EFFECT OF HISTAMINE IN AUTOIMMUNE MYOCARDITIS MICE

The contractile effect of histamine, as well as the H1 receptor popula tion and H2 receptor-mediated cAMP production, were measured in cardia c tissue from control normal and autoimmune myocarditis mice. Histamin e triggered positive chronotropy and negative inotropy at high concent rations in both control and autoimmune auricles, H2 receptors being th e most important mediator of these responses. In contrast, in atria fr om autoimmune myocarditis mice, histamine at lower concentrations caus ed positive inotropy and negative chronotropy. These effects, not veri fied in the normal control atria, are mediated by H1 receptors. The ex pression of H2 and H1 receptors mediating the cardiac response to hist amine was evaluated through histamine-stimulated cAMP level and bindin g of [H-3] mephyramine, respectively. Both control and autoimmune myoc ardium were able to increase cAMP levels, an effect that was inhibited by H2 antagonist drug. The amount of cAMP was significantly higher in control myocardium than in those from autoimmune ones. Saturable bind ing of [H-3] mephyramine occurs in autoimmune myocardium, with distinc t high and low affinity binding sites. In control myocardium non-satur able binding was detected. These results suggest that H1 and H2 recept ors coexist in heart from autoimmune myocarditis mice, whereas only H2 receptors are present in myocardium from control mice. The presence o f H1 receptors in autoimmune myocardium could be an important factor i n the regulation of its physiological behaviour.