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Adrenergic control of the cardiovascular system in the turtle Trachemys scripta
1 Department of Zoophysiology, Aarhus University, Building 131, 8000 Aarhus
C, Denmark
2 Department of Biological Sciences, Simon Fraser University, Burnaby,
British Columbia, V5A 1S6, Canada
* Author for correspondence (e-mail: johannes.overgaard{at}biology.au.dk)
Accepted 8 August 2002
Freshwater turtles, Trachemys scripta, like all non-crocodilian
reptiles, are able to shunt blood between the pulmonary and systemic
circulations owing to their undivided ventricle. The prevailing hypothesis is
that the ratio of pulmonary and systemic resistances is the primary
determinant of cardiac shunting in turtles. In the present study, we have
examined the adrenergic influences on vascular resistances in the pulmonary
and systemic circulations and the associated effects on cardiac shunts in
turtles. To achieve this objective, systemic blood flow and pressures and
pulmonary blood flow and pressures were measured simultaneously in
anaesthetised turtles during bolus injections of 
- and
ß-adrenergic agonists and antagonists. Total cardiac output, systemic
vascular resistance, pulmonary vascular resistance, heart rate and cardiac
stroke volume were derived from these measurements. Anaesthetised turtles
showed cardiovascular characteristics that were similar to those of
non-apnoeic non-anaesthetised turtles, because anaesthesia blocked the
cholinergically mediated constriction of the pulmonary artery that is normally
associated with apnoea. As a result, the anaesthetised turtles exhibited a
large net left-to-right shunt, and the adrenergic responses could be observed
without confounding changes resulting from apnoea. Potent -adrenergic
vasoconstriction and weaker ß-adrenergic vasodilation were discovered in
the systemic circulation. Modest ß-adrenergic vasodilation and possible
weak -adrenergic vasodilation were discovered in the pulmonary
circulation. This adrenergically mediated vasoactivity produced the largest
range of cardiac shunts observed so far in turtles. Regression analysis
revealed that 97% of the variability in the cardiac shunts could be accounted
for by the ratio of the pulmonary and systemic resistances. Thus, we conclude
that, independent of whether the pulmonary vascular resistance is modulated
(as during apnoea) or the systemic resistance is modulated with adrenergic
mechanisms (as shown here), the consequences on the cardiac shunt patterns are
the same because they are determined primarily by the ratios of the pulmonary
and systemic resistance.
Key words: turtle, Trachemys scripta, adrenergic response, cardiac shunting, cardiovascular, pulmonary resistance, systemic resistance
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