The cardiovascular responses of the red-eared slider (Trachemys scripta) acclimated to either 22 or 5 degrees C. II. Effects of anoxia on adrenergic and cholinergic control.

Cardiovascular control in cold-acclimated freshwater turtles during chronic anoxic exposure is not well understood. We tested the hypothesis that the observed bradycardia in Trachemys scripta results from increased cholinergic inhibitory tone and reduced sympathetic activity. Cardiovascular status was measured in vivo in turtles acclimated to either 22 degrees C or 5 degrees C and either acutely exposed (6 h) to anoxia at 22 degrees C or chronically exposed (22 days) to anoxia at 5 degrees C. In 22 degrees C-acclimated turtles, injection of the cholinergic antagonist atropine induced a significant tachycardia under both normoxic and anoxic conditions. However, in 5 degrees C-acclimated turtles, atropine injection had little effect on heart rate. Therefore, cholinergic control of heart rate was suppressed during cold acclimation; instead, temperature effects are more important in bringing about bradycardia, while the intrinsic effects of anoxia and acidosis are probably important during chronic anoxia. Injection of adrenaline caused a pressor response through increased systemic resistance at both acclimation temperatures. This response was blunted by acute and chronic anoxic exposure, suggesting that systemic vasomotor control was altered independently of acclimation temperature. This anoxic blunting may be related in part to the anoxia-induced increase in systemic resistance. Injection of nadolol after atropine decreased systemic cardiac output. The tonic beta-adrenergic cardiac stimulation was attenuated by acute and chronic anoxic exposure. Some of this attenuation of beta-adrenergic control could be attributed to the 39-40 % reduction in cell surface beta-adrenoreceptor density in the ventricles of these turtles that accompanied acute and chronic anoxic exposure. In conclusion and contrary to our original hypothesis, cholinergic and adrenergic control of the cardiovascular system in turtles was attenuated under cold anoxic conditions, perhaps assisting in the depressed physiological state of these animals.