Early reperfusion by thrombolytic therapy or a primary percutaneous coronary intervention (PCI) is the most effective strategy to reduce the size of a myocardial infarct and improve the clinical outcome of acute myocardial infarction. However, the process of restoring blood flow to the ischemic myocardium through revascularization of large epicardial arteries can paradoxically induce significant microvascular injury, called reperfusion injury 1,2. This injury may reduce the beneficial effects of reperfusion. Therefore, various efforts collectively addressed as cardioprotection are made to prevent or ameliorate this detrimental effect 3.
The incretin hormone glucagon-like peptide 1 (GLP-1) secreted by enteroendocrine L-cells, located primarily in the distal ileum and colon 4, was first described as gut hormone, secreted in a nutrient-dependent manner, which reduces postprandial glycemia by stimulation of insulin secretion and glucagon secretion inhibition. Interestingly, GLP-1 levels detected in patient after total gut resection that is extra-gut secretion 5. In addition, GLP-1 receptors are widely dispersed in the human body and extraglycemic effects have been described in several tissues and organs; among these, the heart and vascular tissue confers a cardioprotective effect 6. A large body of evidence suggests that the incretin hormone GLP-1 has a cardioprotective effect 7,8. Epidemiologic data show that treatment with dipeptidyl peptidase-4 (DPP-4) inhibitors, which increases the levels of endogenous GLP-1, is associated with a modest amelioration of cardiovascular risk factors, namely, hyperlipidemia and hypertension 9. A recent retrospective meta-analysis showed that incretin therapy results in a significant reduction in cardiovascular morbidity in patients with type 2 diabetes 10.
GLP-1 receptors have been detected in animal and human myocardium 8 and administration of exogenous GLP-1 activates a variety of receptor-dependent and receptor-independent cardioprotective mechanisms including increased glucose uptake, cAMP and cGMP release, increased coronary flow and functional recovery, and cardiomyocyte viability 8. In addition, experimental data from porcine model of acute ischemia and reperfusion injury showed that the administration of GLP-1 analogue exenatide reduced infarction size significantly 11. More recently, it has been found in patients with acute myocardial infarction undergoing reperfusion treatment that administration of a GLP-1 analogue decreases the infarction size and improves their clinical outcome 12. Taken together, these data strongly suggest that GLP-1 is potentially effective in protecting injured myocardium from chronic and acute ischemia 13. They also raise the question of whether the physiologic reaction to acute cardiac ischemia also results in GLP-1 secretion representing an endogenous protective response. This paper examines for the first time the natural history dynamics of endogenous GLP-1 in association with acute cardiac ischemia.
Patients and methods
This is a prospective, observational, pilot, single-center study including patients with an acute ST-segment elevation myocardial infarction (STEMI) eligible for primary PCI. Patients between 18 and 75 years of age who presented with STEMI with ST-segment elevation of more than 0.2 mV in two or more in anatomically contiguous leads and duration of symptoms of less than 6 h were included. Patients with cardiac arrest, previous acute myocardial infarction, previous PCI or CABG, end-stage kidney disease or hepatic failure, recent stroke, known coagulopathy, pregnancy, inability to sign an informed consent, and receiving chronic treatment with oral DPP-4 inhibitors or parenteral GLP-1 analogues were excluded from the study. Fasting blood samples for GLP-1 determination were taken before the reperfusion procedure (baseline), 1, 3, and 90 days after admission.
This study protocol was approved by the local ethics committee in accordance with the Declaration of Helsinki. All patients provided written informed consent before inclusion in the study.
Blood samples were collected into K2-EDTA tubes (PL6 7BP; BD, Plymouth, UK) by venipuncture and immediately mixed with DPP-4 inhibitor (Millipore Corporation, Billerica, Massachusetts, USA) at a final dilution of 1 : 100 to inhibit the degradation of GLP-1. Samples were cooled and centrifuged as soon as possible and the plasma was separated immediately and frozen at −70°C for later testing.
Determination of total GLP-1 concentrations in plasma samples was performed using the GLP-1 Total ELISA 96-Well Plate Assay kit according to the manufacturer’s instructions (Cat.# EZGLP1T-36K; Millipore Corporation). Plasma sample volume 50 μl in duplicate was used in this spectrophotometric HRP-based sandwich ELISA assay. Absorbance of samples was read at 450 nm in a plate reader. Calculation of the GLP-1 concentrations was carried out from the standard curve generated with reference standards of known concentrations of GLP-1. The limit of sensitivity of this assay was 1.5 pmol/l total GLP-1. The average intra-assay coefficient of variation between duplicates was 6.5%.
The data were analyzed using BMDP (reference). Continuous variables were compared across groups using one-way analysis of variance after applying a log transformation to those values that did not have Gaussian distributions. We applied analysis of variance with repeated measures to determine a significant change between ‘baseline’ and ‘peak’ values. A P-value of 0.05 or less was considered significant.
