Central chemical control of ventilation in the unanesthetized turtle

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Abstract
The cerebral ventricles (lateral-4th) of the fresh water turtle Pseudemys scripta elegans (1-2 kg) were perfused with mock CSF of normal and lower than normal pH. Alteration of the pH, at constant PCO2 [partial pressure of CO2], was accomplished by altering the difference between the number of completely dissociated cations and anions, the strong ion difference (SID). In a dilute salt solution with no other weak acid buffer the SID is equal to the [HCO3-]. Changing the SID of the mock CSF from control values (mean = 32 meq/l) to 15 meq/l resulted in an increase in the ventilation to metabolism ratio (.ovrhdot.VE[ventilation]/.ovrhdot.V02) of 2-13 .times. (mean = 7) that of control. Observations revealed no significant difference in ventilation between perfusion with normal pH mock CSF and preperfusion values. The increase in ventilation with the low pH perfusate was similar in animals equilibrated at 20 and 30.degree. C. Decreasing the SID 2 meq/l from control values caused a 4-fold increase in the mean .ovrhdot.VE/.ovrhdot.V02 (P < 0.001) indicating that the central chemosensors of Pseudemys display a high degree of sensitivity. The induced hyperventilation persisted for the duration of the perfusion with the low pH mock CSF, in spite of a marked respiratory alkalosis of the arterial blood, demonstrating that the central chemosensors are dominant over any peripheral inputs. Since the perfusion was confined to the cerebral ventricles, the central chemosensors of Pseudemys are apparently in functional contact with the ventricular CSF. This is the 1st study which demonstrates the existence of central chemical control of ventilation in an ectothermic vertebrate.

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