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Journal of Experimental Biology 114,661-672 (1985)
Published by Company of Biologists 1985

Ventilatory and Acid-Base Responses to Long-Term Hypercapnia in the Freshwater Turtle, Chrysemys Picta Bellii

Randi B. Silver ¹ and Donald C. Jackson ¹

¹ Division of Biology and Medicine, Brown University, Providence, Rhode Island, 02912, U.S.A.

The response to hypercapnia was studied in western painted turtles, Chrysemys picta bellii at 20°C. Ventilation, metabolic rate, arterial blood gases, blood pH and blood plasma ions were monitored periodically on individual turtles exposed to 5.7% CO₂ for 72 h and then allowed to recover in air.

In response to hypercapnia, there is an immediate 10- to 15-fold increase in ventilation from control levels, which was maintained throughout the entire 5.7% CO₂ breathing period. The first hour of CO₂ breathing caused an increase in Pa_COCO2 from 24–39 mmHg with a concomitant decrease in pH and rise in [HCO₃^-]. [HCO₃^-] rose from 42 to 50mmol1^-1 in the next 24 h of CO₂ breathing and remained at this level for the rest of the hypercapnic period. Small, significant increases in total [Ca²⁺] and total [Mg²⁺] were found; however, no changes were observed in the plasma Na⁺, K⁺ or Cl^- concentrations and the overall change in measured ions could not account for the increased [HCO₃^-]. The maximum change in [HCO₃^-] attained in Chrysemys exposed to a more severe acidosis (14.3% CO₂) for up to 18 h was the same as that seen in the animals breathing 5.7% CO₂ (10mmol1^-1) implying that there is an upper limit for the accumulation of [HCO₃^-] in Chrysemys at 20°C.

The blood pH of turtles recovering in air returned to the control value (7.56–7.74) within the first hour although Pa_COCO2 did not return to the control value. The HCO₃^- ion concentration also remained elevated throughout the 48-h recovery period, which suggests that ionic compensation is a slower process.

The freshwater turtle employs two mechanisms to reduce the severity of an imposed respiratory acidosis: increased ventilation and changes in the strong ion difference. In spite of these responses, blood pH is not restored to the control value.

Key words: Ionic compensation, calcium, chronic respiratory acidosis.

Accepted on July 20, 1984

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