Protocols were approved by the Animal Care Committee of the Toronto General Hospital in accordance with guidelines of the Canadian Council for Animal Care. Male 10- to 12-week-old C57BL/6 mice were obtained from Charles River (Montreal, PQ, Canada) and housed for at least 2 weeks before experimentation.

The volume of individual i.p. injections was 100 ?l. The experimental protocols are summarized in supplemental Fig. 1 (available in an online appendix at http://diabetes.diabetesjournals.org/cgi/content/full/db08-1193). One group of animals (n = 75) was injected with the GLP-1R agonist liraglutide (Novo Nordisk, Novo Alle, Bagsvaerd, Denmark) at a previously used dose of 200 ?g/kg i.p. twice daily (¹⁸) for 7 days before permanent surgical ligation of the left anterior descending (LAD) artery as previously detailed (²⁰). A parallel group of control animals (n = 75) was injected with an equivalent volume of the vehicle, PBS. After this treatment period, some animals (n = 10 per group) were killed just before planned LAD ligation with hearts dissected immediately, weighed, and frozen. A second group of mice received PBS or liraglutide (n = 10 each) for 7 days and were subjected to sham surgery without LAD occlusion. Because a regimen of liraglutide 200 ?g/kg i.p twice daily (LIR 200) induced weight loss in mice, we also studied the effects of LAD ligation in separate groups of mice after administration of a lower dose of liraglutide (75 ?g/kg i.p. twice daily: LIR 75) that did not produce significant weight loss or pair-feeding to induce weight loss in control animals comparable to that seen in mice treated with LIR 200. Separate groups of PBS- and liraglutide-treated mice (n = 13 per group) were killed on day 4 post?myocardial infarction (MI) for biochemical and histological analyses. LV tissues from infarct and peri-infarct zones and remote (noninfarcted) area were separated, snap-frozen, and stored at ?80?C. The remaining mice were monitored for 28 days post-MI for survival analysis and histomorphometry for infarct size. In separate studies, mice were maintained on a high-fat diet (45% Kcal from fat, D-12451; Research Diets) for 1 month, following which diabetes was induced by treatment with streptozotocin (90 mg/kg i.p.). After stratification by degree of hyperglycemia at 3 weeks after streptozotocin injection, mice were randomized to receive treatment for 7 days with placebo (PBS, 100 ?l i.p. twice daily), metformin in chow (6.76 g/kg of mouse diet), or liraglutide 75 ?g twice daily (LIR 75) before sham or LAD-ligation surgery (n = 23/group: 5 sham, 18 LAD-ligation). Diabetic mice were maintained on the high-fat diet until euthanasia on day 28 postsurgery. To investigate the role of the known GLP-1R in the cardiac actions of liraglutide, male Glp1r-/- mice, 10 to 12 weeks of age, in the C57BL/6 background and their littermate (Glp1r+/+) controls (n = 6/group), were injected with either PBS or 75 ?g liraglutide twice daily for 1 week, then killed and their hearts used for Western blot assessment of prosurvival kinases.

Cardiac examinations were performed on deceased mice post-MI. The presence of a large amount of blood or clot around the heart and in the thoracic cavity as well as a perforation of the infarct or peri-infarct area was taken to indicate cardiac rupture.

Before anesthesia, nonfasting blood glucose measurements were obtained through a tail nick using a handheld glucometer and One-Touch glucometer strips (LifeScan Canada, Burnaby, BC, Canada).

Heart-to-body weight ratios were calculated for each animal at the time of terminal euthanasia.

Demarcation of the infarct area was performed in separate groups of PBS- and liraglutide-treated mice using either 2,3,5-triphenyl tetrazolium chloride (TTC) or hematoxylin and eosin (H-E) staining at 2 and 28 days post-MI, respectively, as described (²⁰,²¹). Sections from three levels of each heart (5 ?m: apical, midventricular, and basal) were stained with H-E and scanned, and the circumference of the fibrotic infarct area and entire left ventricle was measured with Image J software (NIH). The mean of measurements of percent infarct size (circumference) was then calculated (infarct circumference/total LV circumference ? 100%).

For analysis after TTC staining, hearts were sectioned perpendicular to the long axis with a thickness of ?2 mm from the apical, midventricular, and basal (immediately below the ligation point) regions and incubated in the prewarmed (30?C) fresh TTC solution for 15 min. They were then weighed and transferred to a 4% paraformaldehyde solution for 1 h before image acquisition with a digital camera. Infarct and total left ventricle areas were then measured by Image J for determination of percent infarct size (area) by calculation (infarct area/total LV area ? 100%).

Matrix metalloproteinase-9 (MMP-9) was assessed in the infarct zone 4 days post-MI as described (²²).

