Gastric acid secretion histamine effects

The histamine H2-receptor (359 amino acids) is best known for its effect on gastric acid secretion. Histamine H2-receptor activation, in conjunction with gastrin and acetylcholine from the vagus, potently stimulate acid secretion from parietal cells. High concentrations of histamine are also present in cardiac tissues and can stimulate positive chronotropic and inotropic effects via H2-receptor stimulation and activation of adenylyl... 

Impromidine is a selective H2 agonist used as a diagnostic aid the potency varies between 9 and 100 times histamine depending on the test system [312, 314,315]. In humans, it produces maximal gastric acid secretion, an effect inhibited by cimetidine [316]. It is a potent inhibitor of histamine methyl transferase and diamine oxidase [315]. The two imidazole units and the... 

These agents are inhibitors of gastric acid secretion (histamine [H-2] antagonists) and offer very effective treatment of peptic ulcers. 

C. Histamine stimulates gastric acid secretion through an effect on Hj-receptors of gastric parietal cells. Although certain antihistamines are metabolized by cytochrome P450 enzymes, histamine does not induce their production. Histamine helps to maintain a wakeful state through an effect on Hj-receptors. Histamine-mediated hronchoconstriction is mediated by Hj-receptors, while histamine-mediated vasodilation occurs as a result of stimulation of Hi- and Hj-receptors. 

Mecftanism of Action AGI Hj-blocker and gastric acid secretion inhibitor that inhibits histamine action at histamine 2 receptors of parietal cells Therapeutic Effect Inhibits gastric acid secretion when fasting, at night, or when stimulated by food, caffeine, or insulin. 

Have potent direct vasodilator actions on vascular smooth muscle Enhance gastric acid secretion through a histamine-like effect Cause hypotension and bradycardia... 

Histamine was synthesized in 1907 and later isolated from mammalian tissues. Early hypotheses concerning the possible physiologic roles of tissue histamine were based on similarities between the effects of intravenously administered histamine and the symptoms of anaphylactic shock and tissue injury. Marked species variation is observed, but in humans histamine is an important mediator of immediate allergic (such as urticaria) and inflammatory reactions, although it plays only a modest role in anaphylaxis. Histamine plays an important role in gastric acid secretion (see Chapter 62) and functions as a neurotransmitter and neuromodulator (see Chapters 6 and 21). Newer evidence indicates that histamine also plays a role in chemotaxis of white blood cells. 

Histamine has long been recognized as a powerful stimulant of gastric acid secretion and, to a lesser extent, of gastric pepsin and intrinsic factor production. The effect is caused by activation of H2 receptors on gastric parietal cells and is associated with increased adenylyl... 

The Hi Receptor and its Ligands. The H receptor mediates effects, through an increase in cyclic adenosine monophosphate (cAMP). such as gastric acid secretion relaxation of airway smooth muscle and of pulmonary vessels increased lower airway mucus secretion esophageal contraclion inhibition of basophil, but not mas cell histamine release inhibition of neutrophil activation and induction or suppressor T cells. There is no evidence that the H- receptor causes significant modulation of lung function in the healthy human subject or in the asthmatic. 

Hi receptor antagonists block the actions of histamine by reversible competitive antagonism at the Hi receptor. They have negligible potency at the H2 receptor and little at the H3 receptor. For example, histamine-induced contraction of bronchiolar or gastrointestinal smooth muscle can be completely blocked by these agents, but the effects on gastric acid secretion and the heart are unmodified. 

Effects of acetylcholine, histamine, prostaglandin I2, and E2, and gastrin on gastric acid secretion by the parietal cells of stomach Gs and Gj are membrane proteins that mediate the stimulatory or inhibitory effect of receptor coupling to adenylyl cyclase. 

The unwanted effects of histamine injections can be partially blocked by Hi histamine receptor antagonists, which were once used to prevent systemic adverse effects of histamine when it was used to stimulate gastric acid secretion, which is mediated by H2 receptors. 

Histamine is an endogenous substance that activates histamine H2, and H3 receptors, and its principal pharmacologic effects involve exocrine glands, extravascular smooth muscles, and the cardiovascular system. H, receptor stimulation increases inositol-1,4,5-triphosphate, which increases intracellular calcium, resulting in vasoconstriction. Activation of H2 receptors increases intracellular cAMP, which mediates gastric acid secretions and cardiovascular effects. H3 receptor stimulation may be involved in feedback inhibition of histamine synthesis and release. 

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