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The Journal of Experimental Biology 206, 857-865 (2003)
doi: 10.1242/jeb.00184

Cardiovascular responses to hypoxia and anaemia in the toad Bufo marinus

Johnnie B. Andersen^1,*, Michael S. Hedrick^1,2 and Tobias Wang¹

¹ Department of Zoophysiology, Institute of Biological Sciences, University of Aarhus, Denmark
² Department of Biological Sciences, California State University, Hayward, CA 94542, USA

^* Author for correspondence (e-mail: johnnie.andersen{at}biology.au.dk)

Accepted 28 November 2002

Amphibians exhibit cardiorespiratory responses to hypoxia and, although several oxygen-sensitive chemoreceptor sites have been identified, the specific oxygen stimulus that triggers these responses remains controversial. This study investigates whether the cardiovascular response to oxygen shortage correlates with decreased oxygen partial pressure of arterial blood (Pa_O2) or reduced oxygen concentration ([O₂]) in toads. Toads, equipped with blood flow probes and an arterial catheter, were exposed to graded hypoxia [fraction of oxygen in the inspired air (FI_O2)=0.21, 0.15, 0.10, 0.07 and 0.05] before and after reductions in arterial [O₂] by isovolemic anaemia that reduced haematocrit by approximately 50%. Toads responded to hypoxia by increasing heart rate (fH) and pulmocutaneous blood flow (_pc) and reducing the net cardiac right-to-left-shunt. When arterial [O₂] was reduced by anaemia, the toads exhibited a similar cardiovascular response to that observed in hypoxia. While arterial CO₂ partial pressure (Pa_CO2) decreased significantly during hypoxia, indicative of increased alveolar ventilation, anaemia did not alter Pa_CO2). This suggests that reductions in [O₂] mediate cardiovascular adjustments, while ventilatory responses are caused by reduced Pa_O2.

Key words: amphibian, Bufo, cardiovascular, respiratory, anaemia, hypoxia, cardiac shunt

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