The electrometric [Delta]pH method and an in vitro radioisotopic HCO3- dehydration assay were used to demonstrate the presence of true extracellular carbonic anhydrase (CA) activity in the blood of the Pacific spiny dogfish Squalus acanthias. An extracorporeal circulation and stopflow technique were then used to characterise the acid­base disequilibrium in the arterial (postbranchial) blood. During the stopflow period, arterial pH (pHa) decreased by 0.028±0.003 units (mean ± s.e.m., N=27), in contrast to the increase in pHa of 0.029±0.006 units (mean ± s.e.m., N=6) observed in seawater-acclimated rainbow trout Oncorhynchus mykiss under similar conditions. The negative disequilibrium in dogfish blood was abolished by the addition of bovine CA to the circulation, while inhibition by benzolamide of extracellular and gill membrane-bound CA activities reversed the direction of the acid­base disequilibrium such that pHa increased by 0.059±0.016 units (mean ± s.e.m., N=6) during the stopflow period. When the CA activity of red blood cells (rbcs) was additionally inhibited using acetazolamide, the magnitude of the negative disequilibrium was increased significantly to -0.045±0.007 units (mean ± s.e.m., N=6). Blockage of the rbc Cl-/HCO3- exchanger using 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS) also increased the magnitude of the negative disequilibrium, in this case to -0.089±0.008 units (mean ± s.e.m., N=6). Exposure of dogfish to hypercapnia had no effect on the disequilibrium, whereas the disequilibrium was significantly larger under hypoxic conditions, at -0.049±0.008 units (mean ± s.e.m., N=6). The results are interpreted within a framework in which the absence of a positive CO2 excretion disequilibrium in the arterial blood of the spiny dogfish is attributed to the membrane-bound and extracellular CA activities. The negative disequilibrium may arise from the continuation of Cl-/HCO3- exchange in the postbranchial blood and/or the hydration of CO2 added to the plasma postbranchially. Two possible sources of this CO2 are discussed; rbc CO2 production or the admixture of blood having 'low' and 'high' CO2 tensions, i.e. the mixing of postbranchial blood with blood which has bypassed the respiratory exchange surface.

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