Physiological Responses of the Crayfish Pacifastacus Leniusculus to Environmental Hyperoxia : II. Role of the Antennal Gland in Acid–Base and Ion Regulation

Handling of electrolytes and acidic equivalents by the antennal gland was monitored in freshwater crayfish (Pacifastacus leniusculus) during control normoxia (⁠⁠; 1 mmHg = 133·3 Pa), 72h of hyperoxia (⁠⁠) and 24 h of recovery. A preparation was developed for direct collection of urinary flow (UFR) which was confirmed using inulin clearance (Ċ_IN)-Renal effluxes of Na⁺, Cl⁻, Mg²⁺, K⁺, Ca²⁺, sulphate, phosphate, titratable and nontitratable acidity and CO₂ were monitored. These were used in conjunction with filtration rates to calculate net rates of reabsorption.

UFR was elevated by around 50 % during hyperoxia. Contributing to this were an increase in Ċ_IN and a reduction in [IN]_u: [IN]_e. Excretion of Na⁺ (Ė_Na) tended to increase initially whereas Ė_Cl tended to decrease after 36h. Ė_K and Ė_Mg showed a similar profile to that of Ė_Na with increases averaging 60 %. The increases in excretion of Ca²⁺, phosphate and sulphate were more pronounced (threefold). In control crayfish, 65–95 % of filtered electrolytes were reabsorbed at the antennal gland except for ammonia which was secreted. Electrolyte reabsorption increased during hyperoxia; percentage increases varied from 70% (Na⁺, Cl⁻) to 150% (sulphate, phosphate) with HCO₃⁻ showing a fourfold increase above control values. Ammonia secretion correspondingly increased. Control urine was acid with respect to the haemolymph and became increasingly acidified during initial hyperoxic exposure. Net renal proton excretion largely reflected (90%) ammonia excretion; both were approximately doubled during hyperoxia. Unlike the situation in mammals, ammonia appears to be more important than phosphate in buffering urine pH. Urinary parameters generally required 24 h for complete recovery when normoxia was reinstated.

Renal net efflux and reabsorption rates were compared with branchial net and unidirectional influx rates from an earlier part of this study. Branchial and renal net effluxes of Na⁺ and Cl⁻ had similar vectors and magnitudes during the control period but tended to counter each other during hyperoxia. Renal reabsorption rates of these ions were threefold greater than branchial influx rates, confirming increased transporting capability. Both epithelia exhibited a net H⁺ efflux during hyperoxia; the kidney contributed only 10% of the whole-animal response. Possible mechanisms of renal postfiltrational electrolyte and acidic equivalent processing are discussed.