Cardiomyocytes from newborn mice were prepared using a modified protocol (²³). For cAMP assay, neonatal mouse cardiac myocytes were preincubated with IBMX (250 ?mol/l; Sigma) for 30 min to inhibit cAMP degradation followed by subsequent treatment periods of 20 min for liraglutide (100 nmol/l), 30 min for the GLP-1R antagonist exendin (Ex)^9?39 (1 ?mol/l), and 15 min for forskolin (100 nmol/l). For the liraglutide + Ex^9?39 treatment, cells were exposed to Ex^9?39 for 30 min before coincubation with fresh Ex^9?39 and liraglutide for an additional 20 min. All experiments were performed in quadruplicate. Samples were collected and analyzed using a cAMP radioimmunoassay kit (Amersham, Little Chalfont, U.K.). For tumor necrosis factor-? (TNF-?) experiments, cells were grown on six-well dishes, serum deprived for 24 h, incubated for 1 h with 10,100 or 1,000 nmol/l doses of liraglutide with or without 10 ?mol/l Ex^9?39 followed by coincubation with liraglutide and TNF-? (100 ng/ml; Sigma) for another 24 h to induce apoptosis (²⁴). As a positive control for the induction of apoptosis, another group of cardiomyocytes was exposed to 0.5 ?mol/l H₂O₂ as described (²⁵). All experiments were performed in triplicate. Western blot analysis using whole cell extracts was used to quantify levels of cleaved caspase 3.

Extracts from cells, whole hearts, and infarct regions were prepared as described (²⁶). Rabbit polyclonal primary antibodies against GLP-1R (LS-A1205; MBL International), Nrf2 (C20; Santa Cruz Biotechnology), heme-oxygenase-1 (HO-1; Stressgen), peroxisome proliferator?activated receptor (PPAR)-?/? (H-74; Santa Cruz), and monoclonal antibodies against Akt, phospho-Akt (Ser473), glycogen synthase kinase (GSK)-3?, phospho-GSK-3? (Ser9), and cleaved-caspase 3 (all from Cell Signaling) as well as goat-polyclonal anti?atrial natriuretic peptide (ANP) antibodies (Santa Cruz) were used per manufacturers' instructions. Mouse monoclonal anti??-actin (Sigma) and anti?transcription factor II D (TFIID) (Santa Cruz) antibodies and a rabbit polyclonal anti-GAPDH (Santa Cruz) were used to evaluate the amount of protein loaded in each sample.

Total RNA was isolated from hearts using TRIZOL (Invitrogen), quantified (2 ?g), and treated with DNase-I. cDNA synthesis was performed by Superscript III reverse transcriptase (Invitrogen) and specific primers for HO-1 (F: GGC-CTT-CTG-GTA-TGG-GCC-TCA-CTG-G; R: GCC-TCT-GAC-GAA-GTG-ACG-CCA-TCT-G) and Nrf2 (F: TCT-CCT-CGC-TGG-AAA-AAG-AA; R: AAT-GTG-CTG-GCT-GTG-CTT-TA). PCR products were visualized on 1.5% agarose gels with ethidium bromide. GAPDH reaction product served as a loading and RT efficiency control.

Isolated hearts were prepared as previously described (²⁷). Isolated hearts underwent a 20-min equilibration phase followed by a 40-min perfusion phase during which LV developed pressure (LVDP) was continuously recorded. Then, 30 min of sustained global ischemia was generated by clamping inflow to the heart followed by reperfusion for 40 min. In some experiments, liraglutide was added to the buffer during the final 20 min of the perfusion phase (i.e., preischemia); in others, it was added to the buffer only during reperfusion (i.e., postischemia). Recovery of LVDP was measured at the end of reperfusion. These experiments were also performed using hearts isolated from mice after 1 day (acute) or 7 days (chronic) of twice-daily liraglutide (200 ?g/kg i.p.).

Image acquisition and data analysis were carried out as described (²⁸,²⁹). Three groups of nondiabetic mice, including 8 control mice not subjected to LAD occlusion, and 15 liraglutide-treated and 15 saline-treated mice were studied on day 28 post-MI using high-frequency ultrasound imaging. Identical procedures were used to image diabetic mice (sham, n = 5/group; LAD-ligated, n = 10?11/group).

All data are expressed as mean ? SE. Survival analysis was done by the Kaplan-Meier method (Fig. 1A and B). Student's t test was used to compare two groups for data shown in Figs. 2F3?4. For analysis of echocardiographic data (Table 1) and specific time points in ischemia-reperfusion (I/R) experiments (Fig. 5) and cAMP and TNF-? studies (Fig. 6), a one-way ANOVA was used to evaluate the difference among groups. If the ANOVA was significant, the Student-Newman-Keuls (SNK) post hoc test was used for pairwise multiple comparisons. Significance was defined as P < 0.05.

