Myocardial loss of glutamate after cold chemical cardioplegia and storage in isolated blood-perfused pig hearts

Effect of glutamate infusion on NT-proBNP after coronary artery bypass grafting in high-risk patients (GLUTAMICS II): A randomized controlled trial

BACKGROUND

Animal and human data suggest that glutamate can enhance recovery of myocardial metabolism and function after ischemia. N-terminal pro-brain natriuretic peptide (NT-proBNP) reflects myocardial dysfunction after coronary artery bypass surgery (CABG). We investigated whether glutamate infusion can reduce rises of NT-proBNP in moderate- to high-risk patients after CABG.

METHODS AND FINDINGS

A prospective, randomized, double-blind study enrolled patients from November 15, 2015 to September 30, 2020, with a 30-day follow-up at 4 academic cardiac surgery centers in Sweden. Patients underwent CABG ± valve procedure and had left ventricular ejection fraction ≤0.30 or EuroSCORE II ≥3.0. Intravenous infusion of 0.125 M L-glutamic acid or saline at 1.65 mL/kg/h started 10 to 20 minutes before releasing the aortic cross-clamp, then continued for another 150 minutes. Patients, staff, and investigators were blinded to the treatment. The primary endpoint was the difference between preoperative and day-3 postoperative NT-proBNP levels. Analysis was intention to treat. We studied 303 patients (age 74 ± 7 years; females 26%, diabetes 47%), 148 receiving glutamate group and 155 controls. There was no significant difference in the primary endpoint associated with glutamate administration (5,390 ± 5,396 ng/L versus 6,452 ± 5,215 ng/L; p = 0.086). One patient died ≤30 days in the glutamate group compared to 6 controls (0.7% versus 3.9%; p = 0.12). No adverse events linked to glutamate were observed. A significant interaction between glutamate and diabetes was found (p = 0.03). Among patients without diabetes the primary endpoint (mean 4,503 ± 4,846 ng/L versus 6,824 ± 5,671 ng/L; p = 0.007), and the incidence of acute kidney injury (11% versus 29%; p = 0.005) was reduced in the glutamate group. These associations remained significant after adjusting for differences in baseline data. The main limitations of the study are: (i) it relies on a surrogate marker for heart failure; and (ii) the proportion of patients with diabetes had almost doubled compared to the cohort used for the sample size estimation.

CONCLUSIONS

Infusion of glutamate did not significantly reduce postoperative rises of NT-proBNP. Diverging results in patients with and without diabetes agree with previous observations and suggest that the concept of enhancing postischemic myocardial recovery with glutamate merits further evaluation.

TRIAL REGISTRATION

ClinicalTrials.gov https://clinicaltrials.gov/ct2/show/NCT02592824. European Union Drug Regulating Authorities Clinical Trials Database (Eudra CT number 2011-006241-15).

Isolated hemoperfused heart model of slaughterhouse pigs

A model of hemoperfused slaughterhouse pighearts is described providing a wide range of applications which leads to a reduction in animal experiments.

The size of a pigheart, heart rate, coronary perfusion, metabolism, etc. are more comparable to conditions in patients than those in hearts of small laboratory animals.

Global heart function can be assessed either by measuring stroke volume, ejection fraction, Emaxetc. in the working model or by measuring intraventricular pressure with balloon catheters in the isovolumetric model. Regional cardiac function can be measured by sonomicrometry and ischemic and non-ischemic areas can be compared. Local metabolic changes are measurable as well with microdialysis.

Cardiac function can be kept on any given functional level by infusion of norepinephrine in spite of the fact that functional parameters are lower without adrenergic drive in vitro than in vivo.

Stable heart function can be maintained for several hours with only 500 to 1000 ml of blood because the blood is permanently regenerated by a special dialysis system. This model can be applied in many research projects dealing with reperfusion injuries, inotropic, antiarrhythmic or arrhythmogenic effects of certain drugs, immunological rejection, evaluation of imaging systems (NMR, echocardiography etc.) or cardiac assist devices.

