The role of the teleost gill epithelium in the acclimation of euryhaline teleost fishes to both freshwater and seawater

euryhaline teleost fishes to both freshwater and seawater environments has been studied extensively. Na+/K+-ATPase is one of the most important enzymes associated with ion regulation in the fish gill because it indirectly energizes the branchial excretion of NaCl from marine teleosts (see McCormick, 1995; Marshall and Bryson, 1998; Evans et al., 1999). The role of Na+/K+-ATPase in freshwater ion balance is not well understood, but it may be important for establishing electrochemical gradients necessary for NaCl and/or Ca2+ uptake (see Perry, 1997). Na+/K+-ATPase is a membrane-bound P-ATPase composed of at least three subunits: α, β and γ (Blanco and Mercer, 1998). This ubiquitous transporter is involved in maintaining routine cell volume and resting membrane potential (Blanco and Mercer, 1998) and is extremely abundant in specialized iontransporting cells. Two well-studied examples in fishes are the marine teleost chloride cell and the elasmobranch rectal gland tubule cell. In these two cell types, Na+/K+-ATPase is localized along the complex basolateral membrane and/or tubule system (Karnaky et al., 1976; Eveloff et al., 1979), the large surface areas of which amplify the possible sites for Na+/K+-ATPase insertion (Karnaky, 1986; Valentich et al., 1995). Many researchers have reported a positive correlation between environmental salinity and the biochemical activity of branchial Na+/K+-ATPase in teleost fishes, especially in salmonids and anguillids (Kultz and Jurss, 1993; Madsen et al., 1995; McCormick, 1995; Uchida et al., 1996, 1997; Crockett, 1999; D’Cotta et al., 2000). However, in other species, branchial Na+/K+-ATPase activity in freshwater-acclimated individuals is equivalent to or exceeds that of seawateracclimated fish (Yoshikawa et al., 1993; Madsen et al., 1994; McCormick, 1995; Jensen et al., 1998; Vonck et al., 1998; Kelly et al., 1999; Marshall et al., 1999). Recently, there have been further insights into the cellular regulation of fish gill Na+/K+-ATPase using heterologous and homologous antibodies and partial cDNA sequences to the αsubunit. For example, changes in branchial Na+/K+-ATPase activity have been positively correlated with Na+/K+-ATPasespecific mRNA and protein levels (Kisen et al., 1994; Cutler et al., 1995; Madsen et al., 1995; Hwang et al., 1998; Jensen et al., 1998; Lee et al., 1998; D’Cotta et al., 2000), suggesting that changes in fish gill Na+/K+-ATPase activity are due to changes in the relative abundance of the enzyme. In mammals, activation of latent Na+/K+-ATPase has also been shown to be responsible for changes in the activity of the enzyme (Ewart 2957 The Journal of Experimental Biology 203, 2957–2966 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 JEB2916