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 58mmHg (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, 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 30mequivl⁻¹ upper limit to bicarbonate compensation: [HCO₃⁻] values over 50mequivl⁻¹ were achieved in both animals, and %pH regulation remained around 70% at the highest ⁠. 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.