The mechanism of L-arginine HCl on vascular endothelial function

Upon entering the body, L-Arginine HCl rapidly dissociates into L-arginine and chloride ions. Among these, L-arginine serves as the sole substrate for nitric oxide (NO) synthesis in vascular endothelial cells. As a core signaling molecule regulating vascular endothelial function, NO mediates multiple downstream physiological pathways activated by L-Arginine HCl via the arginine-NO axis. L-Arginine HCl improves vascular endothelial function through four core dimensions: substrate supply, enzyme activity regulation, oxidative stress resistance, and anti-inflammatory & antithrombotic effects. The specific mechanisms are elaborated as follows:

I. Direct Substrate Supplementation to Drive Endothelium-Derived NO Synthesis

Endothelial nitric oxide synthase (eNOS) in vascular endothelial cells is the key enzyme catalyzing NO production. With L-arginine as the substrate, and in the presence of cofactors including tetrahydrobiopterin (BH₄), calcium ions (Ca²⁺), and flavin mononucleotide (FMN), eNOS catalyzes the decomposition of L-arginine into NO and L-citrulline.

Theoretically, the basal intracellular concentration of L-arginine in endothelial cells (0.1–0.5 mmol/L) is much higher than the Michaelis constant (Kₘ=1–5 μmol/L) of eNOS. However, the arginine paradox exists: intracellular L-arginine is compartmentalized into two pools—the "metabolic pool" and the "eNOS-accessible pool". Most L-arginine is allocated to other metabolic pathways such as protein synthesis and the urea cycle, with only a small fraction available for eNOS-catalyzed reactions. Exogenous supplementation of L-Arginine HCl disrupts this compartmentalization, significantly increasing the concentration of eNOS-accessible L-arginine and directly driving the synthesis and release of NO.

Meanwhile, the L-citrulline generated during the reaction can be recycled back to L-arginine via the arginine-citrulline cycle: in the cytoplasm, citrulline undergoes catalysis by argininosuccinate synthetase (ASS) and argininosuccinate lyase (ASL), consuming aspartic acid and ATP to produce L-arginine. This cyclic substrate utilization sustains the continuous and stable release of NO.

II. Regulating eNOS Activity and Conformation to Enhance NO Synthesis Efficiency

L-Arginine HCl acts not only as a substrate for eNOS but also enhances the catalytic activity and stability of eNOS through allosteric modulation and signaling pathway regulation, via the following specific mechanisms:

1. Allosteric activation of eNOS

L-arginine binds to the substrate-binding domain of eNOS, stabilizing the enzyme’s active homodimeric conformation and preventing its dissociation into inactive monomers, thereby maintaining the catalytic function of eNOS.

2. Promoting eNOS phosphorylation modification

L-Arginine HCl activates the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) signaling pathway. Phosphorylated Akt induces phosphorylation of eNOS at serine residue 1177—a key marker of eNOS activation—which can increase the catalytic efficiency of the enzyme several-fold.

3. Ameliorating eNOS "uncoupling" state

In the absence of the cofactor BH₄, eNOS switches from a "coupled state" to an "uncoupled state". In this state, the enzyme no longer produces NO but generates reactive oxygen species (ROS) such as superoxide anions (O₂⁻), exacerbating endothelial damage. L-Arginine HCl promotes the binding of BH₄ to eNOS, restoring the enzyme’s coupled state, reducing ROS production, and simultaneously increasing NO synthesis.

III. Mediating Downstream NO Effects to Maintain Endothelial Homeostasis

As a core signaling molecule, NO diffuses into vascular smooth muscle cells, platelets, and endothelial cells themselves, mediating a series of physiological effects that improve endothelial function:

1. Regulating vascular tone and improving endothelium-dependent vasodilation

Upon entering vascular smooth muscle cells, NO activates soluble guanylate cyclase (sGC), leading to increased intracellular cyclic guanosine monophosphate (cGMP) levels. cGMP activates protein kinase G (PKG), which promotes calcium pumps in smooth muscle cells to expel Ca²⁺ out of the cells. This reduction in intracellular Ca²⁺ concentration causes smooth muscle relaxation and vasodilation. This process is the core mechanism underlying flow-mediated dilation (FMD)—the gold standard for clinical evaluation of endothelial function. By increasing NO levels, L-Arginine HCl supplementation can significantly improve FMD in patients with hypertension, coronary heart disease, and other conditions, reducing peripheral vascular resistance.

2. Stabilizing the endothelial barrier and inhibiting vascular leakage

Tight junction proteins (e.g., occludin, claudin) between vascular endothelial cells are critical structures for maintaining endothelial barrier integrity. NO stabilizes the distribution and expression of tight junction proteins by inhibiting actin rearrangement in endothelial cells, reducing the leakage of intravascular fluids and macromolecules into the interstitial space. Additionally, NO promotes the synthesis of vascular endothelial growth factor (VEGF) in endothelial cells, accelerating the repair of damaged endothelium and exerting a protective effect against endothelial barrier disruption induced by ischemia-reperfusion and inflammation.

3. Anti-inflammatory and antithrombotic effects to inhibit endothelial activation

An important hallmark of endothelial dysfunction is endothelial activation, characterized by upregulated expression of adhesion molecules (e.g., ICAM-1, VCAM-1), which in turn promotes leukocyte adhesion and platelet aggregation, triggering inflammation and thrombosis. NO inhibits this process through two pathways: first, it increases intracellular cGMP levels in platelets, suppressing platelet adhesion, aggregation, and activation; second, it inhibits the activation of the nuclear factor κB (NF-κB) signaling pathway, reducing the release of adhesion molecules and proinflammatory cytokines (e.g., TNF-α, IL-6), and alleviating endothelial inflammatory responses.

IV. Antioxidative Stress to Attenuate Endothelial Oxidative Damage

Oxidative stress is a core contributor to endothelial dysfunction. ROS can directly inactivate NO (generating inactive peroxynitrite anions, ONOO⁻) while damaging endothelial DNA, proteins, and lipids. L-Arginine HCl exerts antioxidant effects through a dual mechanism:

1. Direct ROS scavenging

NO acts as a free radical scavenger, rapidly reacting with ROS such as superoxide anions, thereby reducing ROS-induced endothelial damage.

2. Enhancing the antioxidant defense system

L-Arginine HCl increases the synthesis of glutathione (GSH) in endothelial cells. GSH is a key intracellular antioxidant that scavenges ROS such as hydrogen peroxide (H₂O₂), mitigating oxidative stress-induced damage to eNOS and maintaining the stability of the NO synthesis pathway.

The core mechanism by which L-Arginine HCl improves vascular endothelial function is to supplement substrates, activate eNOS, and promote NO production, thereby mediating NO downstream effects including vasodilation, barrier stabilization, anti-inflammation, antithrombosis, and antioxidation, ultimately achieving the homeostatic regulation of endothelial function.