Combined oral administration of L-arginine and tetrahydrobiopterin in a rat model of pulmonary arterial hypertension

Impact of l-citrulline on nitric oxide signaling and arginase activity in hypoxic human pulmonary artery endothelial cells

Impaired nitric oxide (NO) signaling contributes to the development of pulmonary hypertension (PH). The l-arginine precursor, l-citrulline, improves NO signaling and has therapeutic potential in PH. However, there is evidence that l-citrulline might increase arginase activity, which in turn, has been shown to contribute to PH. Our major purpose was to determine if l-citrulline increases arginase activity in hypoxic human pulmonary artery endothelial cells (PAECs). In addition, to avoid potential adverse effects from high dose l-citrulline monotherapy, we evaluated whether the effect on NO signaling is greater using co-treatment with l-citrulline and another agent that improves NO signaling, folic acid, than either alone. Arginase activity was measured in human PAECs cultured under hypoxic conditions in the presence of l-citrulline (0-1 mM). NO production and endothelial nitric oxide synthase (eNOS) coupling, as assessed by eNOS dimer-to-monomer ratios, were measured in PAECs treated with l-citrulline and/or folic acid (0.2 μM). Arginase activity increased in hypoxic PAECs treated with 1 mM but not with either 0.05 or 0.1 mM l-citrulline. Co-treatment with folic acid and 0.1 mM l-citrulline increased NO production and eNOS dimer-to-monomer ratios more than treatment with either alone. The potential to increase arginase activity suggests that there might be plasma l-citrulline concentrations that should not be exceeded when using l-citrulline to treat PH. Rather than progressively increasing the dose of l-citrulline as a monotherapy, co-therapy with l-citrulline and folic acid merits consideration, due to the possibility of achieving efficacy at lower doses and minimizing side effects.

Folic acid, either solely or combined with L-citrulline, improves NO signaling and ameliorates chronic hypoxia-induced pulmonary hypertension in newborn pigs

Concomitant with developing pulmonary hypertension (PH), newborn piglets exposed to chronic hypoxia develop pulmonary vascular NO signaling impairments. PH is reduced and NO signaling is improved in chronically hypoxic piglets treated with the NO-arginine precursor, L-citrulline. Folic acid positively impacts NO signaling. We evaluated whether the effect on NO signaling and PH is greater using co-treatment with folic acid and L-citrulline than either alone. From day 3 to day 10 of hypoxia, piglets were treated solely with folic acid, solely with L-citrulline, or co-treated with both. Catheters were placed to measure in vivo hemodynamics. NO production was measured in vitro in dissected pulmonary arteries. Compared to normoxic piglets, pulmonary vascular resistance (PVR) was elevated and NO production was reduced in untreated hypoxic piglets. Regardless of treatment strategy, PVR was less in all three treated groups of hypoxic piglets when compared to the untreated hypoxic group. In addition, for all three groups of treated hypoxic piglets, NO production was higher than the untreated group. Improvements in PVR and NO production did not differ between piglets co-treated with folic acid and L-citrulline and those treated solely with either. Thus, the impact on NO production and PVR was not augmented by combining folic acid and L-citrulline treatments. Nonetheless, treatment with folic acid, either singly or when combined with L-citrulline, increases NO production and inhibits PH in chronically hypoxic newborn piglets. Folic acid merits consideration as a therapy for PH in human infants with chronic heart and lung conditions that are associated with chronic hypoxia.

Vasoconstrictor Mechanisms in Chronic Hypoxia-Induced Pulmonary Hypertension: Role of Oxidant Signaling

Elevated resistance of pulmonary circulation after chronic hypoxia exposure leads to pulmonary hypertension. Contributing to this pathological process is enhanced pulmonary vasoconstriction through both calcium-dependent and calcium sensitization mechanisms. Reactive oxygen species (ROS), as a result of increased enzymatic production and/or decreased scavenging, participate in augmentation of pulmonary arterial constriction by potentiating calcium influx as well as activation of myofilament sensitization, therefore mediating the development of pulmonary hypertension. Here, we review the effects of chronic hypoxia on sources of ROS within the pulmonary vasculature including NADPH oxidases, mitochondria, uncoupled endothelial nitric oxide synthase, xanthine oxidase, monoamine oxidases and dysfunctional superoxide dismutases. We also summarize the ROS-induced functional alterations of various Ca2+ and K+ channels involved in regulating Ca2+ influx, and of Rho kinase that is responsible for myofilament Ca2+ sensitivity. A variety of antioxidants have been shown to have beneficial therapeutic effects in animal models of pulmonary hypertension, supporting the role of ROS in the development of pulmonary hypertension. A better understanding of the mechanisms by which ROS enhance vasoconstriction will be useful in evaluating the efficacy of antioxidants for the treatment of pulmonary hypertension.

