ANGIOTENSIN-II RECEPTOR SUBTYPES IN EEL (ANGUILLA-ANGUILLA)

Previous studies have shown the effects of angiotensin II (Ang II) in teleosts, and Ang II-binding sites have also been localized in tissues from rainbow trout. The purpose of this study was to extend these fin dings and to provide an analysis of Ang II receptor (Ang II-R) isoform s in three tissues obtained from European eel (Anguilla anguilla). Ang II-Rs were identified in eel liver, kidney and intestine membranes by the binding of either 0.5 nmol human I-125-labelled Tyr(4)-Ile(5)-Ang II/l or increasing concentrations (1-120 nmol/l) of [3,5-H-3]Tyr(4)-I le(5)-Ang II. Using an isoelectric focusing technique, two Ang II-bind ing sites were identified in liver membranes. These migrated to isoele ctric points (pI values) 6.5 and 6.7. Seventy per cent of binding to b oth sites was displaced by a 10 000-fold excess of unlabelled human An g II. In both whole plasma membranes and brush border membranes from i ntestine, only one form of the Ang II-R was found, with pI 6.5 and hig h affinity (K-d=3.4 nmol/l) for the [3,5-H-3]Tyr(4)-Ile(5)-Ang II. Sim ilarly, only the isoform focusing at pI 6.5 was observed in renal tubu lar epithelial brush border membranes. Reduction of disulphide bridges with dithiothreitol significantly enhanced Ang II binding to the isof orm at pI 6.5 in liver (P<0.05) and kidney (P<0.01), while in liver th e binding to the isoform of pI 6.7 was significantly reduced (P<0.001) . The data suggest the existence in eel liver of multiple forms of Ang II-R, which may have different functions, while one single form appea red to be present in enterocyte plasma membrane and in renal brush bor der membrane.