Angiotensin II modulates the activity of the Na+/K(+)ATPase in eel kidney

We have previously shown that angiotensin II (Ang II) has a role at the lev el of the eel gill chloride cell regulating sodium balance, and therefore o smoregulation; the purpose of the present study was to extend these finding s to another important osmoregulatory organ, the kidney. By catalytic histochemistry Na+/K(+)ATPase activity was found in both sea w ater (SW)- and freshwater (FW)adapted eel kidney, particularly at the level of both proximal and distal tubules. Quantitation of tubular cell Na+/K(+) ATPase activity, by imaging, gave values in SW-adapted eels which were doub le those found in FW-adapted eels (Student's t-test: P < 0.0001). This was due to a reduced number of positive tubules present in FW-adapted eels comp ared with SW-adapted eels. By conventional enzymatic assay, the Na+/K(+)ATP ase activity in isolated tubular cells from SW-adapted eels showed values 1 .85-fold higher those found in FW-adapted eels (Student's t-test: P < 0.000 1). Perfusion of kidney for 20 min with 100 nM Ang II provoked a significan t increase (1.8-fold) in Na+/K(+)ATPase activity in FW, due to up-regulatio n of Na+/K(+)ATPase activity in a significantly larger number of tubules (S tudent's t-test: P < 0.0001). The effect of 100 nM Ang II in SW-adapted kid neys was not significant. Stimulation with increasing Ang II concentrations was performed on isolated kidney tubule cells: Ang II provoked a dose-depe ndent stimulation of the Na+/K(+)ATPase activity in FW-adapted eels, reachi ng a maximum at 100 nM (1.82-fold stimulation), but no significant effect w as round in SW-adapted eels (ANOVA: P < 0.001 and P > 0.05 respectively). I solated tubule cells stimulated with 100 nM Ang II showed a significant gen eration of inositol trisphosphate (InsP(3)) and an increment in calcium rel ease from intracellular stores. In conclusion, our results suggest that tubular Na+/K(+)ATPase is modulated by environmental salinity, and that Ang II has a role in regulating its ac tivity in FW-adapted eels, probably through an InsP(3)-dependent mechanism.