Simulated sleep apnea alters hydrogen sulfide regulation of blood flow and pressure.

In sleep apnea, airway obstruction causes intermittent hypoxia (IH). In animal studies, IH-dependent hypertension is associated with loss of vasodilator hydrogen sulfide (H2S), and increased H2S activation of sympathetic nervous system (SNS) activity in the carotid body. We previously reported that inhibiting cystathionine γ-lyase (CSE) to prevent H2S synthesis augments vascular resistance in control rats. The goal of this study was to evaluate the contribution of IH-induced changes in CSE signaling to increased blood pressure and vascular resistance. We hypothesized that chronic IH exposure eliminates CSE regulation of blood pressure (BP) and vascular resistance. In rats instrumented with venous catheters, arterial telemeters, and flow probes on the main mesenteric artery, the CSE inhibitor dl-propargylglycine (PAG, 50 mg/kg/day i.v. for 5 days) increased BP in Sham rats but decreased BP in IH rats [in mmHg, Sham (n = 11): 114 ± 4 to 131 ± 6; IH (n = 8): 131 ± 8 to 115 ± 7 mmHg, P < 0.05]. PAG treatment increased mesenteric vascular resistance in Sham rats but decreased it in IH rats (day 5/day 1: Sham: 1.50 ± 0.07; IH: 0.85 ± 0.19, P < 0.05). Administration of the ganglionic blocker hexamethonium (to evaluate SNS activity) decreased mesenteric resistance in PAG-treated Sham rats more than in saline-treated Sham rats or PAG-treated IH rats. CSE immunoreactivity in IH carotid bodies compared with those from Sham rats. However, CSE staining in small mesenteric arteries was less in arteries from IH than in Sham rats but not different in larger arteries (inner diameter > 200 µm). These results suggest endogenous H2S regulates blood pressure and vascular resistance, but this control is lost after IH exposure with decreased CSE expression in resistance size arteries. IH exposure concurrently increases carotid body CSE expression and relative SNS control of blood pressure, suggesting both vascular and carotid body H2S generation contribute to blood pressure regulation.NEW & NOTEWORTHY These results suggest that CSE's protective role in the vasculature is impaired by simulated sleep apnea, which also upregulates CSE in the carotid body. Thus, this enzyme system can exert both pro- and antihypertensive effects and may contribute to elevated SNS outflow in sleep apnea.

Hydrogen sulfide regulation of renal and mesenteric blood flow.

Hydrogen sulfide (H2S) dilates isolated arteries, and knockout of the H2S-synthesizing enzyme cystathionine γ-lyase (CSE) increases blood pressure. However, the contributions of endogenously produced H2S to blood flow regulation in specific vascular beds are unknown. Published studies in isolated arteries show that CSE production of H2S influences vascular tone more in small mesenteric arteries than in renal arteries or the aorta. Therefore, the goal of this study was to evaluate H2S regulation of blood pressure, vascular resistance, and regional blood flows using chronically instrumented rats. We hypothesized that during whole animal CSE inhibition, vascular resistance would increase more in the mesenteric than the renal circulation. Under anesthesia, CSE inhibition [ß-cyanoalanine (BCA), 30 mg/kg bolus + 5 mg·kg-1·min-1 for 20 min iv) rapidly increased mean arterial pressure (MAP) more than saline administration (%Δ: saline -1.4 ± 0.75 vs. BCA 7.1 ± 1.69, P < 0.05) but did not change resistance (MAP/flow) in either the mesenteric or renal circulation. In conscious rats, BCA infusion similarly increased MAP (%Δ: saline -0.8 ± 1.18 vs. BCA 8.2 ± 2.6, P < 0.05, n = 7) and significantly increased mesenteric resistance (saline 0.9 ± 3.1 vs. BCA 15.6 ± 6.5, P < 0.05, n = 12). The H2S donor Na2S (50 mg/kg) decreased blood pressure and mesenteric resistance ,but the fall in resistance was not significant. Inhibiting CSE for multiple days with dl-proparglycine (PAG, 50 mg·kg-1·min-1 iv bolus for 5 days) significantly increased vascular resistance in both mesenteric (ratio of day 1: saline 0.86 ± 0.033 vs. PAG 1.79 ± 0.38) and renal circulations (ratio of day 1: saline 1.26 ± 0.22 vs. 1.98 ± 0.14 PAG). These results support our hypothesis that CSE-derived H2S is an important regulator of blood pressure and vascular resistance in both mesenteric and renal circulations. Furthermore, inhalation anesthesia diminishes the effect of CSE inhibition on vascular tone.NEW & NOTEWORTHY These results suggest that CSE-derived H2S has a prominent role in regulating blood pressure and blood flow under physiological conditions, which may have been underestimated in prior studies in anesthetized subjects. Therefore, enhancing substrate availability or enzyme activity or dosing with H2S donors could be a novel therapeutic approach to treat cardiovascular diseases.