Bioassay of histamine using guinea-pig ileum Lab # 8

Outline Description of ileum Histamine receptors Dose-response curve of histamine

Ileum 3/5 of intestine Empties in the large intestine via ileocecal valve Bile salts, vitamin B12, water and electrolytes absorption Doesn’t have myogenic contraction More sensitive to histamine action

Histamine receptors H1 H2 H3 H4 R subtype Distribution Action Agonist Antagonist H1 Sm. muscle Contraction Hist. Mepyramine Cyproheptadine H2 Gastric mucosa Acid release Cimetidine Ranitidine H3 Presynaptic Autoregulation of histamine release Thioperamide H4 Eosinophils Neutrophils CD4T cells Modulate the production of blood cells & cytokines

Histamine receptors in ileum H1 receptors Receptor type: G-protein-coupled receptor Agonist: Histamine MOA: ++ G-protein → ++phspholipase C → splitting of PIP2 into 1) DAG that increases the opening of calcium channels 2)IP3 which increases calcium mobilization from sarcoplasmic stores; DAG & IP3 lead to increase in the intracellular concentration of calcium and smooth muscles contraction.

Bioassay of histamine using three-point assay W+N W+N W N W+N 0.05 St. 0.1 St. 0.2 St 0.1 T 0.9 St. 0.12 St 0.2 St 0.3 St. 0.4 St. 0.1 T 0.1 St.

Curves On semi-log paper On normal paper * Ht. Ht. * * Log Dose Dose

Conc. = 10 mcg/ml S1= 0.1 ml ~ 0.1 mcg S2 = 0.12 ml ~ 0.12 mcg T = (0.1 + 0.12) /2 = 1.1 mcg