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Angiotensin II Induces Hypertrophy, not Hyperplasia, of Cultured Rat Aortic Smooth Muscle Cells

We have explored the hypothesis that contractile agonists are important regulators of smooth muscle cell growth by examining the effects of one potent contractile agonist, angiotensin II (AII), on both cell proliferation and cellular hypertrophy. AII neither stimulated proliferation of cells made quiescent in a defined serum-free media nor augmented cell proliferation induced by serum or platelet-derived growth factor. However, AII did induce cellular hypertrophy of postconfluent quiescent cultures following 4 days of treatment, increasing smooth muscle cell protein content by 20% as compared with vehicle-treated controls. AII-induced hypertrophy was maximal at 1 μM, had an ED50 of 5 nM, and was blocked by the specific AII receptor antagonist Sar1, Ile8 AII. The cellular hypertrophy was due to an increase in protein synthesis, which was elevated within 6–9 hours following AII treatment, while no changes in protein degradation were apparent. AII was even more effective in inducing hypertrophy of subconfluent cultures, causing a 38% increase in protein content after 4 days of treatment (1 μM) and showing a maximal response at concentrations as low as 0.1 nM. Interestingly, in subconfluent cultures, AII treatment (1 μM, 4 days) was associated with a 50% increase in the fraction of cells with 4C DNA content with the virtual absence of cells in S-phase of the cell cycle, consistent with either arrest of cells in the G2 phase of the cell cycle or development of tetraploidy. These studies show that AII is a potent hypertrophic agent but has no detectable mitogenic activity in cultured rat aortic smooth muscle cells and describe an in vitro model that should be extremely valuable in exploring the cellular controls of smooth muscle cell hypertrophy.