Control of ventilation in the hypercapnic skate Raja ocellata: I. Blood and extradural fluid.

In order to study the role of CO2 and acid-base status in contributing to ventilatory drive, skate were exposed to normoxic hypercapnia (PICO2 = 7.5 Torr) under conditions where the primary O2 drive would remain unaltered. Blood O2 transport was markedly insensitive to CO2, with no Root effect and only a small Bohr effect. Red blood cell pHi was not preferentially regulated, and there was no evidence of RBC swelling or nucleoside triphosphate adjustment. Although there were no changes in arterial O2 levels during hypercapnia, ventilation immediately increased 2.7-fold through large changes in stroke volume and small changes in frequency, and declined only slightly through 24-48 h. PaCO2 equilibrated rapidly with PICO2, driving down arterial pHa, which was 65% corrected through HCO3- accumulation by 24 h. In contrast, the extradural fluid outside the brain equilibrated only very slowly, and was clearly not involved in the ventilatory stimulation. Increased ventilation during hypercapnia may be related to depression in pHa.