ACID-BASE-BALANCE AND ION TRANSFERS IN THE SPINY DOGFISH (SQUALUS-ACANTHIAS) DURING HYPERCAPNIA - A ROLE FOR AMMONIA EXCRETION

The effect of hypercapnia on the acid-base balance and acid-equivalent transfers has been measured in the dogfish Squalus acanthias. Previous reports on Squalus are not in agreement as to the role played by compensatory acid-base transfers between the animal and the water during hypercapnia. Cannulated animals were maintained in a closed circuit, seawater recirculation system. Plasma pH, CCO2 (from which PCO2 and [HCO3-] were calculated), and transfers of NH4+ and HCO3- -equivalent ions between the fish and the water, were measured during 24 hours of hypercapnic exposure (PCO2: 8-10 torr) and a subsequent 8-24 hour normocapnic recovery period. Respiratory acidosis resulted in a plasma pH depression, which was then almost completely compensated (within approximately 0.1 pH unit) over 24 hours by a approximately 20 mM increase in plasma [HCO3-]. In contrast to previous studies on elasmobranch acid-base regulation, hypercapnia induced a rapid 3 x increase in not only HCO3- -equivalent uptake but also branchial ammonia (NH4+) excretion. These transfers combined for a net DELTA-H+ loss to the water of 5.5 mmol kg-1. During the normocapnic period, net DELTA-H+ was reversed to -6.9 mmol kg-1, nearly completely due to HCO3-efflux. Several lines of evidence point to the contribution of gill Na+/NH4+ exchange to the total ammonia excreted during hypercapnia, whereas NH3 diffusion predominates during the recovery period. Likewise, Cl-/HCO3- or Cl-/OH- exchange may enhance the uptake of HCO3- during hypercapnia.