The role of hydrogen peroxide in the contractile response to angiotensin II

In the last years, reactive oxygen species (ROS) have been proposed as medi ators of proliferative/hypertrophic responses to angiotensin II (Ang II), b oth in vivo and in vitro. However, the hypothesis that the Ang II-dependent cell contraction could be mediated by ROS, particularly H2O2, has not been tested. Present experiments were devoted to test this hypothesis and to an alyze the possible mechanisms involved. Catalase (CAT) prevented the increa sed myosin light chain phosphorylation and the decreased planar cell surfac e area (PCSA) induced by 1 muM Ang II in cultured rat vascular smooth muscl e cells (VSMC). This preventive effect of CAT was also detected when 1 muM platelet-activating factor (PAF) was used as a contractile agonist instead of Ang II. Similar results were found when using horseradish peroxidase as an H2O2 scavenger or cultured rat mesangial cells. In vascular smooth muscl e cells, CAT modified neither the binding of labeled Ang II nor the Ang II- induced inositol 1,4,5-trisphosphate (IP3) synthesis. However, it completel y abolished the Ang II-dependent calcium peak, in a dose-dependent fashion. CAT-loaded cells (increased intracellular CAT concentration over 3-fold) d id not show either a decreased PCSA or an increased intracellular calcium c oncentration after Ang II treatment. Ang II stimulated the H2O2 synthesis b y cultured cells, and the presence of CAT in the extracellular compartment significantly diminished the Ang II-dependent increased intracellular H2O2 concentration. The physiological importance of these findings was tested in rat thoracic aortic rings: CAT prevented the contraction elicited by Ang I I. In summary, present experiments point to H2O2 as a critical intracellula r metabolite in the regulation of cell contraction.