A Novel Angiotensin Receptor Subtype in Rat Mesangium Coupling to Adenyiyi Cyclase

The diversity of angiotensin II (Ang II) actions implies multiple receptor subtypes. To characterize these subtypes in rat mesangial cells, we used the angiotensin subtype LA (ATu) antagonist losartan (DuP 753), the subtype 2/1B (AT2/AT,B) antagonist PD 123319, and the AT2 antagonist CGP 421L2A in radioreceptor and adenyiyi cyclase assays. In radioligand binding competition experiments, approximately 25% of the specific binding sites labeled by I-[Sar']Ang II were inhibited by low concentrations of PD 123319 (0.1 to 10 nM), whereas the AT2 antagonist CGP 42112A was inactive at concentrations less than 0.1 fiM. Conversely, losartan inhibited 75% of the binding at low concentrations (0.1 nM to 0.1 fiM), but higher concentrations (up to 10 /iM) were required to inhibit the second component of I-[Sar']Ang II binding. The effects of the different antagonists on the inhibition by Ang II of forskolin-stimulated cyclic AMP production were also analyzed. Ang II inhibited forskolin-stimulated adenyiyi cyclase in a concentrationdependent fashion (IC50, 35±7 nM), and the maximal inhibition of adenyiyi cyclase was 44±2%. In the radioligand binding experiments, both losartan and PD 123319 antagonized the inhibition of adenyiyi cyclase elicited by 0.1 fiM Ang II (IC5,, 0.5±0.2 and L2±0.4 pM, respectively), whereas CGP 42112A was less potent (ICn, 5.7±1.6 /iM). Comparison of binding affinities at ATIB receptor sites with antagonist potencies in the adenyiyi cyclase assay show good agreement for losartan and CGP 42112A, whereas PD 123319 is less potent than expected from membrane binding assays, possibly because of partial agonist properties. These results cannot be accounted for by a single AT, subtype and imply that there are two functionally coupled subtypes of AT, receptor in rat mesangial cell membranes. ATW and AT1B subtypes differ in their ligand specificity profile. The ATU subtype and an additional receptor, possibly the AT,B, are coupled to adenyiyi cyclase through G,. (Hypertension 1993^1:1035-1038)