Cardiovascular disease (CVD) is the main cause of disease burden worldwide, with high morbidity and mortality, and is continuously increasing in most countries [1]. Since Gruentzig successfully performed the first percutaneous transluminal coronary angioplasty (PTCA) in 1977, percutaneous coronary intervention (PCI) has rapidly evolved into the first-line therapy in most patients with acute coronary syndrome [2]. However, PCI causes myocardial injury, inflammatory reaction, and ischemia-reperfusion injury in ischemic and necrotic areas, making some patients suffer from major adverse cardiovascular events (MACEs) after the operation and affecting the therapeutic effect [3]. Therefore, identifying the mechanism and intervention measures of myocardial injury after PCI is essential. Reportedly, stent implantation triggered the acute phase response and systemic inflammation, which was associated with increased plaque burden and pathological features [4].

Phosphocreatine (PCr) is the main form of chemical energy reserve of myocardial cells with a critical role in energy metabolism during myocardial contraction. A declined level of PCr may have adverse effects on myocardial contraction and recovery of cardiac function [5]. Sodium PCr is an efficient energy supplement, which can effectively supply exogenous myocardial energy and play a critical role in reducing myocardial injury and maintaining normal cardiac function [6]. A recent study found that additional sodium PCr treatment improves early cardiac dysfunction and 28-day survival outcomes in patients with septic shock [7]. Thus, PCr is recommended to be involved in inflammation. However, the mechanism and effect of exogenous PCr on myocardial injury after PCI lacks clinical evidence.

In the present study, we hypothesized that exogenous PCr supplementation before operation alleviates systemic inflammation triggered by PCI to protect the myocardium. A series of experiments were designed and performed to test this hypothesis. Interleukin-6 (IL-6) and blood cell count were used to reflect the inflammation. Cardiac troponin I (cTnI) was adopted as a sensitive marker of myocardial injury [8].

CVD is mainly caused by stenosis or occlusion of the lumen due to atherosclerosis, which leads to myocardial ischemia, hypoxia, or infarction. PCI is one of the major methods to treat CVD by restoring coronary circulation and myocardial reperfusion. Some studies have shown that the myocardium is damaged during PCI, which is one of the main factors of cardiovascular adverse events in postoperative patients [9]. The myocardium is a tissue with high oxygen demand and consumption. When coronary artery stenosis occurs, the blood flow decreases, which destroys the balance of oxygen supply and demand. The primary changes caused by ischemia are the destruction of myocardial cells and vascular endothelial cells [10].

The destruction of intima also decreases nitric oxide production, which disrupts the endothelium-dependent vasodilation function. The myocardial cells also press the microvessels due to cell swelling and ischemia-induced injury, which is due to the decrease in adenosine triphosphate (ATP) caused by long-term ischemia and hypoxia, the inactivation of sodium-potassium-exchanging ATPase on the cell membrane, the decrease in calcium ion outflow, and the limitation of endoplasmic reticulum’s absorption of calcium ions, resulting in the overload of calcium ions in myocardial cells. These changes in myocardial cells are accompanied by the activation of intracellular protease, damaging the myofibrils. The reperfusion injury potentiates the ischemic injury. Previous studies have shown that the increase in serum cTnI level is closely related to the occurrence of MACEs after PCI, which guides the therapeutic strategy [11]. The current results showed that the log${}_e$-transformed cTnI increased significantly after PCI and peaked at 24 h. The level of log${}_e$-transformed cTnI in the PCr group was lower than that of the control group at 4 h, 12 h, 24 h, and 48 h post-PCI, indicating that PCr intervention significantly alleviates the PCI-induced myocardial injury.

