Recently, heme oxygenase-1 (HO-1) has been shown to be present in vascular smooth muscle cells. In the present study, we examined the effect of angiotensin II (Ang II) on HO-1 in rat vascular smooth muscle cells. After treatment with 100 nmol/L Ang II, HO-1 mRNA levels were decreased, with a nadir at 2 hours (39 +/- 9% of the control level, P < .01). This downregulation was completely blocked by the Ang II type 1 receptor antagonist losartan. Western blot analysis showed that HO-1 protein is also significantly downregulated, with a nadir at 4 hours (52 +/- 6% of the control level, P < .01). Heme oxygenase activity was also significantly decreased at 4 hours (control, 0.35 +/- 0.86 nmol bilirubin/mg per hour; Ang II, 0.10 +/- 0.06). This downregulation was observed in serum-starved cells to a similar extent as in serum-supplemented cells. Inhibitors of protein kinase C, lipoxygenase, cyclooxygenase, cytochrome P450 monooxygenase, and phospholipase A2did not block this downregulation. However, this effect was not observed in the absence of calcium and presence of EGTA (2 mmol/L). Furthermore, a 2-hour incubation with calcium ionophore or arginine vasopressin decreased HO-1 mRNA levels, suggesting that an increase of intracellular calcium mediates the downregulation. In conclusion, Ang II decreases HO-1 mRNA in a calcium-dependent manner in vascular smooth muscle cells, which may provide a novel mechanism for the modulation of vascular tone and oxidative stress. (Hypertension. 1997;29:790-795.)