A Paradoxical Vasodilatory Nutraceutical Intervention for Prevention and Attenuation of Migraine-A Hypothetical Review

Studies suggest that migraine pain has a vascular component. The prevailing dogma is that peripheral vasoconstriction activates baroreceptors in central, large arteries. Dilatation of central vessels stimulates nociceptors and induces cortical spreading depression. Studies investigating nitric oxide (NO) donors support the indicated hypothesis that pain is amplified when acutely administered. In this review, we provide an alternate hypothesis which, if substantiated, may provide therapeutic opportunities for attenuating migraine frequency and severity. We suggest that in migraines, heightened sympathetic tone results in progressive central microvascular constriction. Suboptimal parenchymal blood flow, we suggest, activates nociceptors and triggers headache pain onset. Administration of NO donors could paradoxically promote constriction of the microvasculature as a consequence of larger upstream central artery vasodilatation. Inhibitors of NO production are reported to alleviate migraine pain. We describe how constriction of larger upstream arteries, induced by NO synthesis inhibitors, may result in a compensatory dilatory response of the microvasculature. The restoration of central capillary blood flow may be the primary mechanism for pain relief. Attenuating the propensity for central capillary constriction and promoting a more dilatory phenotype may reduce frequency and severity of migraines. Here, we propose consideration of two dietary nutraceuticals for reducing migraine risk: L-arginine and aged garlic extracts.

Combination therapy improves vascular volume in female rats with pulmonary hypertension

Pulmonary arterial hypertension (PAH) is a female predominant disease in which progressive vascular remodeling and vasoconstriction result in right ventricular (RV) failure and death. Most PAH patients utilize multiple therapies. In contrast, the majority of preclinical therapeutic studies are performed in male rats with a single novel drug often markedly reversing disease in the model. We sought to differentiate single drug therapy from combination therapy in female rats with severe disease. One week after left pneumonectomy, we induced PH in young female Sprague-Dawley rats with an injection of monocrotaline (45 mg/kg). Female rats were then randomized to receive combination therapy (ambrisentan plus tadalafil), ambrisentan monotherapy, tadalafil monotherapy, or vehicle. We measured RV size and function on two serial echocardiograms during the development of disease. We measured RV systolic pressure (RVSP) invasively at day 28 after monocrotaline before analyzing the vascular volume with microcomputed tomography (microCT) of the right middle lobe. RVSP was significantly lower in female rats treated with combination therapy, and combination therapy resulted in increased small vessel volume density measured by microCT compared with untreated rats. Combination-treated rats had the smallest RV end-diastolic diameter on echocardiogram as compared with the other groups. In summary, we report a female model of pulmonary hypertension that can distinguish between one and two drug therapies; this model may facilitate better preclinical drug testing for novel compounds.

Vascular Biology of Superoxide-Generating NADPH Oxidase 5-Implications in Hypertension and Cardiovascular Disease

SIGNIFICANCE

NADPH oxidases (Noxs), of which there are seven isoforms (Nox1-5, Duox1/Duox2), are professional oxidases functioning as reactive oxygen species (ROS)-generating enzymes. ROS are signaling molecules important in physiological processes. Increased ROS production and altered redox signaling in the vascular system have been implicated in the pathophysiology of cardiovascular diseases, including hypertension, and have been attributed, in part, to increased Nox activity. Recent Advances: Nox1, Nox2, Nox4, and Nox5 are expressed and functionally active in human vascular cells. While Nox1, Nox2, and Nox4 have been well characterized in models of cardiovascular disease, little is known about Nox5. This may relate to the lack of experimental models because rodents lack NOX5. However, recent studies have advanced the field by (i) elucidating mechanisms of Nox5 regulation, (ii) identifying Nox5 variants, (iii) characterizing Nox5 expression, and (iv) discovering the Nox5 crystal structure. Moreover, studies in human Nox5-expressing mice have highlighted a putative role for Nox5 in cardiovascular disease.

CRITICAL ISSUES

Although growing evidence indicates a role for Nox-derived ROS in cardiovascular (patho)physiology, the exact function of each isoform remains unclear. This is especially true for Nox5.

FUTURE DIRECTIONS

Future directions should focus on clinically relevant studies to discover the functional significance of Noxs, and Nox5 in particular, in human health and disease. Two important recent studies will impact future directions. First, Nox5 is the first Nox to be crystallized. Second, a genome-wide association study identified Nox5 as a novel blood pressure-associated gene. These discoveries, together with advancements in Nox5 biology and biochemistry, will facilitate discovery of drugs that selectively target Noxs to interfere in uncontrolled ROS generation.

