making it more effective in that regard.

Efficacy of Medications

• Efficacy refers to a drug's ability to produce the desired therapeutic effect, independent of the dosage administered.

• The effectiveness of Famotidine and Ranitidine against conditions like gastritis or peptic ulcer disease illustrates that efficacy is paramount over dosage considerations.

Intrinsic Activity Explained

• Intrinsic activity measures how effectively a drug-receptor complex can elicit a pharmacological response once the drug binds to its receptor.

Conclusion on Pharmacodynamics

• Understanding key concepts such as mechanism of action, antagonistic functions, and pharmacological interactions is essential for grasping pharmacodynamics.