The positive inotropic effect of angiotensin II - Role of endothelin-1 and reactive oxygen species

Many effects believed to be because of angiotensin II ( Ang II) are attributable to the action of endothelin ( ET)-1, which is released/produced by Ang II. We investigated whether Ang II elicits its positive inotropic effect ( PIE) by the action of endogenous ET-1, in addition to the role played by reactive oxygen species ( ROS) in this mechanism. Cat cardiomyocytes were used for: ( 1) sarcomere shortening measurements; ( 2) ROS measurements by epifluorescence; ( 3) immunohistochemical staining for preproET-1, BigET-1, and ET-1; and ( 4) measurement of preproET- 1 mRNA by RT-PCR. Cells were exposed to 1 nmol/L Ang II for 15 minutes. This low concentration of Ang II increases sarcomere shortening by 29.2 +/- 3.7% ( P < 0.05). This PIE was abrogated by Na+/H+ exchanger or Na+/Ca2+ exchanger reverse mode inhibition. The production of ROS increased in response to Ang II treatment ( Delta ROS respect to control: 68 +/- 15 fluorescence units; P < 0.05). The Ang II-induced PIE and ROS production were blocked by the Ang II type 1 receptor blocker losartan, the nonselective ET-1 receptor blocker TAK044, the selective ETA receptor blocker BQ-123, or the ROS scavenger N-(2-mercapto-propionyl) glycine. Exogenous ET-1 ( 0.4 nmol/L) induced a similar PIE and increase in ROS production to those caused by Ang II. Immunostaining for preproET- 1, BigET-1, and ET-1 was positive in cardiomyocytes. The preproET- 1 mRNA abundance increased from 100 +/- 4.6% in control to 241.9 +/- 39.9% in Ang II-treated cells ( P < 0.05). We conclude that the PIE after exposure to 1 nmol/L Ang II is due to endogenous ET-1 acting through the ETA receptor and triggering ROS production, Na+/H+ exchanger stimulation, and Na+/Ca2+ exchanger reverse mode activation.