Compensation of Progressive Hypercapnia in Channel Catfish and Blue Crabs

Channel catfish (Ictalurus punctatus Rafinesque) were progressively acclimated to CO2 partial pressures of 7.5, 15, 30, 45 and 58 mmHg (1, 2, 4, 6 and 8% CO2 in air) and blue crabs (Callinectes sapidus Rathbun) to 15, 30 and 45 mmHg, with 24 h at each partial pressure. Measurements of both conventional acid-base parameters (pH, PCO2. total CO2) and ‘strong’ ion concentrations (Na+, K+, Mg2+, Ca2+ and Cl−) were made at various times during each treatment. Intracellular [Na+], [K+] and [Cl−] were determined for red and white muscle in control and hypercapnic (8%) catfish. Extracellular [HCO3−] and strong ion difference (SID) both rose during hypercapnic compensation, with correlation coefficients (r) of 0.97 (P<0.01) for catfish and 0.41 (NS) for blue crabs. Since [HCO3−] is calculated from two rapid measurements, and SID from four separate procedures, the former appears to be the measurement of choice. The results also dispel the notion of a 30 mequiv1−1 upper limit to bicarbonate compensation: [HCO3−] values over 50mequiv1−1 were achieved in both animals, and %pH regulation remained around 70% at the highest PCO2 Due to superior intracellular buffering, the large change in extracellular SID did not lead to a measurable change in the concentrations of the major intracellular ions. The primary gill filaments of hypercapnic catfish showed a 30% increase in numbers and a 75% increase in area of apical crypts of chloride cells. The chloride cell ‘patches’ in crab gills increased in staining density after hypercapnia but did not enlarge. Note: Present address: Department of Biology, Dalhousie University, Halifax, NS, Canada B3H4JI.