antihistamines Flashcards

1
Q

describe the histamine receptors

A

H1-4, all GPCRs

H1: Gq type, cause acute allergic reaction, activation causes increase in intracellular calcium

H2: Gs type, causes gastric acid secretion, activation stimulates adenyl cyclase

H3: Gi/o type, involved in neurotransmitter modulation, activation of receptor inhibits adenyl cyclase

H4: Gi/o type (suspected), involved in immunomodulation, activation causes increase in intracellular calcium

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2
Q

what are actions of histamine of smooth muscle

A

causes relaxation in arteriolar and arterial smooth muscle via H1

causes contraction of smooth muscle in veins, bronchial and gastrointestinal smooth muscle and uterus via H1

causes relaxation in rat uterus via H2

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3
Q

what are actions of histamine on gastric acid secretions

A

causes gastric acid secretion via H2 receptors

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4
Q

what are clinical uses of histamine antagonists

A

H1 antagonists: e.g diphenhydramine (crosses blood brain barrier); sedation, treatment of motion sickness, allergies (not preffered because of drowsiness)

H1 antagonists not entering CNS: e.g cetirizine; allergies (only partially effective)

H2 antagonists: e.g ranitidine; suppression of gastric secretion for healing of duodenal ulcers

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5
Q

what causes peptic ulcer formation

A

formed by imbalance between tissue damaging chemicals such as gastric acid and peptic enzymes and tissue protective agents such as;

mucus secretion, bicarbonate secretion

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6
Q

what factors may contribute to peptic ulcers

A

NSAIDs, alcohol, gastric hyperactivity, gastric reflux, ischaemia, shock

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7
Q

what are stimulators of gastric acid secretion

A

acetylcholine, gastrin (peptide hormone), histamine

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8
Q

where is gastric acid secreted from

A

parietal cells in the oxyntic glands

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9
Q

what drugs are used to treat peptic ulcers

A

H2 receptor antagonists, proton pump inhibitors, antacids, coating agents, prostaglandins, anticholinergic agents

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10
Q

what is mechanism for proton pump inhibitors

A

e.g omeprazole
decrease acid secretion by blocking the H+/K+ ATPase of parietal cells

they are weak bases, stronger inhibitors of gastric acid secretion than H2 antagonists and are long lasting (days)

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11
Q

how are antacids used to treat peptic ulcers

A

neutralise gastric acid, however do not raise pH sufficiently to acheive ulcer healing, used to relieve pain from erosion and ulceration

e.g aluminium hydroxide

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12
Q

how are coating agents used in treatment of peptic ulcers

A

agents act as protective barrier for mucosa
e.g sucralfate (aluminium hydroxide and sulphated sucrose): forms complex matrix with mucus which limits proton diffusion and actions of pepsin

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13
Q

how do anticholinergic agents treat peptic ulcers

A

decrease acid secreation by blocking muscarinic receptors on parietal cells

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14
Q

name some 1st and 2nd generation H1 antagonists and some H2 antagonists, what is difference between 1st and 2nd gen H1 antagonists

A

H1 1st gen: diphenhydramine
H1 2nd gen: cetrizine, astemizole
H2: cimetidine, ranitidine

1st gen; cross CNS barrier, have atropine like actions (muscarinic antagonists)

2nd gen; do not cross SNS

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15
Q

what are effects of histamine on neurotransmitter release

A

histaminergic neurones involved in vasopressin release from posterior pituitary

involved in emesis, consciousness and temperature regulation

presynaptic H3 autoreceptors act in neuromodulation

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16
Q

what are effects of histamine on heart

A

has positive chronotropic effects (increases heart rate) via H2 receptors

has positive ionotropic actions (increases ion flow into heart; increasing strength of contraction) via H2 and H1 receptors

reduced AV conduction via H1 receptors

17
Q

how does histamine increase vascular permeability

A

endothelial stimulation via H1 receptors cause endothelial cells to round up, causing intracellular spaces to form

18
Q

what is effect of histamine on glands

A

stimulates gastric secretions via H2 receptors

stimulates secretion from adrenal medulla, pancreas, salivary, lacrymal and bronchial glands, all via H1 receptors

19
Q

what is effect of histamine on pain

A

it stimulates sensory C fibres in skin