The Effects of Modulating Endothelial Nitric Oxide Synthese (eNOS) Activity and Coupling in Extracorporeal Shock Wave Lithotripsy (ESWL)

Introduction: ESWL is a clinical therapy to break down kidney and uretal stones into smaller fragments that are more easily eliminated through the urinary tract. High-energy shock waves are focused on the stone to cause shear stress and cavitation bubbles which synergistically ablate the stones. While ESWL is the preferred treatment for kidney stones over invasive surgeries, the repetitive shock waves necessary to break up the stones may also cause damage to the renal vasculature endothelium and that can lead to chronic hypertension [1]. Previous studies have found that ESWL can cause endothelial dysfunction which is characterized decreased nitric oxide (NO) bioavailability and increased production of reactive oxygen species (ROS) such as superoxide (O2-) [2]. Normally, endothelial nitric oxide synthase (eNOS) is in a coupled state which forms NO in the presence of essential cofactor tetrahydrobiopterin (BH4) and molecular oxygen. Oxidative stress, such as that caused by ESWL-induced ROS, can cause BH4 to be oxidized to dihydrobiopterin (BH2). When the BH2:BH4 ratio is increased, eNOS becomes uncoupled and produces O2- instead of NO [2, 3] (Figure 1). O2- is short-lived and converted to hydrogen peroxide (H2O2) in blood by superoxide dismutase. Protein kinase C epsilon (PKCε) has previously been found to regulate eNOS activity via phosphorylation at serine-1177. Cell-permeable PKCε peptide activator (PKCε+) increases eNOS activity while PKCε inhibitor (PKCε-) reduces eNOS activity [2]. Using a combination of eNOS cofactors BH4 or BH2 with eNOS activity regulators PKCε+ or PKCε-, we can explore the role of modulating eNOS to reduce oxidative stress and endothelial dysfunction caused by ESWL

The Effects of Dihydrobiopterin and Tetrahydrobiopterin on Hydrogen Peroxide and Nitric Oxide Release During Extracorporeal Shockwave Lithotripsy

Extracorporeal shockwave lithrotripsy (ESWL) is an effective, non-invasive clinical therapy utilized to break up stones in the kidney and urinary tract. A lithotripter generates high-energy acoustic pulses and propagates those shock waves through a lens on a region that focuses on the location of the stone, in turn breaking up the stone. The successive pulses generate shearing forces and cavitation bubbles. Cavitation bubbles are the formation and implosion of liquid free zones. The cavitation bubbles implode rapidly to create their own shockwaves that also put pressure on the stone. After treatment, fragmentation of the stone allows the debris to be cleared by the flow of the urinary tract. The problem is that to break up the kidney stone, it requires many repetitive shock waves that not only hit the kidney stone but also the surrounding tissue. Although lithotripsy provides a safer alternative to invasive treatments for removing harmful stones, ESWL may cause prolonged vasoconstriction after ESWL treatment, reducing renal blood flow, and subsequent endothelial dysfunction, which may cause kidney damage leading to acute to chronic hypertension clinically. ESWL-induced vascular oxidative stress and further endothelial dysfunction may be mediated by reduced levels of endothelial-derived nitric oxide (NO) and/or increased reactive oxygen species. Previous studies have shown that ESWL can induce oxidative stress, which can cause an increase in blood hydrogen peroxide (H2O2) and a decrease in endothelial-derived NO bioavailability release. Under normal conditions, tetrahydrobiopterin (BH4) is the cofactor to promote eNOS coupling, and endothelial-derived NO is produced. When the dihydrobiopterin (BH2) to tetrahydrobiopterin (BH4) ratio is increased during oxidative stress, such as ESWL, BH2 promotes eNOS uncoupling and produces superoxide (SO) instead of NO. (1,2) (Figure 1) SO is then later converted to H2O2 by superoxide dismutase. BH4 and BH2 bind to eNOS with equal affinity, therefore the ratio will determine whether eNOS principally produces NO or SO