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pubmed:abstractText |
Because angiotensin II (Ang II) has been found at high concentrations in the proximal tubule fluid and because tubular brush border membranes exhibit a marked capacity for degrading Ang II, we thought it of interest to examine the binding sites for Ang II (3-8) (referred to as Ang IV), a metabolite of Ang II, downstream in the nephron. We studied the binding of [125I]-Ang IV and also of [125I]-Sar1, Ala8, Ang II to SV-40 transformed human collecting duct cell (HCD) membranes. No specific binding site for [125I]-Sar1, Ala8, Ang II and no Ang II-dependent cytosolic calcium response could be observed. Moreover, no signal for the human type I Ang II receptor (hAT1) mRNA was present in HCD cells. In contrast, [125I]-Ang IV bound specifically to HCD cell membranes. Mean Kd and Bmax values derived from saturation binding studies were 5.6 +/- 2.0 nM and 1007.6 +/- 140.2 fmol/mg protein, respectively. The rank order of affinity for competitive Ang II-related peptides was: Ang IV > Ang III > Ang II > Ang II (4-8) > Ang II (1-7). [125I]-Ang IV binding was not modified by nonpeptide AT1 (losartan) or AT2 (PD123177) antagonists. GTP gamma S and dithiotreitol did not affect [125I]-Ang IV binding. Ang IV stimulated cAMP production by intact HCD cells in the presence of forskolin but did not modify cGMP production or cytosolic calcium concentration. Taken together, these results indicate that HCD cells represent a target site for Ang IV but do not possess Ang II receptors.
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