The S-100B substudy of the GLUTAMICS trial: glutamate infusion not associated with sustained elevation of plasma S-100B after coronary surgery

BACKGROUND & AIMS

Concerns have been raised about potential neurological injury related to exogenous glutamate. In cardiac surgery glutamate has been administered as a putative cardioprotective agent by cardioplegia or intravenous infusion. In the GLUTAMICS trial, in addition to surveillance of clinical neurological injuries, a prespecified subgroup was analyzed with regard to postoperative S-100B levels to detect potential subclinical neurological injury related to glutamate infusion.

METHODS

Sixty-nine patients operated on for unstable coronary syndrome were randomized to intravenous infusion of glutamate (n=35) or saline (n=34) perioperatively. Plasma levels of S-100B were obtained on the third postoperative day.

RESULTS

S-100B in the glutamate group and the control group were 0.079+/-0.034microg/L and 0.090+/-0.042microg/L respectively (p=0.245). There were no patients with stroke or mortality. Three patients in the control group and two in the glutamate group had postoperative confusion. These patients had significantly elevated S-100B compared with those without confusion (0.132+/-0.047vs 0.081+/-0.036microg/L; p=0.003). Overall, 21 patients had S-100B above reference level (> or =0.10microg/L) and these patients had significantly more calcifications in the ascending aorta on epiaortic scanning.

CONCLUSIONS

Intravenous glutamate infusion during surgery for unstable coronary artery disease did not initiate a sustained elevation of plasma S-100B. Thus, no evidence for subclinical neurological injury related to glutamate infusion was found. In contrast, postoperative elevation of plasma S-100B was linked to calcification of the ascending aorta and postoperative confusion.

L-glutamate and glutamine improve haemodynamic function and restore myocardial glycogen content during postischaemic reperfusion: A radioactive tracer study in the rat isolated heart

1. L-Glutamate and glutamine have been suggested to have cardioprotective effects. However, the issue is controversial and the metabolic mechanisms underlying a beneficial effect are not well understood. 2. In the present study we investigated the effects of L-glutamate and glutamine on haemodynamic recovery, the rate of de novo glycogen synthesis and myocardial glucose uptake during postischaemic reperfusion. 3. Hearts from male Wistar rats (250-300 g) were divided into three groups as follows: (i) control (n = 12); (ii) L-glutamate (n = 12); and (iii) glutamine (n = 12). Hearts were mounted in a Langendorff preparation and perfused with oxygenated Krebs'-Henseleit solution at 80 mmHg and 37C. Global ischaemia for 20 min was followed by 15 min reperfusion, during which L-glutamate (50 mmol/L) or glutamine (20 mmol/L) were administered. Left ventricular developed pressure (LVDP), de novo synthesis of glycogen using [14C]-glucose and myocardial glucose uptake using D-[2-3H]-glucose were measured. 4. L-Glutamate and glutamine increased postischaemic LVDP (P < 0.01 vs control hearts for both). L-Glutamate and glutamine increased de novo glycogen synthesis by 78% (P < 0.001) and 55% (P < 0.01), respectively. At the end of reperfusion, total myocardial glycogen content was increased by both L-glutamate and glutamine (5.7 +/- 0.3 and 6.2 +/- 0.7 micromol/g wet weight, respectively; P < 0.05 and 0.01, respectively) compared with that in control hearts (3.6 +/- 0.4 micromol/g wet weight). Neither L-glutamate nor glutamine affected myocardial glucose uptake during reperfusion. 5. Improved postischaemic haemodynamic recovery after L-glutamate and glutamine supplementation during reperfusion is associated with increased de novo glycogen synthesis, suggesting a favourable modulation of intracellular myocardial carbohydrate metabolism.

Captopril-induced glutamate release at the start of reperfusion after cold cardioplegic storage of pig hearts

OBJECTIVE

We sought to evaluate the effects of captopril on glucose-related metabolism during hypothermic cardioplegic storage and subsequent reperfusion.

