Regulation of pH in the isolated perfused gills of the shore crab Carcinus maenas

Isolated posterior gills (no. 7) of shore crabs Carcinus maenas acclimated to brackish water of a salinity of 10 S were bathed and perfused with 50% sea water (200 mmol/l Na super(+)), and the internal perfusate collected during subsequent periods of 5 min. During a single passage through the gill the pH of the perfusion medium decreased from ca. 8.1 to ca. 7.7, a result implying that the gill possesses structures which "recognize" unphysiologically high pH values in the haemolymph and regulates them down to physiological values of ca. 7.7. The findings obtained suggest that pH regulation by crab gills depends on the oxidative metabolism of the intact branchial epithelium and that carbonic anhydrase plays a central role in this process. Na super(+)/H super(+) exchange, anion exchange or cotransport and active proton secretion seem not to be involved. While unimpaired active ion uptake is a prerequisite for pH regulation, ion transport itself is independent of it.