Hypothesis / aims of study
Neurogenic injury can have severe consequences to multiple physiological functions including mobility, autonomic regulation of the urinary bladder and vascular dysfunction, which is believed to occur in response to inflammation, oxidative stress, and vascular endothelial cell damage. A hallmark of vascular damage is the impaired responsiveness to nitric oxide (NO•), a key mediator of vascular smooth muscle relaxation. NO• activates soluble guanylate cyclase (sGC) to generate cyclic guanosine monophosphate (cGMP) that stimulates a multitude of downstream pathways. There are indications that increasing NO•-cGMP signaling can be beneficial in ameliorating spinal cord injury (SCI) induced detrusor overactivity [1]. However, approved agents such as phosphodiesterase (PDE) type 5 inhibitors have limited efficacy due to decreased NO• bioavailability and/or oxidative stress-induced inhibition of sGC, preventing cGMP generation. Small molecule sGC activators can circumvent these pathological changes as they are heme mimetics that allosterically activate sGC in the absence of its heme prosthetic group and NO• (Fig 1). We have measured the benefits of cinaciguat on the mitochondrial respiration and spinal cord regrowth in spinal cord contused (SCC) mice.
Study design, materials and methods
Mitochondria were isolated from the spinal cords of control, SCC and SCC C57Bl/6 mice treated with cinaciguat (10 mg/kg/day/7 days). The mitochondria were mechanically dissociated, isolated by differential centrifugation, and placed in a gas-tight vessel containing a Clark-type oxygen microelectrode to measure state 3 (succinate + ADP) and state 4 (succinate alone) respiratory rates. The respiratory control ratio (RCR), a measure of the “tightness of coupling” between electron transport and oxidative phosphorylation, was determined from the ratio of state 3 to state 4 respiration rates. A RCR of 2-4 is considered good for complex II substrates [2]. Measures were expressed as mean ± standard deviation (SD). Between group differences were assessed by a one-way ANOVA with Tukey’s post-hoc multiple comparison analysis (Prism 9, GraphPad) and significance was determined as p<0.05.
Interpretation of results
The unique feature of sGC activators is their ability to induce cGMP generation when the heme moiety is oxidized or absent and the NO• signaling pathway is compromised. This is especially important in SCI accompanied by systemic inflammation and chronic oxidative stress that promotes heme oxidation and inactivation of sGC. Our data show SCC resulted in significant alterations in mitochondrial RCR in spinal cord tissue which was normalized by treatment with cinaciguat.