The effect is species dependent, with rodents exhibiting profound vasoconstriction while in humans the effect is relatively minor and practicably negligible due to the predominance of intense small vessel vasodilatation. Increased Automaticity which may lead to arrhythmias. H 2 receptor occupation will result in relaxation. Histamine induced smooth muscle contraction predominates, presumably due to a higher affinity for and greater number of H 1 receptors. Other Smooth Muscle Effects are negligible in humans.
Histamine is one of the primary mediators of gastric acid secretion.
Activation of H 2 receptors will increase acid, pepsin, and intrinsic factor secretion from these glands. At extremely high doses of histamine, an increase in the secretion of adrenaline from the adrenal medulla will occur. All of these effects are mediated by the H 1 receptor. The degree of reactivity of the bronchus to varying doses of histamine correlates to the severity of asthma in the patient. These tests MUST be well controlled in case of massive reactivity and total bronchial obstruction.