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The Journal of Experimental Biology 204, 4361-4366 (2001)
© 2001 The Company of Biologists Limited

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

F. Seebacher1,* and C. E. Franklin2

1 School of Biological Sciences A08, The University of Sydney, NSW 2006, Australia and
2 Department of Zoology and Entomology, The University of Queensland, Brisbane Qld 4072, Australia

*Author for correspondence (e-mail: fseebach{at}bio.usyd.edu.au)

Accepted 2 October 2001

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 ß-adrenergic antagonist. Cholinergic and ß-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 ß-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 ß-adrenergic blockade. The results showed that cholinergic and ß-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.

Key words: thermoregulation, heart rate, neural control, cholinergic, adrenergic, reptile, lizard, Pogona barbata.




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© The Company of Biologists Ltd 2001