Depression of aerobic metabolism and intracellular pH by hypercapnia in land snails, Otala lactea

The pulmonate land snail Otala lactea undergoes simultaneous hypercapnia, hypoxia, extracellular acidosis and metabolic depression during dormancy. We tested the effects of ambient hypercapnia and hypoxia on oxygen consumption (VO2) and on extracellular and intracellular pH of active (i.e. non-dormant) individuals. Active snails reduced VO2, by 50% within l h when exposed to 65mmHg (1 mmHg = 133.3Pa) ambient PCO2, and by 63% in 98mmHg. These levels of CO2 are within the range that occurs naturally in the lung and blood during dormancy. VO2 of hypercapnic snails remained below that of controls for the duration of exposure (up to 9 h) and returned to control levels within 1 h when CO2 was removed. Both pHe and whole-body pHi (measured using [14C]DMO) fell with increasing haemolymph PCO2 by approximately 0.7logPCO2 Critical (VO2- limiting) ambient PO2 of active snails was 90mmHg in the absence of CO2 and dropped to 50 mmHg when VO2 was reduced 45% by exposure to CO2. Estimated critical PO2 at the lower VO2 typical of dormancy is well below the typical lung PO2 of dormant Otala, suggesting that PO2 in the lung does not normally limit oxygen consumption during dormancy. These results support the hypothesis that hypercapnia or resulting respiratory acidosis depresses metabolic rate during dormancy, and argue against a limitation of VO2 by hypoxia