Pharmacology Of Histamine Phosphate

Indication	Histamine phosphate is indicated as a diagnostic aid for evaluation of gastric acid secretory function.
Pharmacodynamics	Histamine stimulates gastric gland secretion, causing an increased secretion of gastric juice of high acidity. This action is probably due mainly to a direct action on parietal and chief gland cells.
Mechanism of action	Histamine acts directly on the blood vessels to dilate arteries and capillaries; this action is mediated by both H 1- and H 2-receptors. Capillary dilatation may produce flushing of the face, a decrease in systemic blood pressure, and gastric gland secretion, causing an increased secretion of gastric juice of high acidity. Increased capillary permeability accompanies capillary dilatation, producing an outward passage of plasma protein and fluid into the extracellular spaces, an increase in lymph flow and protein content, and the formation of edema. In addition, histamine has a direct stimulant action on smooth muscle, producing contraction if H 1-receptors are activated, or mostly relaxation if H 2-receptors are activated. Also in humans, the stimulant effect of histamine may cause contraction of the intestinal muscle. However, little effect is noticed on the uterus, bladder, or gallbladder. Histamine has some stimulant effect on duodenal, salivary, pancreatic, bronchial, and lacrimal glands. Histamine also can bind to H3 and H4 receptors which are involved in the CNS/PNS neurotransmitter release and immune system chemotaxis, respectively.
Absorption	Readily absorbed after parenteral administration
Volume of distribution	Not Available
Protein binding	Not Available
Metabolism	Primarily hepatic. Histamine is rapidly metabolized by methylation and oxidation. Methylation involves ring methylation and catalyzation by the enzyme histamine-N-methyltransferase, producing N-methylhistamine, which is mostly converted to N-methyl imidazole acetic acid. 2 to 3% excreted as free histamine, 4 to 8% as N-methylhistamine, 42 to 47% as N-methyl imidazole acetic acid, 9 to 11% as imidazole acetic acid, and 16 to 23% as imidazole acetic acid riboside
Route of elimination	Not Available
Half life	Not Available
Clearance	Not Available
Toxicity	LD50=807 mg/kg (mouse, oral). Side effects can lead to hypertension, hypotension, headache, dizziness, nervousness and tachycardia. Large overdoses can lead to seizures