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Histamine H1‐Receptor in Heart: Unique Electrophoretic Mobility and Autoradiographic Localization

Histamine H1‐receptors, visualized in the guinea pig heart by autoradiography using [125I]iodoboipyramine as a specific probe, are abundant in the nodal tissue and cardiac vessels but also occur heterogeneously in the myocardium. Following photoaffinity labeling with [125I]iodoazidophenpyramine and electrophoresis, the ligand binding domain of the heart H1‐receptor was shown to be present on a major 68‐kDa and a less abundant 54‐ to 58‐kDa protein. The 68‐kDa protein displayed a molecular size higher in heart than in all other tissues (56 kDa). This indicates the existence of at least two isoforms of the H1‐receptor; the cardiac isoform, however, was pharmacologically indistinguishable from the common isoform studied in cerebellar membranes using available ligands. Its distinct electrophoretic properties suggest that the cardiac isoform may have a unique function.