Week 4 L3 - Histamine, allergy, and antacids

Dan

How does neurotransmission work and what is its targets?

Mechanism: Nerve terminals release neurotransmitters within synapses.

Targets: They act upon a number of effector tissues (e.g., skeletal muscle in the neuromuscular junction) or various tissues in the autonomic nervous system.

What is the path of an endocrine hormone from secretion to action, and what two physiological criteria must it meet to affect distant tissues?

Path: Hormones are released from endocrine tissues (e.g., anterior pituitary) and released directly into the bloodstream.

Criteria for Action: The hormone needs to be stable and present in a high enough concentration to affect distant tissues.

How does the paracrine system operate regarding local hormones, and what is its specific spatial/distance limitation?

Mechanism: Local hormones or signaling molecules are released from specific cell types embedded in tissues that also contain effector cells.

Limitation: Local transmitters can only travel short distances, meaning they only have effects local to the site of release.

Define the autocrine system, and state which cell type utilises this mode of signaling as a central feature of blood clotting.

Definition: Transmitters are released from cells and act back upon the exact same cell type.

Clinical Example: Platelets are autocrine cells, which is central to blood clotting (studied in Haematology).

Local transmitters involved in the paracrine system are central mediators in which three specific physiological or pathological processes?

1. Mediators of allergy

2. Acid secretion

3. Bronchial asthma

Using histamine as an example of a local paracrine transmitter, what structural rule must apply to the cells that release it?

Because histamine acts locally to the site of release, the cells that release it must be located within the immediate tissue microenvironment of the target effector cells.

What specific name is given to the specialized paracrine cells responsible for synthesising and releasing histamine?

Mast cells

What two other distinct classes of paracrine molecules are synthesized and released by mast cells alongside other paracrine cell types?

1. Prostanoids

2. Leukotrienes

What are the four distinct modes of transmission?

1. Autocrine (looping back to the same cell)

2. Paracrine (acting on immediate neighbouring cells)

3. Endocrine (dumping hormone molecules directly into the bloodstream)

4. Neuroendocrine (a neuron terminating directly into the bloodstream)

Where are mast cells primarily found in the human body, and what are three specific physiological locations explicitly listed?

General Location: Found in aerated tissues.

Examples: 1. Lungs 2. Skin 3. Mucosa of the GI tract (e.g., stomach

Describe the initial event that occurs when a mast cell is first exposed to an allergen, prior to any mediator release.

Exposure to allergens stimulates the binding of IgE antibodies to specific receptors (Fc-epsilon Receptors) on the surface of the mast cells, "priming" the cell for a future secondary response.

What structural event must happen to the membrane-bound IgE antibodies upon a subsequent exposure to the same allergen, and what direct cellular action does this trigger?

Mechanism: The allergen dimerises (cross-links) and activates the membrane-bound IgE antibodies.

Result: This physical cross-linking directly stimulates the degranulation of histamine from the cell.

Which two distinct leukocyte types found within the circulating bloodstream exhibit a similar immunological action to mast cells upon activation?

1. Basophils

2. Neutrophils

(Note: these cells can release alternative mediators like Platelet-Activating Factor [PAF] through specific Fc receptors).

What specific amino acid acts as the precursor for histamine synthesis, and what enzyme catalyzes its transformation?

Precursor: Histidine.

Enzyme: Histidine decarboxylase.

Where is histamine stored inside the cell, and what two specific molecules is it bound to within those compartments prior to release?

It is stored within granules, bound securely to heparin and ATP until allergen binding triggers degranulation

What two enzymes act in combination to metabolise histamine, and what final inactive molecule terminates its transmitter action?

Enzymes: N-methyl transferase and monoamine oxidase.

End Product: N-methyl imidazoleacetic acid.

What two primary histamine receptor subtypes mediate its local actions, and what are the two main characteristic physiological responses explicitly listed on the slide?

Subtypes: H_1​ and H₂​ receptors.

