Abstract
Two populations of histaminergic H1 receptors with distinct high and low affinity binding sites were characterized by the specific H1 receptor antagonist [3H]mepyramine in autoimmune myocardium. No saturable binding of the radiolabelled H1 antagonist was observed in normal myocardium. Reaction of autoimmune myocardium with specific H1 agonist (2-thiazolylethylamine (ThEA)) triggered positive inotropy and negative chronotropy, which were inhibited by mepyramine. Inhibitors of phospholipase C and protein kinase C attenuated both the inotropic and chronotropic effects of ThEA, suggesting the participation of phosphoinositide hydrolysis in this phenomenon. The latter was verified by measurement of polyphosphoinositide hydrolysis in autoimmune myocardium following the reaction of ThEA with histaminergic H1 receptors. We conclude that functional H1 histaminergic receptors could involve a distinctive mechanism operating in autoimmune myocardium as a result of cardiac antigen immunization.
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