Clonidine stimulates force of contraction via histamine H₂ receptors in the human atrium.

"Declarations Ethical approvalAnimals: The investigation conformed to the Guide for the Care and Use of Laboratory Animals as published by the National Research Council (2011). The animals were handled and maintained according to the approved protocols of the Animal Welfare Committees of the University of Halle-Wittenberg, Halle, Germany, and the University of Regensburg, Regensburg, Germany. Humans: This study in patients complies with the Declaration of Helsinki and has been approved by the local ethics committee (hm-bü 04.08.2005). Consent to participateWritten informed consent was obtained for the use of right atrial tissues from patients undergoing cardiac surgery. Consent to publishAll authors declare that they have seen and approved the submitted version of this manuscript. Competing interestsThe authors declare no competing interests. Competing interests The authors declare no competing interests."

"Funding Open Access funding enabled and organized by Projekt DEAL. S.P. was supported by the Fonds der Chemischen Industrie (No. 661688)."