Responses:

1. Characteristic inflammatory responses (associated with response to potential pathogens).

2. Gastric acid secretion.

Which type of cell signalling involves hormones travelling through the bloodstream to distant targets?

Endocrine

What best describes the paracrine system?

Signals act locally on nearby cells

What cells are primarily responsible for releasing histamine in allergic responses?

Mast cells

Histamine is synthesised from which amino acid?

Histidine

Which histamine receptor subtype mediates vasodilation in the vasculature, and what three downstream hemodynamic consequences does this cause?

Receptor: H₁ receptor.

Haemodynamic Impact: Leads to a fall in peripheral resistance, a decrease in venous return, and a subsequent reduction in cardiac output.

Less pressure in blood vessels causes lower blood pressure

What occurs in the microcirculation upon H₁ receptor stimulation, and what are the two local physiological results?

It causes increased permeability in the microcirculation, leading to:

1. Egress of white blood cells to infiltrate regions of potential pathogenic insult.

2. Loss of water, resulting in local oedema (swelling).

What effect does histamine have on sensory nerve endings via the H₁ receptor, and what classic "bite or sting" triad contextualises this?

Effect: Direct stimulation of sensory nerve endings, leading to local pain perception (and itching via the itch-sensitive neurons shown in the diagram).

Clinical Triad: Swelling, redness, and itch.

Contrast the receptor subtype and mechanical action of histamine on gastric tissue versus its action on airway tissue.

In Gastric Tissue: Drives the stimulation of gastric acid secretion mediated specifically by H₂ receptors.

In Airway Tissue: Drives the contraction of bronchial smooth muscle mediated by H₁ receptors.

Gastric acid secretion is under both autonomic and hormonal control. Name the specific neural transmission system and the hormone that simulate this process.

Autonomic Control: Simulated by cholinergic transmission (Acetylcholine/ACh from cholinergic nerves).

Hormonal Control: Simulated via gastrin (released from blood vessels).

What specific cell type coordinates gastric acid secretion, what receptors are activated on its surface by ACh and gastrin, and what is the ultimate molecular pump activated?

Cell Type: Parietal cells.

Receptors: Muscarinic receptors (MR, activated by ACh) and gastrin receptors (GR, activated by gastrin).

Ultimate Target: Activation of these receptors coordinates the stimulation of the proton pump (H+/K+ ATPase).

What localised cellular event releases histamine into the stomach lining, what receptor subtype does it bind to on parietal cells, and what is its intracellular effect?

Source: Mast cell degranulation releases histamine locally in the stomach lining.

Target Receptor: It acts upon H₂ receptors (H₂R) on parietal cells.

Effect: Promotes the catalytic activity of the proton pump, driving gastric acid (H+) secretion into the lumen.

What is the precise pharmacological mechanism of action of classic antacid treatments like cimetidine and ranitidine?

They act as competitive H₂ receptor antagonists, blocking histamine from binding to parietal cells and thereby reducing down-regulated proton pump activity.

What drug class does omeprazole belong to, and what are its precise target and binding kinetics?

Class/Target: Proton pump inhibitor (PPI).

Kinetics: It directly and irreversibly inhibits the (H+/K+ ATPase) proton pump, blocking the final common pathway of gastric acid secretion.

What specific type of drug is betazole, and how does cimetidine experimentally prevent its physiological action at the receptor level?

Betazole: An acid secretion-promoting H₂ receptor agonist.

Mechanism: Cimetidine is a reversible H₂ antagonist that directly outcompetes betazole for the H₂ receptor binding sites on parietal cells, successfully preventing downstream gastric acid secretion.

What are the primary clinical indications for prescribing H₂ antagonists, and what is an example of an over-the-counter (OTC) preparation mentioned?

Indications: Used over-the-counter for indigestion, or prescribed clinically to manage chronic acid reflux and peptic ulcers.

OTC Example: Zantac.

If a patient's symptoms remain uncontrolled by H₂ antagonists, what class of drug can be added or substituted, and how do its binding kinetics compare?

