Evidence that induced histamine is an intrinsic regulator of the microcirculatory system

Evidence is presented that mammals possess an inducible form of histidine decarboxylase, associated anatomically with the microcirculation, which synthesizes histamine at a rate determined by the needs of the tissues for blood under various environmental conditions. Some supporting evidence is: a) microcirculatory changes and histidine decarboxylase activation follow the same stimuli, occur at the same anatomical sites, have parallel time relationships, and are both essentially autonomous; b) histamine can reproduce all the microcirculatory changes of the slow phase of inflammation; c) the inducible enzyme has been found in every tested vascular tissue of the mouse; d) such diverse stimuli as increased room temperature, infection, and injection of reticuloendothelial system activators elicit changes in histidine decarboxylase activity consistent with microcirculatory homeostasis; e) certain poorly understood observations on the glucocorticoids, e.g., their vasoconstrictive action and the increased need for them during inflammation or infection, are compatible with the existence of a vasodilator substance, associated with small blood vessels, which may be produced at an increased rate either locally or systemically.