Virtually all of the histamine found in individual organs and tissues is synthesized locally and stored in subcellu-lar secretory granules. Within the tissues, the mast cells are the principal sites of storage; in the blood, the ba-
sophils serve this function. Histamine is also present in neurons of the CNS, where it acts as a neurotransmitter.
Histamine is synthesized from the amino acid histi-dine by an action of the enzyme histidine decarboxylase (Fig. 38.1). Following synthesis, histamine is either rapidly inactivated or stored in the secretory granules of mast cells and basophils as an inactive complex with proteases and heparin sulfate or chondroitin sulfate.
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