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First published online January 17, 2007
Journal of Experimental Biology 210, 421-431 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02653

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Effects of extracellular changes on spontaneous heart rate of normoxia- and anoxia-acclimated turtles (Trachemys scripta)

Jonathan A. W. Stecyk^1,* and Anthony P. Farrell²

¹ Department of Zoology, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada
² Faculty of Land and Food Systems and Department of Zoology, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada

^* Author for correspondence (e-mail: jstecyk{at}interchange.ubc.ca)

Accepted 13 November 2006

Heart rate (f_H) of the anoxia-tolerant freshwater turtle (Trachemys scripta) during prolonged anoxia exposure is 2.5- to 5-times lower than the normoxic rate, but whether alterations in blood composition that accompany prolonged anoxia contribute to this bradycardia is unknown. We examined how temperature acclimation, oxygen deprivation, acidosis, hyperkalemia, hypercalcemia and adrenaline affect chronotropy in the turtle myocardium. We monitored spontaneous contraction rates of right-atrial preparations obtained from 21°C- and 5°C-acclimated turtles that had been exposed to either normoxia or anoxia (6 h at 21°C; 2 weeks at 5°C). Sequential exposures to saline solutions were designed to mimic, in a step-wise manner, the shift from a normoxic to anoxic extracellular condition (for normoxia-acclimated preparations) or the reverse (for anoxia-acclimated preparations). Our results clearly show that prolonged anoxia exposure re-sets the intrinsic f_H of turtles at both temperatures, with reductions in intrinsic f_H in the range of 25%–53% compared with normoxia. This intrinsic change would contribute to the bradycardia observed with prolonged anoxia. Further, we found negative chronotropic effects of extracellular anoxia, acidosis and hyperkalemia, and positive chronotropic effects of hypercalcemia and adrenaline. The exact nature of these extracellular effects depended, however, on the acclimation temperature and the prior exposure of the animal to anoxia. With normoxia-acclimated preparations at 21°C, combined anoxia and acidosis (pH reduced from 7.8 to 7.2) significantly reduced spontaneous f_H by 22% and subsequent exposure to hyperkalemia (3.5 mmol l^–1K⁺) further decreased f_H. These negative chronotropic effects were ameliorated by increasing the adrenaline concentration from the tonic level of 1 nmol l^–1 to 60 nmol l^–1. However, in anoxia-acclimated preparations at 21°C, anoxia alone inhibited f_H (by 30%). This negative chronotropic effect was counteracted by both hypercalcemia (6 mmol l^–1 Ca²⁺) and adrenaline (60 nmol l^–1). At 5°C, only the combination of anoxia, acidosis (pH reduced from 8.0 to 7.5) and hyperkalemia (3.5 mmol l^–1 K⁺) significantly reduced spontaneous f_H (by 23%) with preparations from normoxia-acclimated turtles. This negative chronotropic effect was fully reversed by hypercalcemia (10 mmol l^–1 Ca²⁺). By contrast, spontaneous f_H of anoxia-acclimated preparations at 5°C was not affected by any of the extracellular changes. We conclude that prior temperature and anoxia experiences are central to determining f_H during prolonged anoxia in Trachemys scripta both as a result of the re-setting of pacemaker rhythm and through the potential influence of extracellular changes.

Key words: acidosis, adrenaline, anoxia, calcium, cardiovascular, intrinsic heart rate, potassium, red-eared slider, temperature, Trachemys scripta, turtle

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This article has been cited by other articles:

				J. A. W. Stecyk, C. Bock, J. Overgaard, T. Wang, A. P. Farrell, and H.-O. Portner Correlation of cardiac performance with cellular energetic components in the oxygen-deprived turtle heart Am J Physiol Regulatory Integrative Comp Physiol, September 1, 2009; 297(3): R756 - R768. [Abstract] [Full Text] [PDF]

				J. A. W. Stecyk, V. Paajanen, A. P. Farrell, and M. Vornanen Effect of temperature and prolonged anoxia exposure on electrophysiological properties of the turtle (Trachemys scripta) heart Am J Physiol Regulatory Integrative Comp Physiol, July 1, 2007; 293(1): R421 - R437. [Abstract] [Full Text] [PDF]