ION-TRANSPORT IN GILLS OF THE EURYHALINE FISH GILLICHTHYS MIRABILIS IS FACILITATED BY A PHOSPHOCREATINE CIRCUIT

The function of creatine kinase (CK) isozymes in energy metabolism and the short-term regulation of active ion transport in gills of the eur yhaline teleost Gillichthys mirabilis was investigated. After a transf er of fish from regular seawater [36 parts/thousand (ppt)] to hypersal ine water (60 ppt), the plasma osmolality increased significantly from 361.0 +/- 5.2 to 434.2 +/- 20.6 mosmol/kgH(2)O within 2 h and was reg ulated down to 391.8 +/- 11.3 mosmol/kgH(2)O within 12 h. Although the ATP concentration in the gill tissue remained unchanged, the creatine concentration increased significantly from 17.3 +/- 3.2 to 37.6 +/- 5 .9 nmol/mg protein within 2 h after the salinity change. CK and Na+-K-adenosinetriphosphatase (Na+-K+-ATPase) activities were unchanged 48 h after transfer. Independent of salinity, the activities of CK were t hree to sewn times those of the Na+-K+-ATPase, and the creatine concen tration in the gill was at least one order of magnitude higher than th e ATP concentration. The occurrence of muscle-type CK (CK-M), brain-ty pe CK, and mitochondrial CK was demonstrated. CK-M was predominant in gills (59 +/- 7.1% of total CK activity). Evidence for a direct functi onal coupling between CK and Na+-K+-ATPase was obtained with permeabil ized gill cells, by using the CK inhibitor iodoacetamide, which abolis hes the competitive channeling of ADP from the external pyruvate kinas e reaction to the endogeneous CK reaction in a coupled in situ Na+-K+- ATPase assay. Our results show the significance and the central regula tory role for energy metabolism and adaptive ionoregulation of a phosp hocreatine-CK circuit in situations of high and fluctuating energy dem ands for euryhaline fishes.