METHODS

We compared hearts from control pigs with hearts from pigs treated with increasing oral doses of captopril for 3 weeks (12.5-150 mg daily), an intravenous bolus (25 mg) before operation, and captopril-containing cardioplegic solution (1 mg/L). The hearts were excised after infusion of cold crystalloid cardioplegic solution and stored in saline solution (4 degrees C-6 degrees C). In one series we studied myocardial blood flow and arteriovenous differences in oxygen, glucose, lactate, glutamate, and alanine during 60 minutes of postcardioplegic blood reperfusion. In this series captopril-treated hearts were reperfused with captopril-containing blood (1 mg/L). In another series we obtained biopsy specimens from the left ventricle throughout 30 hours of hypothermic cardioplegic storage and monitored tissue content of energy-rich phosphates, glycogen, glutamate, and alanine.

RESULTS

Captopril increased glutamate and alanine release 11- to 17-fold at the start of reperfusion (P <.001). Furthermore, captopril increased myocardial oxygen and glucose uptake during reperfusion (P <.001 for both), whereas lactate release and myocardial blood flow were unaffected by captopril. At the start of reperfusion, there was a positive correlation between glutamate release and glucose uptake in captopril-treated hearts (r = 0.66, P =.05). We found no statistically significant differences between captopril and control hearts in tissue content of adenosine triphosphate, glycogen, glutamate, alanine, or lactate during 30 hours of cardioplegic storage.

CONCLUSIONS

The metabolic effects of captopril are strictly related to reperfusion, during which oxidative metabolism of glucose is improved. The captopril-induced increase in glutamate and alanine release at the start of reperfusion after cardioplegic storage may reflect a switch in metabolism of glucose-related amino acids.

In situ detection of myocardial infarction in pig by measurements of aspartate aminotransferase (ASAT) activity in the interstitial fluid

Microdialysis probes permeable to large molecules (m.w. cut-off > 200 kD) were introduced into the myocardium of anaesthetized pigs in order to evaluate their potential for early detection of myocardial ischaemia and enzyme markers for infarction. The left anterior descending coronary artery was occluded for 30 min and the myocardium was reperfused for 3 h. The concentrations of aspartate aminotransferase (ASAT), lactate, glucose and selected free amino acids were measured. The levels in the interstitium of ischaemic and non-ischaemic myocardium were compared with those in plasma from the coronary sinus as well as from a peripheral vein. Twelve probes were inserted in six pigs and withdrawn after 8-72 hours of sampling. No complications occurred. Simultaneous 100% increase of ASAT and lactate was found in myocardial dialysates after 30 min of ischaemia. ASAT activity remained at that level until the end of reperfusion. The plasma peak ASAT level was not attained until after 3 h. Glutamate was the only amino acid which increased significantly in the myocardial interstitium during ischaemia, peaking after 30 min of reperfusion. Dialysates from the unaffected myocardium showed no effects on lactate, ASAT or glutamate. The use of myocardial microdialysis for pre- and postoperative recordings in man is discussed.

Haemodynamic and metabolic effects of gallopamil as additive to calcium-containing and calcium-free cardioplegic solutions in mature pig hearts

Myocardial haemodynamic and metabolic effects of the calcium-channel blocker gallopamil as additive to calcium-containing (St Thomas Hospital, STH) and calcium-free (Bretschneider procaine-containing, BRT) crystalloid cardioplegic solutions were evaluated. Adult pig hearts (weight 0.033 kg) were randomized to four groups and perfused with 1 litre of cold (4 degrees C) cardioplegic solution; group A: BRT without gallopamil, n = 9, group B: BRT with gallopamil (0.4 microM), n = 8, group C: gallopamil-free STH, n = 8, and group D: STH with gallopamil (0.4 microM), n = 8. After storage at 4 degrees C for 6 hours the hearts were reperfused with blood/Ringer solution in a modified Langendorff model for 60 min. Developed left ventricular pressure, rate-pressure product and +dP/dt were lower in gallopamil-treated hearts during reperfusion (p < 0.05), as were oxygen extraction and oxygen uptake (p < 0.05) and lactate release (p < 0.05). Myocardial blood flow was greater in gallopamil-treated hearts (p < 0.05). In hearts comparable in size and anatomy to the human heart, gallopamil added to both cardioplegic solutions reduced cardiac function and oxygen uptake despite increased myocardial blood flow. The findings suggest reduced myocardial protection after addition of gallopamil to cardioplegic solutions.