Twelve male patients, average age 61.9±12 (range 44–90 years), were enrolled consecutively upon arrival to the ICCU (Table 1). All patients had at least one cardiovascular risk factor including hypertension, hyperlipidemia, past or active smoking, and type 2 diabetes mellitus. Myocardial infarction (MI) was the first evidence of atherosclerotic heart disease in all patients. The majority of patients (50%) presented with inferior wall MI and only 8% presented with lateral wall MI. Recovery following primary PCI was uneventful in all patients, except patient 3, who presented with cardiogenic shock and pulmonary edema and required mechanical ventilation along with intra-aortic balloon counter-pulsation support for 2 days.
GLP-1 serum levels sampled within 24 h after admission increased in eight patients (66%). In six of these patients, peak GLP-1 levels were observed after 24 h, whereas in two patients peak levels were observed 72 h after admission. The highest levels of GLP-1 were observed in patient 3, who presented with cardiogenic shock (Table 2 and Fig. 1b). Mean group GLP-1 levels 24 h after arrival as well as peak levels determined within 72 h were significantly increased from 27±7.1 to 39.5±11.4 and 43.4±11.1 pmol/l as compared with the mean levels before performance of PCI (P<0.04 and P<0.006, respectively). Mean group GLP-1 levels determined 3 months after discharge were comparable to levels at arrival (Fig. 1a). No correlation was found between GLP-1 levels and any of the clinical and laboratory parameters and indicators of MI severity, namely, TIMI score and ejection fraction. However, history (present and past) of both hypertension and smoking was correlated with significantly lower levels of GLP-1 as compared with normotensive (60.1±20.075 vs. 26.78±5.75) and nonmokers versus smokers (58.27±17.13 and 22.68±4.413 pmol/l, P<0.01 and P<0.04, respectively).
We report for the first time that the levels of GLP-1 increase significantly during the first 72 h following acute STEMI in patients treated with primary PCI. This novel preliminary report is particularly important as the physiologic GLP-1 response to acute MI was not examined by the numerous studies addressing the possible role of GLP-1 in cardioprotection. The significance of the increase in GLP-1 in association with acute MI is not entirely clear. It is possible that this increase represents a nonspecific acute-phase response to tissue injury rather than a specific cardiac repair mechanism. It is also possible that this increase is related to the reperfusion treatment itself. However, it is also likely that this incretin has an as yet undefined role in the maintenance of normal homeostasis of cardiac tissue under less stressful conditions and during acute MI the secretion of this hormone abruptly increases in order to prevent further ischemic damage. This concept is supported by studies in animal models in which the administration of GLP-1 analogue was shown to reduce the infarct size 12 when administered in adjunction to reperfusion therapy and by a more recent report showing the beneficial effect of GLP-1 analogue administration in patients with STEMI 13. Thus, our data provide a missing physiological link of GLP-1 response to acute MI within the general concept of GLP-1 and cardioprotection. The highest GLP-1 and creatine phosphokinase levels were found in patient 3, who developed cardiogenic shock and required mechanical ventilation along with intra-aortic balloon counterpulsation support. However, in contrast to this single patient, the increase in GLP-1 in all other patients did not correlate with any of the clinical and laboratory parameters or indicators of MI severity, namely, TIMI score and ejection fraction. This may indicate that unlike the increase in cardiac enzymes such as creatine phosphokinase, which indicates tissue destruction, increase in GLP-1 in this setting is independent of tissue damage and acute decrease in cardiac function. It is also noteworthy that patients 1, 7, and 8 did not show an increase in the GLP-1 levels at 24 h. This may be because of an individual patient-dependent GLP-1 secretory response. Alternatively, it may reflect relatively longer ischemic periods from the start of symptoms to arrival to reperfusion and decrease in GLP-1 levels. This latter variable was not established in our patients.
Taken together, these findings raise a question on the clinical value of GLP-1 determination in acute MI. Nevertheless, it is noteworthy that significantly lower GLP-1 levels before and after acute MI were found in the patients with a history of hypertension and active or former smoking, which are associated with increased incidence of ischemic heart disease. This association strengthens the clinical correlation between GLP-1 and ischemic heart disease and may indicate that determination of GLP-1 levels can be potentially exploited in the assessment of future cardiovascular risk.
The small number of patients and lack of longitudinal follow-up of our study leave several intriguing questions unanswered in terms of the endogenous GLP-1 response in patients with acute coronary syndrome. These include the effect of primary PCI as compared with fibrinolytic reperfusion or conservatively managed MI patients, the effect of urgent PCI versus elective PCI, the correlation between the duration of ischemic pain and GLP-1 levels as well as whether GLP-1 levels differ in patients with acute non-STEMI and STEMI. Most importantly, the prognostic value of GLP-1 levels in patients with acute MI remains unclear. Further studies enrolling larger groups of patients with different types of major coronary events and longer follow-up are needed to answer these questions.
This pilot study shows for the first time that GLP-1 levels increase in patients with acute STEMI. This novel finding supports a possible role for GLP-1 in the physiologic response to acute cardiac ischemia while its clinical and prognostic significance await further studies.
Conflicts of interest
There are no conflicts of interest.
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