Physiological Responses of the Rock Crab Cancer irroratus (Say) to Environmental Hyperoxia. I. Acid-Base Regulation

Abstract

Hemolymph acid-base and ion levels and acidic equivalent exchange with the experimental water were monitored in rock crabs (Cancer irroratus Say) exposed to control normoxia (PIo2 ≃ 150 torr), 48 h of hyperoxia (Po2 ≃ 512 torr), and 24 h of subsequent normoxic recovery. Hemolymph CO₂ binding was assessed in vitro on oxygenated and deoxygenated samples. Settled normoxic crabs had the following acid-base status: pH—8.01; Pco2—2.1 torr, and [HCO₃⁻]-9.3 meq·liter⁻¹ and appeared to be in proton balance with the experimental water. Within 6 h of exposure to hyperoxia, the hemolymph exhibited a significant acidosis (0.15 pH units) that was due to respiratory CO2 (Pco2 increased by 50%). This acid-base imbalance was restored between 6 and 24 h by a 50% increase in [HCO₃⁻]. This coincided with a 13-fold increase in excretion of acidic equivalents (primarily titratable) into the experimental water that continued for up to 10 h. Numerically, there was excellent correspondence between acid efflux and the magnitude of the extracellular proton load. Although crabs continued to hypoventilate, preexperimental Pco2 was reestablished by 24 h, suggesting that the original hypercapnia had been due to a perfusion limitation. Between 24 and 48 h, circulating [HCO₃⁻] was also restored to resting levels accompanied by an equivalent net base excretion into the bathing medium. When normoxia was reinstated, hemolymph exhibited a reduction in Pco2 plus a further reduction in [HCO₃⁻] at constant Pco2, making pH alkalotic. The base efflux that persisted into the recovery period significantly exceeded that required to restore extracellular acid-base balance. It is suggested that this base load originated intracellularly and that the reduced cardiac output during hyperoxia limited its removal into the hemolymph. Circulating lactate exhibited a similar washout effect during recovery. Hemolymph of C. irroratus had a low nonbicarbonate buffer value (β) corresponding to reduced protein concentration. However, the Haldane effect, which theoretically could account for the release of 2.13 mmol CO2 per mmol O2 bound, was larger than observed in other decapods.