The highly sensitive cardiac troponin I (cTnI) describes the damage of tiny myocardium, and its molecular weight is smaller than that of myocardial enzymology index, with high sensitivity and specificity. When the myocardium is damaged, the intracellular adenosine triphosphate (ATP) level decreases, leading to acidosis and increased permeability of the cell membrane; also, cTnI is easily released into the blood [12]. The supply of energy is vital to relieve myocardial ischemia and PCI-induced injury. PCr is a high-energy phosphate compound found in the brain cells, myocardium and skeletal muscle. It participates in ATP synthesis, forms actin through the hydrolysis effect, and supplies energy for the contraction of the myocardium. The decline in the PCr level in the myocardium affects the oxidative metabolism of myocytes, causes myocardial injury, and slows down the repair process [13]. In addition, as an endogenous substance, PCr can directly enter cardiac myocytes, providing energy for calcium-transporting ATPases and sodium-potassium-exchanging ATPase, facilitating the transport of intracellular Ca${}^{2+}$ into sarcoplasmic reticulum that promotes the recovery of myocardial contractility and attenuating the injury [14]. Exogenous PCr is also beneficial in maintaining the ATP level, reducing reactive oxygen species, enhancing membrane stability, preventing damage to the myocardium, and diminishing the cTnI content transferred into the blood [15]. Sodium PCr also reduces the degradation of 5’-mono-, 5’-di-, and 5’-triphosphorylated nucleotides and adenine nucleotides that maintain the state of high energy in cardiac myocytes, prevent PLT aggregation, improve microcirculation, and preserve myocardial function [16].

Recent studies have reported that inflammation is involved in the development of atherosclerotic plaque, and the release of inflammatory mediators activates the complements, promotes leukocyte aggregation, damages the intima of blood vessels, and induces the formation of plaque and the progression of nontarget lesions [4]. IL-6 is an acute-phase protein of inflammation. Typically, the level of IL-6 is low in the body. Damaged or infected tissue results in a sharp increase in the synthesis of IL-6 and its release into the blood, which in turn promotes the monocytes to release tissue factors, thus increasing the risk of local thrombosis [17]. IL-6 is also involved in the formation of atherosclerosis, causing an intimal reaction in arteries, aggravating inflammation, and possibly causing vasospasm [18]. The higher the level of IL-6, the more serious the inflammation in patients with CVD, which further increases the risk of cardiovascular adverse events that are detrimental to the prognosis of patients [19]. The current results showed that log${}_e$-transformed IL-6 increased after PCI, especially at 12 h and 24 h. However, log${}_e$-transformed IL-6 of the PCr group was lower than that of the control group, and the difference was significant at 48 h after the operation. This phenomenon suggested that the inflammation of the patients increased after PCI. Furthermore, sodium PCr regulates the inflammatory factors stimulated by PCI, restores the serum IL-6 level to normal, and improves the prognosis of patients.

Some debris (foam-like macrophages, aggregated platelets, cholesterol crystals, intimal debris, and thrombus fragments) fell to the distal end of blood vessels due to the rupture of unstable plaques and physical injuries, such as the entry of guidewires, balloon dilatation, and stent implantation during PCI [20]. Although a small amount of debris does not affect the forward blood flow, it induces the aggregation of inflammatory mediators and releases vasoconstrictors, such as thromboxane A2, serotonin, and tumor necrosis factor. Some studies have shown that shed debris can induce NEU aggregation, activate PLTs, and promote thrombosis. The primary mechanism of myocardial damage lies in the aggregation of NEUs and PLTs and the release of reactive oxygen species (ROS). Activated NEUs and PLTs release a large number of vasoconstrictors, inflammatory factors, ROS, and proteases, which further aggregate the NEUs and PLTs, aggravate the inflammatory reaction, form a vicious circle, cause the destruction and edema of the myocardium, and eventually lead to reperfusion injury [21].

