Differential sensitivity of guanylyl cyclase and mitochondrial respiration to nitric oxide measured using clamped concentrations

Nitric oxide (NO) signal transduction may involve at least two targets: the guanylyl cyclase-coupled NO receptor (NOGCR), which catalyzes cGMP formation, and cytochrome c oxidase, which is responsible for mitochondrial O-2 consumption and which is inhibited by NO in competition with O-2. Current evidence indicates that the two targets may be similarly sensitive to NO, but quantitative comparison has been difficult because of an inability to administer NO in known, constant concentrations. We addressed this deficiency and found that purified NOGCR was about 100-fold more sensitive to NO than reported previously, 50% of maximal activity requiring only 4 nm NO. Conversely, at physiological O-2 concentrations (20-30 muM), mitochondrial respiration was 2-10-fold less sensitive to NO than estimated beforehand. The two concentration-response curves showed minimal overlap. Accordingly, an NO concentration maximally active on the NOGCR (20 nm) inhibited respiration only when the O-2 concentration was pathologically low (50% inhibition at 5 muM O-2). Studies on brain slices under conditions of maximal stimulation of endogenous NO synthesis suggested that the local NO concentration did not rise above 4 nm. It is concluded that under physiological conditions, at least in brain, NO is constrained to target the NOGCR without inhibiting mitochondrial respiration.