Investigation of the pseudobranch organ in rainbow trout (Oncorhyncus mykiss): endogenous substrates and activities of carbonic anhydrase, lactate dehydrogenase, and 3-hydroxy-acyl CoA dehydrogenase.

The pseudobranch in rainbow trout was investigated in order to clarify its metabolic and physiological function. Contents of lactate, glycogen, and glycerides and activities of carbonic anhydrase (CA), lactate dehydrogenase (LDH), and 3-hydroxy-acyl CoA dehydrogenase (HAD) were measured and compared with those in the gills and liver. Contents of lactate and glycerides, and activities of CA and HAD were higher in the pseudobranch than in the gills and liver. The results show that the pseudobranch is well equipped with enzymes (HAD and CA) suggested to be coupled to fatty acids breakdown and synthesis respectively. Lactic acid, possibly formed from glycogen and the glycerol component in glycerides, may be involved in the increase in blood P0 2 (Root effect) that is necessary to meet the oxygen needs of the eye. Pseudobranch carbonic anhydrase is probably involved in this process.