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

Abstract

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.

Arrhythmogenic Effect of Sympathetic Histamine in Mouse Hearts Subjected to Acute Ischemia

“…This result was evidently a postsynaptic effect, since famotidine did not affect sympathetic HA release. This result is in accordance with previous studies, in which certain H 2 receptor antagonists have been found to exhibit a cardiac protecting effect in ischemic heart diseases, including antiarrhythmia (6,7,9,34). Moreover, results in the present study confirm the mediating role of the H 2 receptor (35,36) and indicate the therapeutic potential of its antagonist in treatment of ischemia-induced ventricular arrhythmias.…”

“…Cardiac H 2 receptors mediate positive chronotropic and inotropic effects (9,(31)(32)(33). In the present study, the selective H 2 receptor antagonist famotidine was found to slightly decrease the incidence of ischemia-induced ventricular arrhythmia and markedly reduced the duration.…”

Improved cardiac filling facilitates the postprandial elevation of stroke volume inPython regius

“…Pentobarbital anaesthesia does, however, mute autonomic regulation in reptiles (e.g. Crossley et al, 1998;Skals et al, 2005), and the typical withdrawal of parasympathetic tone is likely to explain the higher f H of the anaesthetized pythons than in previous studies on fully recovered pythons (Enok et al, 2012;Skovgaard et al, 2009). The lack of body movement during anaesthesia may also affect venous return, but will do so similarly in both groups and therefore have little effect on a comparison of cardiovascular parameters.…”

“…While the tachycardia stems from a combination of NANC (non-adrenergicnon-cholinergic) factors and a withdrawal of cholinergic tone (Enok et al, 2012;Skovgaard et al, 2009), the rise in V S remains more enigmatic. The postprandial increase of V S in pythons was originally ascribed to a rapid and extensive growth of the heart allowing for an elevated end-diastolic volume (Andersen et al, 2005;Secor and White, 2010), but a number of recent studies clearly demonstrate that heart mass does not increase during digestion (Hansen et al, 2013;Henriksen et al, 2014;Jensen et al, 2011;Slay et al, 2014).…”

In situcardiac perfusion reveals interspecific variation of intraventricular flow separation in reptiles

“…Wood et al, 1977;Overgaard et al, 2002a;Wang and Hicks, 2008) and 30 ml min −1 kg −1 for snakes and the bearded dragon at room temperature (e.g. Skovgaard et al, 2009)] by adjusting filling pressure.…”