Autacoids 1: Histamine

Question 1

site of H1 receptor

Answer 1

• BVs
• Smooth muscles
• Skin (sensory nerve endings)
• CNS

Question 2

Site of H2 receptor

Answer 2

• BVs
• gastric parietal cells
• Heart
• Mast cells

Question 3

H1 receptor mechanism

Answer 3

Gq
+ causes vasodilatation by releasing Nitric oxide (which increases cAMP)

Question 4

H2 receptor mechanism

Answer 4

Gs (increases cAMP)

Question 5

Action of H1 receptor

Answer 5

• Vasodilatation
• Bronchoconstriction & GIT spasm
• Skin: itching, urticaria & pain
• function related to appetite in CNS

Question 6

Action of H2 receptor

Answer 6

• Vasodilatation
• Increase HCL secretion
• Negative feedback onto mast cells
• Increase Heart rate & contractility

Question 7

Physiological antagonist of Histamine

Answer 7

Adrenaline

Question 8

Examples of H2 blockers

Answer 8

• Cimetidine
• Ranitidine

-tidine

Question 9

Action of H2 blockers

Answer 9

decrease gastric secretions (used for peptic ulcer)

Question 10

Action of H1 blockers

Answer 10

Antihistaminics

Question 11

Antihistaminic are divided into ____

Answer 11

• 1st gen
• 2nd gen

Question 12

Enumerate 1st gen Antihistaminics

Answer 12

• Dimenhydrinate
• Diphenhydramine
• Carbinoxamine
• Chlorpheniramine
• Clemastine
• Cyclizine
• Mecilizine
• Promethazine
• Hydroxyzine
• Cyproheptadine

Question 13

Enumerate 2nd gen Antihistaminics

Answer 13

Lora, ceti, fexo
* Loratadine
* Cetirizine
* Fexofenadine

Question 14

difference between 1st gen & 2nd gen antihistaminics

Answer 14

• 1st gen can cross BBB, thus have a strong sedative effect
• 1st gen autonomic receptor blocking effect (atropine like + blocks Alpha receptors)
• 1st gen has short duration

Question 15

1st gen antihistaminics effects not releated to Histamine receptors

Answer 15

• Sedation
• Atropine like action: urine retention, dry mouth, blurred vision
• Antiemetic: for motion sickness
• Alpha blocker: postural hypotension
• serotonin receptor blocker (Cyproheptadine)

Question 16

2nd gen antihistaminics effects not releated to Histamine receptors

Answer 16

not present

Question 17

theraputic uses of Antihistaminics

Answer 17

• Allergic reactions (except for bronchial asthema)
• Motion sickenss (by blocking muscarinic receptors & H1 of vomiting center)
• Carcinoid Syndrome = ectgopic secretion of serotonin (block Serotonin (5HT) receptors)

Question 18

Explain why Antihistaminics are ineffective in bronchal asthma

Answer 18

• bronchal asthma involves other mediators aside from histamine, mainly Leukotriens
• It’s Atropine like (opposes bronchodilatation)

Question 19

which 2 antihistaminics are especially used in motion sickness

Answer 19

• Diphenhydramine
• Promethazine

Question 20

which antihistaminic is especially used in Carcinoid syndrome

Answer 20

Cyproheptidine

Question 21

side effects of Antihistaminics

Answer 21

• Sedation: so not used by drivers or Children as may cause convulsions
• Atropine like action: dry mouth, urine retention, blurred vision
• Orthostatic hypotension

early 2nd gen antihistaminics may cause prolonged QT interval & serous arrhythmias = torsade de pointes, especially when given with CYP450 inhibitors (the enzyme that metabolizes early 2nd gen antihistaminics)

Question 22

Drug interaction of 1st gen antihistaminics

Answer 22

can potentiate sedative effect of Hypnotics

Question 23

Drug interactions of 2nd gen antihistaminics

Answer 23

arrhythmia when given with CYP450 inhibtors