FUNCTIONAL-STUDIES BUT NOT RECEPTOR-BINDING CAN DISTINGUISH SURMOUNTABLE FROM INSURMOUNTABLE AT(1) ANTAGONISM

Our study demonstrated that inhibition of angiotensin II-(Ang II) medi ated contractions of rabbit aorta by structurally diverse nonpeptide A T(1) antagonists could distinguish surmountable from insurmountable AT (1) antagonism. Cl-996, L158809, EXP 3174 and SKF 108834 produced conc entration-related rightward shifts in Ang II response curves and reduc ed the maximal contraction to Ang II, characteristic of insurmountable antagonism. In contrast, DuP 753 and SKF 108566, produced parallel ri ghtward shifts in Ang II contractile curves without affecting the maxi mal response which is consistent with the definition of surmountable o r competitive antagonism. In addition, Cl-996 demonstrated potent inhi bition of Ang II-stimulated inositol phosphate accumulation in rat aor tic smooth muscle cells, behaving as an insurmountable antagonist. How ever, DuP 753 was a surmountable antagonist of Ang II-stimulated inosi tol phosphate accumulation. Repeated washing of rabbit aorta preincuba ted with either Cl-996 or EXP 3174 did not restore the blunted Ang II contractions. In contrast, both DuP 753 and the structurally dissimila r SKF 108566 at a concentration of 100 nM showed complete recovery of Ang II responses within 2 hr of repeated washing. Surprisingly, repeat ed rinsing of rabbit aorta for up to 5 hr after incubation with 1 mu M DuP 753 failed to restore responses to Ang II. In addition, Scatchard analysis of [I-125] Ang II saturation binding experiments revealed a competitive and rapidly reversible nature of AT(1) receptor antagonism for all the AT(1) antagonists examined. Taken together, the results o f this study provide evidence for a competitive and rapidly reversible binding interaction of structurally diverse nonpeptide antagonists at the AT(1) receptor. The data also suggest that incomplete recovery of sensitivity to Ang II as demonstrated by functional washout experimen ts may be a result of compound sequestration in tissue components vs, the AT(1) receptor itself which may prolong the duration of action of the antagonist. Thus, the ability of a compound to produce insurmounta ble antagonism may in part be due to a slow removal of that agent from tissue compartments, cells or matrix surrounding the AT(1) receptor w hich may be influenced by such factors as compound lipophilicity and k inetics of distribution and metabolism.