NEUs are the main source of ROS during reperfusion and the key inflammatory cells of myocardial injury. In the event of severe inflammation, apoptosis decreases the LYM count while increasing the NEU count [22]. This variation trend was also observed in the present study. The current results showed that after PCI, the counts of WBC, NEU, and NEUR are higher than those before PCI, while those of LYM, EOS, BAS, and MON and the ratios are lower than the baseline. Moreover, the NEUR and LYMR altered at 12 h after PCI. This phenomenon was consistent with the mechanism of reperfusion injury. Therefore, preventing NEU-triggered inflammation during PCI perioperative period may be a key to treating myocardial injury. In this study, NEUR and LYMR in the PCr group were significantly improved compared to those in the control group at 48 h after the operation. This indicated that the intervention of exogenous PCr alleviates the inflammation related to NEUs after PCI, thus protecting the myocardium.

PDW and MPV are crucial features of PLT activation [23]. The current results described that PDW is increased after PCI, while MPV, PLT count, and PCT are decreased. PCr intervention had no effect on the PLT features. These phenomena may be essential to detect sensitive indicators of PLT reactivity [24, 25].

HGB is a specific protein that transports oxygen in RBCs. Anemia is related to serious cardiovascular diseases (such as thromboembolism and hemorrhage), which in turn increases the mortality of anemia patients during hospitalization [26]. The disease is also related to inflammation, which increases the release of erythropoietin and cytokines, leading to vascular endothelial damage, accelerating atherosclerosis, activating platelets, putting the body in a hypercoagulation state, and increasing the risk of thrombosis [27]. The current results showed that RBC, HGB, HCT, MCH, and MCHC were lower after PCI than those before PCI, which was similar to the previous studies. Compared to the control group, these indicators in the PCr group improved slightly, albeit not significantly. Thus, additional studies with larger sample sizes may be needed. Interestingly, the plasma concentration of free hemoglobin (fHGB) was significantly higher in patients with acute myocardial infarction (AMI) between 12 h and 24–48 h time points after stent implantation [28], suggesting that the decrease in HGB may be related to perioperative sterile inflammation and hemolysis.

Nevertheless, the present study has the following limitations. Firstly, this is a single-center, unblinded study, and no confounding factors have been identified to influence the results. Secondly, measuring troponin post-intervention is not clear, and indirect variables for myocardial injury and inflammation are used. Finally, no long-term follow-up was carried out in this study and can be continued in future studies to further evaluate the prediction of the risk of adverse cardiovascular events.

In conclusion, a series of operations during PCI in coronary arteries leads to vascular endothelial damage and vasospasm, increases the risk of thrombosis, affects the blood perfusion of distal vessels, aggravates myocardial ischemia and inflammation, and cause myocardial injury and adverse cardiovascular events. Exogenous PCr regulates the levels of serum cTnI, IL-6, NEUR, and LYMR in patients undergoing PCI, which is crucial for improving myocardial perfusion, alleviating inflammation, and regulating the functions of granulocytes and mononuclear cells. Additional studies are warranted to clarify the detailed mechanism and the long-term prognosis.

MYL performed the analysis, processed the experimental data, and wrote the article. YQX designed and directed the project, performed the statistical analysis, and edited the manuscript. YPS and CL were involved in planning and supervising of the experiments. YPS, ZHW and YW aided in interpreting the results and edited the manuscript. XHL, ZZ and RXZ performed the measurements. LYQ and ML were involved in planning the project and contributed to sample preparation. YLX and HQC aided in interpreting the results. All authors approved the final version to be submitted.

This study complied with the Declaration of Helsinki, was approved by the Medical Ethics Committee of Qilu Hospital of Shandong University and registered at the Chinese Clinical Trial Registry Center (registration number: ChiCTR-IOQ-15007475). All participants provided written informed consents.

We thank all study coordinators and patients for their invaluable contribution.

This study was supported by the National Natural Science of China (grant numbers 81570356, 81700725, 82100472), the Key Research and Development Program of Shandong Province (grant number 2017GSF218101), the Natural Science Foundation of Shandong Province (grant number ZR2017BH003), the Science and Technology Foundation of Jinan City (grant number 201805057) and the Medical and Health Science and Technology Development Foundation of Shandong Province (grant number 2016WS0366).

The authors declare no conflict of interest.