ANG II receptor expression and function during phenotypic modulation of rat aortic smooth muscle cells

Angiotensin II (ANG II) receptors were investigated in primary cultured rat aortic smooth muscle cells (SMC) that expressed either a proliferative phenotype (during the growth phase) or a contractile phenotype (at postconfluence). For each phenotype, alpha-smooth muscle actin expression, 125I-labeled ANG II specific binding, D-myo-inositol 1,4,5-triphosphate [Ins(1,4,5)P3] production, and ANG II-mediated increases in intracellular calcium (Cai2+) were studied. In both phenotypes, 1) ANG II-specific high-affinity binding (KD 0.5 +/- 0.1 nM and Bmax 196 +/- 106 pmol/mg protein in proliferative state, KD 1.5 +/- 0.3 nM and Bmax 560 +/- 299 pmol/mg protein in postconfluent state) was entirely inhibited by the selective AT1-antagonist losartan as well as by [Sar1,Ala8]ANG II and ANG III; 2) the AT2-antagonist CGP 42112A was ineffective, except at very high concentrations (> or = 10 microM); 3) the specific binding of ANG II was inhibited by guanosine 5'-[gamma-thio]triphosphate; and 4) ANG II induced a losartan-sensitive increase in Ins(1,4,5)P3. In postconfluent cultures, ANG II elicited a rapid biphasic elevation in Cai2+, which was abolished by losartan, whereas in growing cultures, this response was either absent or greatly attenuated. It is concluded that AT1-receptors coupled to phospholipase C via a G protein are expressed in the proliferative as well as in the contractile SMC phenotype and that their coupling to Cai2+ release is impaired in the proliferative phenotype. No evidence for AT2-receptor expression during phenotypic modulation of SMC was found.