CLASSIFICATION AND BIOLOGICAL DISTRIBUTION OF HISTAMINE-RECEPTOR SUBTYPES

The distribution and classification of histamine receptors in mammalia n and avian tissues have been summarized in Tables 1-4. It is evident that histamine receptors are present on a number of morphologically di stinct cell types and the proportion of cells bearing H-1- and H-2-rec eptors varies not only with the species but also with the cell source. The pharmacological receptors mediating mepyramine-sensitive histamin e responses have been defined as H-1-receptors. Receptors mediating me pyramine-resistant, but burimamide or metiamide-sensitive histamine re sponses have been classified as H-2-receptors. Histamine responses med iated via H-2-receptors seem to involve the adenylcyclase system resul ting in elevation of intracellular cyclic-AMP level, which is suscepti ble to burimamide blockade but insensitive to beta-adrenergic blocking agents. This mode of action of histamine involving H-2-receptors and the adenyl cyclase system has been shown to stimulate the mammalian he art; promote gastric acid secretion; inhibit antigen-induced histamine release from leucocytes and inhibit lymphocyte-mediated cytotoxicity. It can further be concluded that both H-1- and H-2-receptors are wide ly distributed throughout the animal body in the gastro-intestinal, re productive, respiratory and cardiovascular systems, nervous system and on mast cells and blood leucocytes. In these tissues, histamine recep tors play an important role in physiological, immunological and immuno pathological processes. Interaction of histamine with both H-1- and H- 2-receptors in varying proportions modulates the overall manifestation of cardiovascular and respiratory syndromes during certain immunopath ological conditions (e.g. inflammation, allergy and anaphylaxis). Hist amine receptors also appear to play an important role in the developme nt of immune-competence and immunity.