The vasoactive peptide AngiotensinII (AngII) is an important factor in the cardiovascular system, exerting most of its effects through AngII receptor type 1 (AT(1)). Ib, a new nonpeptide AT(1) receptor antagonist, has been observed to play a positive role in the treatment of hypertension in preclinical tests. In this study, the inhibitory effects of Ib on AngII-induced vascular contraction in vitro were investigated, and its molecular mechanisms were further explored. In endothelium-denuded aortic rings from rabbits, Ib produced a rightward shift in the concentration-response curve for AngII with a decrease in the maximal contractile response and the pD'(2) was 7.29. In vascular smooth muscle cells (VSMCs), the specific binding of [I-125]AngII to AT(1) receptors was inhibited by Ib in a concentration-dependent manner with IC50 value of 0.96 nM. Ib could inhibit both AngII-induced Ca2+ mobilization from internal stores and Ca2+ influx. Moreover, the translocation of PKC-alpha stimulated by AngII was inhibited by Ib. Thus, the inhibitory effects of Ib might be related with the depression on AngII-induced increase in [Ca2+](i) and translocation of PKC-alpha through blocking AT(1) receptors. (C) 2007 Elsevier Inc. All rights reserved.