Role of the sodium-hydrogen exchanger in ischemia-reperfusion injury in diabetes

Clinical impact of admission hyperglycemia on in-hospital mortality in acute myocardial infarction patients

BACKGROUND

Acute hyperglycemia on admission is common in acute myocardial infarction (AMI) patients regardless of diabetic status, and is known as one of prognostic factors. However, the effect of hyperglycemia on non-diabetic patients is still on debate.

METHODS

A total of 12,625 AMI patients (64.0±12.6years, 26.1% female) who were enrolled in Korea Acute Myocardial Infarction Registry-National Institute of Health between November 2011 and December 2015, were classified into 4367 diabetes (65.4±11.6years, 30.4% female) and 8228 non-diabetes (63.3±13years, 23.9% female). Patients were analyzed for in-hospital clinical outcome according to admission hyperglycemic status.

RESULTS

In diabetic patients, independent predictors of in-hospital mortality were old age, high HbA₁C, pre-Thrombolysis In Myocardial Infarction (TIMI) flow 0, left ventricle ejection fraction<40%, cardiogenic shock and ventricular tachycardia. In non-diabetic patients, independent predictors of in-hospital mortality were old age, high admission glucose (≥200mg/dL), pre TIMI flow 0, failed percutaneous coronary intervention, low left ventricle ejection fraction<40%, cardiogenic shock, stent thrombosis and decreased Hb≥5g/dL. In hospital mortality was significantly higher in diabetic patients compared to non-diabetic patients (5.0% vs. 3.4%, p<0.001). However, non-diabetic patients with hyperglycemia have significantly higher mortality compared to diabetic patients (17.4% vs. 7.2%, p<0.001). Comorbidity including cardiogenic shock (p<0.001), cerebral hemorrhage (p=0.012), decreased Hb≥5g/dL (p=0.013), atrioventricular block (p<0.001) and ventricular tachycardia (p=0.007) was higher in non-diabetic with hyperglycemia than in diabetic patients.

CONCLUSIONS

These findings underscore clinical significance of admission hyperglycemia on in-hospital mortality in non-diabetic AMI patients.

Acute hyperglycemia in patients with acute myocardial infarction

Acute hyperglycemia is a common feature during the early phase after acute myocardial infarction (AMI), regardless of diabetes status. Numerous studies have demonstrated that patients with AMI and hyperglycemia on admission have high rates of mortality. It has been reported that there is a linear positive relation between admission blood glucose levels and mortality after AMI. However, recent studies showed that the relationship is U-shaped in patients with a history of diabetes. Diabetic patients with moderate hyperglycemia (glucose 9-11 mmol/L) had the lowest mortality and not only severe hyperglycemia (glucose ≥ 11 mmol/L) but also euglycemia (glucose < 7 mmol/L) was associated with higher mortality. Although it has been debated whether acute hyperglycemia is causally related to adverse outcomes after AMI or is simply an epiphenomenon of severely damaged myocardium, multiple physiological studies have demonstrated that hyperglycemia has a direct detrimental effect on ischemic myocardium through several mechanisms, including oxidative stress, inflammation, apoptosis, endothelial dysfunction, hypercoagulation, platelet aggregation and impairment of ischemic preconditioning. Current guidelines recommend the use of an insulin-based regimen to achieve and maintain glucose levels < 10.0 mmol/dl, and emphasize the avoidance of hypoglycemia. However, the optimal management goal of glucose levels for patients with acute hyperglycemia remains uncertain. Further studies are warranted into the appropriate management in patients with AMI and acute hyperglycemia.

Effects of excess salt and fat intake on myocardial function and infarct size in rat

Important risk factors for cardiovascular disease include excess dietary intake of saturated fat and (or) salt. This study tested the hypothesis that excess intakes of saturated fat (e.g., beef tallow) and salt cause greater myocardial cell death following ischemia-reperfusion injury than each risk factor alone. Male rats were divided into four groups: basal fat diet (4.5% as calories; control), high fat diet (40% as calories; FAT), basal fat diet and high salt (1% NaCl solution; SALT) and high fat diet and high salt (FATSALT). The gain in body weight was significantly higher for FAT and FATSALT groups than those of either the control or the SALT group. Five weeks of exposure to the dietary regimens did not significantly affect the coronary flow rate and except for the salt-fed group, had no effect on the rate-pressure-product of the isolated heart perfused in Langendorff mode. Although infarct size was not affected by the high fat diet, it was reduced by the high salt regimen relative to the high fat diet or the control groups. When rats were fed the FAT and SALT combination, the effect of salt feeding on infarct size was not observed. In addition, the FATSALT group displayed a more marked deterioration in contractile function following ischemia-reperfusion injury than the other groups. In conclusion, short-term intake of a high fat diet, which significantly increases body weight, does not worsen ischemia-reperfusion injury although the treatment prevents the reduction of infarct size associated with high salt feeding.

