Identification and characterization of a functional mitochondrial angiotensin system.

The renin-angiotensin (Ang) system regulates multiple physiological functions through Ang II type 1 and type 2 receptors. Prior studies suggest an intracellular pool of Ang II that may be released in an autocrine manner upon stretch to activate surface membrane Ang receptors. Alternatively, an intracellular renin-Ang system has been proposed, with a primary focus on nuclear Ang receptors. A mitochondrial Ang system has not been previously described. Here we report that functional Ang II type 2 receptors are present on mitochondrial inner membranes and are colocalized with endogenous Ang. We demonstrate that activation of the mitochondrial Ang system is coupled to mitochondrial nitric oxide production and can modulate respiration. In addition, we present evidence of age-related changes in mitochondrial Ang receptor expression, i.e., increased mitochondrial Ang II type 1 receptor and decreased type 2 receptor density that is reversed by chronic treatment with the Ang II type 1 receptor blocker losartan. The presence of a functional Ang system in human mitochondria provides a foundation for understanding the interaction between mitochondria and chronic disease states and reveals potential therapeutic targets for optimizing mitochondrial function and decreasing chronic disease burden with aging.

Is Kir6.1 a subunit of mitoK(ATP)?

The subunit composition of the mitochondrial ATP-sensitive K(+)-channel (mitoK(ATP)) is unknown, though some suspect a role for the inward rectifier, Kir6.1, based largely on antibody studies of heart mitochondria. To ascertain the molecular identity of mitoK(ATP) we therefore sought to purify this putative mitochondrial Kir6.1, and conclusively identify the subunits by mass spectrometry. Immunoblots, conducted with two commercially available antibodies, revealed two distinct signals in isolated heart mitochondria, of 51 and 48kDa, respectively. Localization was confirmed by either immuno-gold electron microscopy or by immunofluorescence. Each putative Kir6.1 species was extracted, purified, and identified by LC-MS/MS. The 51kDa band was identified as NADH-dehydrogenase flavoprotein 1, while the preponderant protein in the 48-kDa band was mitochondrial isocitrate dehydrogenase (NADP form). 1D-, 2D-, and native gel analyses were consistent with these assignments. The data suggest it is premature to assign Kir6.1 a role in mitoK(ATP) on the basis of immunoreactivity alone.