Oxidative stress induces BH4 deficiency in male, but not female, SHR

We previously published that female spontaneously hypertensive rats (SHR) have significantly greater nitric oxide (NO) bioavailability and NO synthase (NOS) enzymatic activity in the renal inner medulla (IM) compared with age-matched males, although the mechanism responsible remains unknown. Tetrahydrobiopterin (BH₄) is a critical cofactor required for NO generation, and decreases in BH₄ as a result of increases in oxidative stress have been implicated in the pathogenesis of hypertension. As male SHR are known to have higher levels of oxidative stress compared with female SHR, we hypothesized that relative BH₄ deficiency induced by oxidative stress in male SHR results in lower levels of NOS activity in renal IM compared with females. Twelve-week-old male and female SHR were randomized to receive tempol (30 mg/kg/day via drinking water) or vehicle for 2 weeks. Tempol treatment did not affect blood pressure (BP) in either sex, but reduced peroxynitrite levels only in males. Females had more total biopterin, dihydrobiopterin (BH₂), and BH₄ levels in renal IMs than males, and tempol treatment eliminated these sex differences. Females had greater total NOS activity in the renal IM than males, and adding exogenous BH₄ to the assay increased NOS activity in both sexes. This sex difference in total NOS and the effect of exogenous BH₄ were abolished with tempol treatment. We conclude that higher oxidative stress in male SHR results in a relative deficiency of BH₄ compared with females, resulting in diminished renal NOS activity in the male.

Evaluation of the effects of RP5063, a novel, multimodal, serotonin receptor modulator, as single-agent therapy and co-administrated with sildenafil, bosentan, and treprostinil in a monocrotaline-induced pulmonary arterial hypertension rat model

Pulmonary arterial hypertension (PAH), a condition that is defined by pulmonary vasculature constriction and remodeling, involves dysfunctional signaling of the serotonin (5-HT) receptors, 5-HT_2A/2B/7. In a rat model of monocrotaline (MCT)-induced PAH, the effectiveness of RP5063 (RP), a dopamine and 5-HT receptor modulator, was evaluated as monotherapy and as an adjunct to standard PAH treatments. After a single 60 mg/kg dose of MCT, rats received vehicle (MCT+Veh; gavage twice-daily [b.i.d.]), RP (10 mg/kg; gavage b.i.d.), bosentan (B; 100 mg/kg; gavage BID), sildenafil (S; 50 mg/kg; gavage, BID), treprostinil (T; 100 ng/kg/min over 24 h intravenous), RP+B, RP+S, and RP+T for 28 days. Single-agent RP limited the functional and structural effects of PAH seen in the MCT+Veh group, with significant improvements in pulmonary hemodynamics, right ventricular (RV) hypertrophy, SO₂, and pulmonary blood vessel structural changes. These effects appeared comparable with those associated with B, S, and T. Adjunctive RP treatment resulted in significantly lower mean pulmonary arterial pressures, RV systolic pressure. It also improved SO₂ measurements, as compared with MCT+Veh (P < 0.05), and diastolic pulmonary artery pressure (P < 0.05), as compared with single-agent B and S therapy (Bonferroni method adjusting for multiplicity). RP+S appeared to show the most consistent and extensive effects on pulmonary hemodynamics, respiratory parameters, and histopathologic changes. These results corroborate earlier preclinical findings supporting the efficacy of single-agent RP in PAH. RP, as mono and adjunctive therapy compared with induced-control, mitigated the functional and structural effects of MCT-induced PAH.

Efficacy and tolerability of pharmacological interventions for pulmonary arterial hypertension: A network meta-analysis

PURPOSE

This network meta-analysis (NMA) is designed to compare the efficacy and tolerability of various therapies and combinations for pulmonary arterial hypertension (PAH).

METHOD

We conducted a systematic search in databases PubMed, Embase, and Cochrane Library. Treatment efficacy and tolerability were compared by synthesizing direct and indirect evidence. The surface under the curve ranking area was utilized to rank multiple interventions.

RESULT

A total of 43 randomized clinical trials were included in our NMA. With regard to efficacy outcomes, including 6 min walking distance (6MWD), functional class amelioration (FCA), death, clinical worsening (CW), pulmonary vascular resistance (PVR), mean pulmonary artery pressure (mPAP), cardiac index (CI), and mean right atrial pressure (mRAP), endothelin receptor antagonists (ERA), phosphodiesterase 5 inhibitor (PDE-5Is), ERA combined with PDE-5Is (EAP), and prostacyclin analogs (PGI) combined with ERA (PAE) performed better than others. Meanwhile PAP and PGE demonstrated better than others in tolerability. Overall, EAP and PAE showed good efficacy and were well-tolerated among all therapies.

CONCLUSION

Overall, we recommend EAP as the optimal choice for patients with PAH in clinical practice and PAE as suboptimal in view of their desirable performance in efficacy. Most of the combination therapies performed better than monotherapies.