Hydrogen sulfide, then nitric oxide and vasoprotection

Vascular dysfunction is typically associated with hypertension and contributes to the progression of cardiovascular disease. Altered vascular tone that constitutes an exaggerated vasoconstrictor response or an attenuated ability for vasorelaxation may potentially lead to a chronic increase in vascular resistance and an elevation in blood pressure. A key component regulating vascular tone is the nitric oxide (NO) system whereby activation of NO synthase (NOS) generates NO; the most prominent NOS isoform in the vasculature is endothelial NOS (eNOS or NOS type III) [1,2]. A well characterized signaling pathway for NOS activation and NO release is the PI3 kinase–Akt kinase axis that results in phosphorylation of the Ser¹¹⁷⁷ residue of eNOS for assembly and activation [2]. Subsequent release of endothelial NO activates soluble cGMP cyclase in the smooth muscle to form cGMP, an essential cofactor for protein kinase G (PKG). The activation of PKG leads to the phosphorylation of multiple substrates that facilitates vasorelaxation [3,4]. The importance of the NO system in blood pressure regulation reflects in part, the sustained hypertensive response to administration of NOS inhibitors such as l-NAME or the targeted deletion of eNOS, as well as the loss of NO tone in various forms of hypertension and other cardiovascular diseases [1,5].

In a very extensive study comprising both hypertensive patients and a preclinical model of renovascular hypertension, Xia et al. [6] sought to establish the contribution of another gas molecule hydrogen sulfide (H₂S) to vascular tone. They report that the H₂S system resides upstream of NOS/NO and identify the signaling pathways for H₂S in the endothelium [6]. The authors assessed two cohorts with established hypertension and normal blood pressure with respect to circulating H₂S levels and the vascular response to H₂S in isolated renal arteries from both groups. Additional studies determined the influence of exogenous H₂S on blood pressure and vascular reactivity in the Goldblatt model (two-kidney, one-clip) of renovascular hypertension. The study established that both clinical and experimental hypertension were associated with significantly lower levels of circulating H₂S [6]. In the human renal arteries, protein levels of the H₂S-generating enzyme cystathionine γ-lyase (CSE) were also lower in the hypertensive vessels that appear to reflect an attenuated synthesis of the enzyme. Moreover, H₂S exposure (12 h) of the isolated renal arteries from the patients with hypertension reversed the decline in CSE expression suggesting a positive feedback role of H₂S to maintain vascular synthesis. Of particular interest, H₂S exposure attenuated protein and mRNA expression of the angiotensin-II type-1 receptor (AT₁R), a novel mechanism to preserve vascular function in the presence of an activated renin–angiotensin–aldosterone system (RAAS). H₂S exposure also restored the attenuated acetylcholine (Ach)-dependent response of the renal vessels from the hypertensive group. Restoration of the Ach response by H₂S was associated with augmented levels of the peroxisome proliferator activator receptor (PPARδ), as well as an increase in phosphorylated eNOS, Akt and AMP kinase [6]. The link to NOS–NO signaling was corroborated in human endothelial cells; exogenous H₂S restored NO levels following Ang-II exposure and addition of selective inhibitors/antagonists to PI3 kinase, Akt, AMP kinase and PPARδ reversed the NO response following H₂S treatment in these cells [6].

In the renovascular hypertensive rats, circulating levels of H₂S were reduced over 70% and protein expression of CSE was similarly reduced in the renal vessels. Exogenous treatment partially reversed the increase in mean arterial pressure; however, this was apparent only after 12 weeks of H₂S treatment and continued to the end of the study at 20 weeks. The lag time for H₂S to lower blood pressure was not addressed in the study and it is unclear whether this reflects effects on the endogenous H₂S system or other regulatory systems within the vasculature. In this regard, the reduction in blood pressure by H₂S was associated with lower AT₁R expression, but higher CSE levels may infer a restored capability for local H₂S formation to impact the vascular RAAS [6]. Moreover, chronic H₂S treatment restored the blunted Ach response in the isolated vessels of the hypertensive rats. Blockade of PI3, Akt and AMP kinases, as well as NOS and PPARδ again abrogated the protective effects of H₂S administration that suggest that the vascular actions of H₂S are primarily mediated by NO. The preservation of NO tone by the chronic administration of H₂S is consistent with the downregulation of the AT₁R in lieu of the inhibitory effects of NO on various components of the RAAS. Furthermore, the chronic administration of a PPARδ agonist reduced blood pressure in spontaneously hypertensive rats (SHR); the PPARδ agonist also reduced indices of inflammation (TNFα, IL-1β, ICAM-1, IL-6) and attenuated the constrictor effects of Ang II and endothelin-1, but did not lower mRNA levels of the AT₁R or the ET_AR [7].

The current data revealing that the vascular effects of H₂S lie upstream from the generation of NO contrast with the reported direct effects of H₂S to either enhance vasorelaxation or stimulate vasoconstriction. The direct vascular effects of H₂S appear to be dependent on the concentration of H₂S applied to the vessel; application of H₂S at doses less than 30 μmol/l are constrictive whereas concentrations greater than 100 μmol/l stimulate relaxation of the vessel [8,9]. The direct constrictor actions of H₂S appear to be mediated through a reduction in NO and an increase in oxidative stress [8–10]. In contrast, higher H₂S concentrations induce activation of K_ATP channels that may reflect an increase in intracellular calcium levels to elicit direct vasorelaxant effects [9,11]. The current data support past studies that assessed the chronic effects of H₂S on blood pressure and vascular function in various models of hypertension. Blood pressure-lowering effects of H₂S were previously reported in the spontaneously hypertensive rat (SHR), DAHL salt-sensitive rat and chronic Ang II-infused mice; the effects of H₂S in these studies were associated with a reduction in oxidative stress in the vasculature [12]. Tissue-wide deletion of the H₂S-generating enzyme CSE was also associated with a marked increase in blood pressure, as well as evidence of vascular dysfunction [12]. Although the current study did not assess the effect of acute (12 h) or chronic (20 weeks) H₂S treatment on vascular redox balance, stimulation of NO tone by H₂S may also reflect an increase in NO bioavailability via the activation of scavenging enzymes to lower oxidative stress or direct interactions with NO to form novel hybrid forms [8,10,13].

The present results demonstrate a significant vasoprotective effect of H₂S that may contribute to the blood pressure-lowering actions of this molecule in renovascular hypertension. Although this study on the cardiovascular role of H₂S primarily focused on the vascular actions of the molecule, H₂S exerts significant actions on the heart as well [9,14]. Indeed, H₂S has direct actions on the cardiomyocyte that involve NO signaling and mitochondrial protection to preserve cardiac function [2,15]. Moreover, there is evidence for the role of H₂S in other cardiovascular-relevant tissues such as the brain, kidney, gut microbiome and immune cells; and these targets should be continued areas of investigation, particularly the extent for the NO-dependent and NO-independent actions of H₂S [9,10,12]. Finally, the study by Xia and colleagues documented that the H₂S effects likely involve the downregulation of the AT₁R in both hypertensive patients and experimental hypertension, but the impact of H₂S tone on the alternative ACE2–Ang-(1–7)-AT₇–MasR axis that functionally opposes the ACE-Ang II-AT₁R should be evaluated as well [5].