Acid-base stress and central chemical control of ventilation in turtles

Central chemical control of ventilation in turtles, Chrysemys (Pseudemys) scripta, has been characterized by alterations in breathing frequency (f). To assess the effects of acid-base stress on the central ventilatory response, three groups of animals were subjected to 2 h of anoxia (98% N2-2% CO2), 2 h of hypercapnia (8% CO2 in air), or 2 h of anoxia plus hypercapnia (92% N2-8% CO2). Ventilatory responses were measured continuously, and arterial blood and cerebrospinal fluid (CSF) acid-base variables were determined after 2 h. Results show that central ventilatory responses (f) were significantly elevated in CSF acidosis caused by CO2 over those caused by the accumulation of CSF fixed acid alone (anoxia). The increases in f did not correlate with decreases in estimated CSF pH, since it was much lower in anoxia (7.19) than in hypercapnia (7.37). Our results strongly suggest that the central nervous system acidosis caused by anoxia produces centrally mediated ventilatory change in a fundamentally different way from hypercapnia.