HTR4 and cardiac arrhythmia: These arrhythmias could be due to the stimulation of receptors, as ergometrine binds to and stimulates human H2-histamine and 5-HT4-serotonin receptors (Jacob et al., 2023a) and because H2-histamine and 5-HT4-serotonin receptors can cause cardiac arrhythmias (review: Neumann et al., 2021a; Neumann et al., 2023c).