The genes of Na,K-ATPase, a selfreview

Retention of ion channel genes expression increases Japanese medaka survival during seawater reacclimation

Euryhaline teleosts exhibit varying acclimability to survive in environments that alternate between being hypotonic and hypertonic. Such ability is conferred by ion channels expressed by ionocytes, the ion-regulating cells in the gills or skin. However, switching between environments is physiologically challenging, because most channels can only perform unidirectional ion transportation. Coordination between acute responses, such as gene expression, and long-term responses, such as cell differentiation, is believed to strongly facilitate adaptability. Moreover, the pre-acclimation to half seawater salinity can improve the survivability of Japanese medaka (Oryzias latipes) during direct transfer to seawater; here, the ionocytes preserve hypertonic acclimability while performing hypotonic functions. Whether acclimability can be similarly induced in a closed species and their corresponding responses in terms of ion channel expression remain unclear. In the present study, Japanese medaka pre-acclimated in brackish water were noted to have higher survival rates while retaining higher expression of the three ion channel genes ATP1a1a.1, ATP1b1b, and SLC12a2a. This retention was maintained up to 2 weeks after the fish were transferred back into freshwater. Notably, this induced acclimability was not found in its close kin, Indian medaka (Oryzias dancena), the natural habitat of which is brackish water. In conclusion, Japanese medaka surpassed Indian medaka in seawater acclimability after experiencing exposure to brackish water, and this ability coincided with seawater-retention gene expression.

Functional expression of the colonic H+,K+-ATPase alpha-subunit. Pharmacologic properties and assembly with X+,K+-ATPase beta-subunits

The functional and pharmacological properties of the alpha-subunit of the colonic H+,K+-ATPase (alphaC) were studied in Xenopus laevis oocytes. alphaC was injected with different rat beta-subunits, the beta-subunit of the gastric H+,K+-ATPase (betaG, the only H+, K+-ATPase beta-subunit identified in rat), or the beta1-subunit of the Na+,K+-ATPase (beta1) (associated with the basolateral Na+, K+-ATPase, but also expressed in the epithelial apical membranes of rat distal colon) (Marxer, A., Stieger, B., Quarini, A., Kashgarian, M., and Hauri, H. P. (1989) J. Cell Biol. 109, 1057-1069). The effect of the different beta-subunits was studied by measuring 86Rb+ uptake (a K+ congener) in the presence or absence of Sch-28080 and ouabain. Significant Na+-independent 86Rb+ uptake was observed only when alphaC was coexpressed with one of the beta-subunits. The expressed alphaCbeta1 and alphaCbetaG complexes were not inhibited by Sch-28080, were only partially sensitive to ouabain (IC50 = 400-600 microM, in the presence of external 1 mM KCl), and exhibited comparable K+ activation kinetics. Coexpression of alphaC with epitope-tagged betaG or beta1, followed by immunopurification of the alphabeta complexes, confirmed stable assembly of alphaCbetaG and alphaCbeta1 complexes. Since the beta1-subunit, but not the alpha1-subunit, of Na+,K+-ATPase is expressed in the apical membrane of rat colonocytes, our data support the view that, in rat distal colon, the beta1-subunit may play a surrogate role as the beta-subunit for the colonic H+,K+-ATPase.