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Diphenhydramine, sold under the brand name Benadryl among others, is an antihistamine and sedative. It is a first-generation H 1 -antihistamine and it works by blocking certain effects of histamine , which produces its antihistamine and sedative effects.
H 1 antagonists, also called H 1 blockers, are a class of medications that block the action of histamine at the H 1 receptor, helping to relieve allergic reactions.Agents where the main therapeutic effect is mediated by negative modulation of histamine receptors are termed antihistamines; other agents may have antihistaminergic action but are not true antihistamines.
The dihydrogen cation or hydrogen molecular ion is a cation (positive ion) with formula +. It consists of two hydrogen nuclei ( protons ), each sharing a single electron . It is the simplest molecular ion .
By definition, an acid is an ion or molecule that can donate a proton, and when introduced to a solution it will react with water molecules (H 2 O) to form a hydronium ion (H 3 O +), a conjugate acid of water. [4] For simplistic reasoning, the hydrogen ion (H +) is often used to abbreviate the hydronium ion.
H1 receptors are linked to allergic responses, H2 to gastric acid regulation, H3 to neurotransmitter release modulation, and H4 to immune system function. There are four known histamine receptors: H 1 receptor – Primarily located on smooth muscle cells , endothelial cells, and neurons.
A histamine agonist is a drug which causes increased activity at one or more of the four histamine receptor subtypes.. H 1 agonists promote wakefulness. [1]H 2: Betazole and Impromidine are examples of agonists used in diagnostics to increase histamine.
Cimetidine was the prototypical histamine H 2 receptor antagonist from which later drugs were developed. Cimetidine was the culmination of a project at Smith, Kline & French (SK&F; now GlaxoSmithKline) by James W. Black, C. Robin Ganellin, and others to develop a histamine receptor antagonist that would suppress stomach acid secretion.
The medication has negligible anticholinergic activity, with 130,000-fold selectivity for the histamine H 1 receptor over the muscarinic acetylcholine receptors (for comparison, diphenhydramine had 20-fold selectivity for the H 1 receptor). [3] It was patented in 1943 and came into medical use in 1949. [4]