Histamine induces postprandial tachycardia through a direct effect on cardiac H2-receptors in pythons

1 Zoophysiology, Department of Biological Sciences, University of Aarhus, Denmark; and 2 Department of Molecular Biology, University of Aarhus, Denmark Submitted 31 May 2008 ; accepted in final form 13 December 2008 The intrinsic heart rate of most vertebrates studied, including humans, is elevated during digestion, suggesting that a nonadrenergic-noncholinergic factor contributes to the postprandial tachycardia. The regulating factor, however, remains elusive and difficult to identify. Pythons can ingest very large meals, and digestion is associated with a marked rise in metabolism that is sustained for several days. The metabolic rise causes more than a doubling of heart rate and a fourfold rise in cardiac output. This makes the python an interesting model to investigate the postprandial tachycardia. We measured blood pressure and heart rate in fasting Python regius , and at 24 and 48 h after ingestion of a meal amounting to 25% of body wt. Digestion caused heart rate to increase from 25 to 56 min, whereas blood pressure was unchanged. The postprandial rise in heart rate was partially due to a doubling of intrinsic heart rate. The H 2 -antagonist did not affect heart rate of fasting snakes but decreased heart rate by 15–20 min at 24 h into digestion, whereas it had no effects at 48 h. Thus, the histaminergic tone on the heart rose from none to 30% at 24 h but vanished after 48 h. In anesthetized snakes, histamine caused a systemic vasodilatation and a marked increase in heart rate and cardiac output mediated through a direct effect on H 2 - receptors. Our study strongly indicates that histamine regulates heart rate during the initial phase of digestion in pythons. This study describes a novel regulation of the vertebrate heart. reptile; digestion; heart rate; blood flow; blood pressure Address for reprint requests and other correspondence: N. Skovgaard, Zoophysiology, Dept. of Biological Sciences, Univ. of Aarhus, Bldg. 1131, 8000 Aarhus C, Denmark (e-mail: nini.jensen{at}biology.au.dk )