Effect of hypertension and hypertension-glucose intolerance on myocardial ischemic injury

Systemic hypertension and type 2 diabetes mellitus are major risk factors for myocardial infarction. Yet, glucose intolerance, a prelude stage to type 2 diabetes, is associated with reduced infarct size. Since chronic hypertension adversely affects the myocardium, we tested the hypothesis that the coexistence of systemic hypertension and glucose intolerance reverses the cardioprotection associated with impaired glucose tolerance. Hearts from 9-month-old animals were subjected to a 40-minute occlusion of the left coronary artery followed by 2 hours of reperfusion. Before ischemia, similar values for the four experimental groups were observed for the coronary flow, heart rate, and maximum ventricular pressure. During the course of the ischemia-reperfusion insult, the two hypertensive groups displayed greater reductions in contractility than their normotensive counterparts. Infarct size was lower in the normotensive glucose-intolerant rat than in the normotensive control rat. Surprisingly, the hypertrophied hearts of the hypertensive and hypertensive glucose-intolerant rats displayed reduced infarct size (P<0.05). However, raising the afterload pressure from 100 to 160 cm H2O increased infarct size in the two hypertensive groups. This narrowed the differential between the hypertensive glucose-intolerant (160 cm H2O) and the normotensive control (100 cm H2O) rats. Nonetheless, at the higher afterload pressure, infarct size was less in the hypertensive glucose-intolerant rats than in their hypertensive counterparts. In conclusion, the impairment in contractile function despite the reduction in infarct size underscores the increased susceptibility of the hypertrophied, hypertensive heart to ischemic injury. Furthermore, exacerbation of cell death at elevated afterload pressure indicates the potential benefit of aggressive antihypertensive therapy.

Effects of omega-3 polyunsaturated fatty acids on cardiac sarcolemmal Na(+)/H(+) exchange

Myocardial ischemia-reperfusion activates the Na(+)/H(+) exchanger, which induces arrhythmias, cell damage, and eventually cell death. Inhibition of the exchanger reduces cell damage and lowers the incidence of arrhythmias after ischemia-reperfusion. The omega-3 polyunsaturated fatty acids (PUFAs) are also known to be cardioprotective and antiarrhythmic during ischemia-reperfusion challenge. Some of the action of PUFAs may occur via inhibition of the Na(+)/H(+) exchanger. The purpose of our study was to determine the capacity for selected PUFAs to alter cardiac sarcolemmal (SL) Na(+)/H(+) exchange. Cardiac membranes highly enriched in SL vesicles were exposed to 10-100 microM eicosapentanoic acid (EPA) or docosahexanoic acid (DHA). H(+)-dependent (22)Na(+) uptake was inhibited by 30-50% after treatment with > or =50 microM EPA or > or =25 microM DHA. This was a specific effect of these PUFAs, because 50 microM linoleic acid or linolenic acid had no significant effect on Na(+)/H(+) exchange. The SL vesicles did not exhibit an increase in passive Na(+) efflux after PUFA treatment. In conclusion, EPA and DHA can potently inhibit cardiac SL Na(+)/H(+) exchange at physiologically relevant concentrations. This may explain, in part, their known cardioprotective effects and antiarrhythmic actions during ischemia-reperfusion.

Cardiovascular complications of non-insulin-dependent diabetes: the JCR:LA-cp rat

Diabetes is a serious medical and financial burden on western societies. It is the seventh leading cause of death in the United States and Canada. The disease is due to a primary defect in glucose tolerance and carbohydrate metabolism resulting from either a deficiency of insulin (Insulin-dependent (type I) diabetes mellitus - IDDM) or a state of insulin resistance (Non-insulin-dependent (type II) diabetes mellitus - NIDDM). NIDDM comprises greater than 80% of total diabetic cases. Associated with the primary metabolic defects are equally deleterious secondary complications affecting the renal, ocular, nervous and cardiovascular systems. The cardiovascular complications account for a major proportion of diabetic mortality. As such, it is of paramount importance to develop or find an animal model expressing complications homologous to the human condition. Many models of NIDDM are available to the diabetic researcher but choosing an accurate one can be difficult. The following compares the advantages and limitations of one such model, the JCR:LA-cp rat to other NIDDM models commonly used today.