Regulation of ventilation and acid-base status in the elasmobranch Scyliorhinus stellaris during hyperoxia-induced hypercapnia.

Exposure of the elasmobranch Scyliorhinus stellaris to environmental hyperoxia (PO2 of 500 mm Hg) resulted in a considerable rise of arterial PCO2 from 1.9 mm Hg during normoxia to about 11 mm Hg after 6 days as an expression of the primarily oxygen-oriented regulation of gill ventilation. In contrast to the typical pattern during environmental hypercapnia, however, arterial plasma pH was hardly affected by the considerable hyperoxia-induced hypercapnia. At elevated arterial PO2 values (200-300 mm Hg) gill ventilation was apparently not adjusted exclusively for the oxygen demands of the organism, but was matched to the requirements of acid-base regulation such that the rise in PCO2 could be compensated for by a net gain of bicarbonate-equivalent ions from the environment. This fine adjustment of gill ventilation to the bicarbonate-equivalent uptake rate extended the process of adaptation to about 6 days and resulted in an almost complete pH compensation during the entire process of PCO2 increase. These data suggest that during conditions of reduced oxygen-related respiratory drive the regulation of gill ventilation is primarily dependent upon the acid-base parameters.