Temperature alters the inotropic, chronotropic and proarrhythmic effects of histamine in atrial muscle preparations from humans and H-receptor overexpressing mice.

We investigated whether hypothermia and hyperthermia can alter the efficacy and potency of histamine at increasing the force of cardiac contractions in mice that overexpress the human H receptor only in their cardiac myocytes (labelled H-TG). Contractile studies were performed in an organ bath on isolated, electrically driven (1 Hz) left atrial preparations and spontaneously beating right atrial preparations from H-TG mice and wild-type (WT) littermate control mice. The basal beating rate in the right atrial preparations from H-TG mice was lowered by hypothermia (23 °C) and elevated by hyperthermia (42 °C). Furthermore, the efficacy of histamine (0.01-100 µM) at exerting positive inotropic effects was more severely attenuated in the left and right H-TG mouse atria under hypothermia and hyperthermia than under normothermia (37 °C). Similarly, the inotropic response to histamine was attenuated under hypothermia and hyperthermia in isolated electrically stimulated (1 Hz) right atrial preparations obtained from humans undergoing cardiac surgery. The phosphorylation state of phospholamban at serine 16 at 23 °C was inferior to that at 37 °C in left atrial preparations from H-TG mice in the presence of 10 µM histamine. In contrast, in human atrial preparations, the phosphorylation state of phospholamban at serine 16 in the presence of 100 µM histamine was lower at 42 °C than at 37 °C. Finally, under hyperthermia, we recorded more and longer lasting arrhythmias in right atrial preparations from H-TG mice than in those from WT mice. We conclude that the inotropic effects of histamine in H-TG mice and in human atrial preparations, as well as the chronotropic effects of histamine in H-TG mice, are temperature dependent. Furthermore, we observed that, even without stimulation of the H receptors by exogenous agonists, temperature elevation can increase arrhythmias in isolated right atrial preparations from H-TG mice. We propose that H receptors play a role in hyperthermia-induced supraventricular arrhythmias in human patients.