Control of heart rate during thermoregulation in the heliothermic lizard Pogona barbata: importance of cholinergic and adrenergic mechanisms

Abstract

SUMMARY During thermoregulation in the bearded dragon Pogona barbata, heart rate when heating is significantly faster than when cooling at any given body temperature (heart rate hysteresis), resulting in faster rates of heating than cooling. However, the mechanisms that control heart rate during heating and cooling are unknown. The aim of this study was to test the hypothesis that changes in cholinergic and adrenergic tone on the heart are responsible for the heart rate hysteresis during heating and cooling in P. barbata. Heating and cooling trials were conducted before and after the administration of atropine, a muscarinic antagonist, and sotalol, a β-adrenergic antagonist. Cholinergic and β-adrenergic blockade did not abolish the heart rate hysteresis, as the heart rate during heating was significantly faster than during cooling in all cases. Adrenergic tone was extremely high (92.3 %) at the commencement of heating, and decreased to 30.7 % at the end of the cooling period. Moreover, in four lizards there was an instantaneous drop in heart rate (up to 15 beats min–1) as the heat source was switched off, and this drop in heart rate coincided with either a drop in β-adrenergic tone or an increase in cholinergic tone. Rates of heating were significantly faster during the cholinergic blockade, and least with a combined cholinergic and β-adrenergic blockade. The results showed that cholinergic and β-adrenergic systems are not the only control mechanisms acting on the heart during heating and cooling, but they do have a significant effect on heart rate and on rates of heating and cooling.

Autonomic control of heart rate exhibits diurnal shifts in a crocodilian

“…If the mammalian find-ings are related to results from this study, then C. latirostris shows large diurnal shifts in the relative amounts sympathetic and parasympathetic activity acting upon the cardiac pacemaker, with parasympathetic tonus being greatest around mid-morning when the animals were sun basking and both T ds and f H were rapidly increasing. This result is interesting as parasym- pathetic action has an inhibitory action upon the reptilian heart (Seebacher and Franklin, 2001), and C. latirostris must have had other mechanisms stimulating the heart as f H was increasing during these periods.…”

“…Similar to other heliothermic reptiles, crocodilians utilise the cardiovascular system to aid in the transfer of heat between the skin surface and the body core (Grigg and Alchin, 1976;Seebacher and Franklin, 2001). Direct heating of the dorsal surface causes rapid increases in f H and changes in peripheral blood flow (Grigg and Alchin, 1976).…”

Anguimorpha as a model group for studying the comparative heart morphology among Lepidosauria: Evolutionary window on the ventricular septation

“…Moreover, there is no such high oxygen consumption level in Teiidae as in varanids and helodermatids (Albuquerque & Garland Jr, 2020), although the species in the group Teiidae are described mostly as active foragers similarly to varanids. Pogona vitticeps (Agamidae) was chosen as the last outgroup, and a member of the crown group Toxicofera with closer phylogenetic relation to Anguimorpha than the group Teiidae (Pyron et al, 2013) but with a different ecological niche (Köhler et al, 2003), especially in comparison to varanids (Pianka & King, 2004) and at the same time being heliothermic animal (Seebacher & Franklin, 2001) in contrast to Sphenodon . The group Anguimorpha represents one of the most unified squamate clade (Mesquita et al, 2016; Pianka, 1995; Pianka & King, 2004), which means successful uniform basic body plan, especially in varanids (Ast, 2001; Pianka & King, 2004).…”

Catecholamines are key modulators of ventricular repolarization patterns in the ball python (Python regius)

“…Anecdotal observations on a few species of reptiles, however, show that the T-wave is transiently positive when body temperature changes ( McDonald and Heath, 1971 ; Mullen, 1967 ; Valentinuzzi et al, 1969 ; Zaar et al, 2004 ). In ectotherms, the change in metabolism when body temperature changes is associated with altered autonomic tone on the heart and vasculature ( Galli et al, 2004 ; Seebacher and Franklin, 2001 ). If the change in T-wave polarity at higher body temperatures is due to altered autonomic tone, it could indicate evolutionary conservation of autonomic modulation of repolarization.…”