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In a dose response curve to angiotensin II (Ang II), we determined that the maximal positive inotropic effect (PIE) elicited by Ang II (100 nM) was partially blocked by the non-selective endothelin (ET) receptor blocker TAK044. Moreover, the PIE induced by a smaller dose of Ang II (1 nM) was completely abolished by TAK044, indicating that at this dose this effect was mediated by the action of endogenous ET released/formed by Ang II. Since this dose of Ang II better resembles physiological concentration of interstitial Ang II, we focused our study on the intracellular pathways triggered by 1 nM Ang II. We used isolated cat cardiomyocytes for measuring sarcomere shortening (SS) with a video-camera and preproET-1 mRNA levels by real time RT-PCR. These parameters were measured in control and after 15 min treatment with 1 nmol/L Ang II. Ang II increased the level of preproET-1 mRNA from 100±3.4 % (n=15) to 173.3±28.1 % (n=11, p<0.05) as an indicator of the replenishment of the ET-1 intracellular pool that might have been reduced after the release of this peptide by Ang II. This increase in the level of preproET-1 mRNA was abrogated by the AT1 receptor blocker losartan (1 mmol/L; 90.5±21.4 %, n=4), the NADPH oxidase inhibitor apocinin (0.3 mmol/L; 77.7±15.8 %, n=5) and the reactive oxygen species (ROS) scavenger mercapto-propionyl-glycine (MPG, 1 mmol/L; 115.7±15.6 %, n=3). The PIE of Ang II (29.2±3.1 %, n=15, p<0.05) was abrogated by losartan (-1±4.9 %, n=6), by TAK044 (-4.5±4.9 %, n=6), by apocinin (8.7±7.3 %, n=8) and by MPG (-8.4±3.8 %, n=6). The fact that apocinin and MPG abrogated both the increase in ET-1 mRNA and the PIE induced by 1 nM Ang II allow us to conclude that Ang II through the release/formation of ET-1 and by stimulating NADPH oxidase, induces an increase in O2-. that triggers the increase in contractility.