Anaerobic metabolism, gas exchange, and acid-base balance during hypoxic exposure in the channel catfish, Ictalurus punctatus

Gill ventilation, blood gas and acid‐base values, ṀO2, ṀCO2 and the gas exchange ratio have been measured before, during, and after exposure to hypoxia in the channel catfish, Ictalurus punctatus. I. punctatus maintains ṀO2 at control levels to a PIO2 as low as 60 mm Hg, through a profound branchial hyperventilation. Concomitantly, however, a lactic acidosis usually develops, indicating a significant anaerobic glycolysis. Both a metabolic acidosis and respiratory alkalosis occur in Ictalurus during hypoxic exposure, with the former usually predominating. ṀCO2 doubles and the gas exchange ratio (RE) increases from 0.8 at control levels to 2.0 at hypoxic levels, indicating that, in addition to anaerobic glycolysis, nonglycolytic pathways producing CO2 also operate during hypoxic exposure. Analysis reveals that at least half of the increased ṀCO2 during hypoxic exposure is due strictly to lactate acidfication of the tissue HCO3− pool, rather than from metabolic production of molecular CO2. Thus, the actual respiratory quotient (RQ) only rises to 1.5 during hypoxic exposure. Within one hour of a return to air saturated water, a large lactate “flush” and severe plasma acidosis occur, and control levels for these and other values are not reachieved for 2–6 hours after hypoxic exposure. Complete analysis of O2 and CO2 exchange in the catfish exposed to short‐term hypoxia thus must consider both the time course of acid‐base disturbance and the evolution of CO2 from the acidified tissue bicarbonate pool.