Escalation Drug: A Proton Pump Inhibitor (PPI) like omeprazole.

• Kinetics Comparison: Unlike reversible H₂ antagonists, PPIs are irreversible drugs.

• Note: Weaker formulations (e.g., Nexium) are also available over the counter.

By what mechanism do H₁ antagonists reduce acute allergic reactions, and what four local pro-inflammatory processes do they downregulate?

They act as H₁ receptor antagonists (antihistamines), blocking local histamine action to reduce:

1. White blood cell egress (infiltration)

2. Swelling (edema)

3. Itch/pain (sensory nerve stimulation)

4. Nasal and airway irritation

Why do initial first-generation H₁ antagonists cause prominent central nervous system side effects like drowsiness, and what is a specific clinical drug example?

Mechanism: First-generation agents readily cross the blood-brain barrier and block central H₁ receptors involved in maintaining alertness, cognition, and sleep-wake cycles.

• Example: Diphenhydramine (marketed as a sedative under the name Nytol).

What four non-histaminic receptor systems or channels are inadvertently targeted by old-generation antihistamines, and what is one associated adverse effect?

1. Muscarinic receptors: Causes dry mouth, urinary retention, and sinus tachycardia.

2. Serotonin receptors: Increases appetite and weight gain.

3. alpha-Adrenergic receptors: Causes dizziness and postural hypotension.

4. Cardiac ion channels (K+ channels): Delays ventricular repolarization (QT interval arrhythmia).

Name two newer-generation antihistamines commonly used to treat hay fever, and explain why they lack the prominent sedative properties of older agents.

Drug Examples: Loratadine and acrivistine.

• Pharmacokinetics: These second- and third-generation agents are highly ionized or structurally modified to not cross the blood-brain barrier, allowing them to selectively target peripheral H₁ receptors without causing central sedation.

Which specific structural layer of the bronchiole cross-section contracts and relaxes under direct autonomic control?

The smooth muscle layer.

How do tissue-resident mast cells respond to a perceived respiratory invasion, what specific class of molecules do they release, and what is the uncontrolled pathological result?

Action: They release chemical mediators known as spasmogens (predominantly histamine).

Pathology: This drives uncontrolled smooth muscle contraction, culminating in acute bronchospasm.

What is the role of infiltrating inflammatory cells in a hypersensitive airway?

They infiltrate local areas of perceived invasion in response to chemotactic signaling and drive sustained, local transmitter secretion to amplify the inflammatory state.

What is the physiological function of the bronchiole's epithelial lining, and what happens to it during chronic, uncontrolled asthma?

Function: It forms a barrier that protects the basement membrane.

Chronic Pathology: Over time, it can become severely damaged, desquamated, and structurally compromised.

Describe the clinical characteristics, onset timeline, and primary chemical mediators driving Phase 1 (early phase) of an allergen-induced asthma response.

Pathology: Immediate bronchospasm leading to a sharp drop in FEV1.

• Timeline: Rapid onset shortly after allergen exposure with a short duration (recovering within 2 hours).

• Mediators: Driven by the action of spasmogens on target receptors on bronchial smooth muscle, including histamine, prostanoids, and leukotrienes.

Describe the onset timeline, physiological cause, and patient impact of Phase 2 (late phase) of an allergen-induced asthma response.

Timeline: Later onset (typically beginning around 3–4 hours post-inhalation) and has a significantly longer duration.

Pathology: Results from complex inflammatory signalling rather than simple immediate spasmogens, leading to severe hyper-reactivity of the airways.

Patient Impact: Often has a greater, more severe impact on the patient's breathing capacity over time.

What is the role of H₂ receptor antagonists such as cimetidine?

Block histamine-induced acid secretion

Which receptor subtype is primarily responsible for gastric acid secretiom?

H₂

Which type of drug irreversibly inhibits gastric acid secretion by targeting the proton?

Proton pump inhibitors