Immediate perioperative mortality (within 24 h of LAD ligation) was 8.3% in PBS- (n = 85) and 5% in LIR 200?treated (n = 60) mice (P = 0.43) with no deaths in the sham-operated animals (n = 20). By day 28 post-MI, LIR 200?treated mice exhibited reduced mortality compared with PBS-treated controls (12 of 60 versus 46 of 60; P = 0.0001) (Fig. 1A). Because mice treated with LIR 200 also exhibited weight loss compared with PBS-treated controls (?1.65 ? 0.05 versus +0.72 ? 0.02 g; P = 0.0001), we carried out additional experiments to determine whether the effects of LIR on survival post-MI were dependent on weight loss. A separate group of mice was pair-fed to achieve weight loss comparable to that of animals receiving LIR 200 for 7 days; no difference in survival was noted between mice treated with LIR 200 versus pair-fed controls (Fig. 1A). We next assessed a range of liraglutide doses on food intake and body weight and then determined the effect of pretreatment with a weight-neutral dose, LIR 75, on post-MI survival. Mice treated with LIR 75 for 7 days did not experience significant weight loss (0.30 ? 0.06 g; P = 0.85) but exhibited a marked improvement in survival after LAD ligation (Fig. 1A, P = 0.0001). Similarly, pretreatment of diabetic mice for 1 week with LIR 75 also reduced mortality after LAD occlusion (Fig. 1B, P = 0.04); in contrast, comparable treatment with metformin that produced equivalent reduction in glycemia (supplemental Table 1, available in the online appendix) did not improve survival after MI (P = 0.5, not significant). Together, these data suggest that pretreatment with liraglutide enhances survival after MI independent of effects on body weight or blood glucose.

To understand the mechanisms by which LIR improves outcomes after MI in mice, we performed postmortem examinations of all animals. Excluding day 1, postmortem analysis in nondiabetic mice revealed that all spontaneous death events within 10 days post-MI were associated with evidence of cardiac rupture. Timing and incidence of cardiac rupture differed among the groups, occurring as early as day 3 post-MI and peaking in incidence at day 5 post-MI in PBS-treated mice (Fig. 1C). By contrast, mice pretreated with LIR 200, and to an even greater extent LIR 75, exhibited fewer and later cardiac rupture events post-MI (Fig. 1C).

Cardiac rupture is known to occur more frequently with larger infarcts (³⁰) and is believed to manifest an imbalance between inflammatory and fibrotic responses as well hemodynamic forces acting on the infarct (³¹,³²). To explore these possibilities in more detail, we first analyzed H-E?defined infarct size at day 28 post-MI in nondiabetic mice. Liraglutide (LIR 200) significantly reduced infarct size compared with PBS-treated mice (20.9 ? 1.7 versus 28.8 ? 3.3; percent total LV circumference, P = 0.02) (Fig. 2). Furthermore, cardiac hypertrophy, as manifested by heart weight/body weight (HW/BW) ratio, was significantly reduced in LIR 200?treated mice (Fig. 2). To determine if these results partly reflected survivor bias, we also examined TTC-defined infarct size at day 2 post-MI. This analysis revealed no significant difference in infarct size between LIR 200? (n = 14) and PBS-treated (n = 15) groups (40.2 ? 4.2 versus 39.3 ? 3.8; P = 0.86). Despite significantly higher survival in liraglutide-treated diabetic mice (Fig. 1B), no differences were observed in either infarct size or HW/BW ratios in diabetic animals at day 28 post-MI (data not shown).

To determine the mechanisms underlying liraglutide-induced improvements in survival and infarct remodeling post-MI, we studied the expression of selected genes and proteins known to modulate cardiomyocyte survival. LIR 200 administered twice daily for 7 days to normal healthy mice (without MI) increased phosphorylation of the prosurvival kinase Akt (pAkt/Akt: 2.10 ? 0.01-fold over control, P = 0.002, Fig. 3A). Similarly, liraglutide increased phosphorylation and thereby reduced the activity of GSK3?, a known Akt substrate, and increased levels of the nuclear receptor PPAR?/? and the redox-sensitive basic leucine zipper transcription factor Nrf2. Liraglutide also induced mRNA and protein levels of HO-1 (Fig. 3A), a protein strongly implicated in cardioprotection in response to ischemic injury (³³). To assess the importance of the known GLP-1R for the actions of liraglutide, we administered LIR 75 for an identical 7-day treatment period in mice with genetic absence of a functional GLP-1R and littermate controls. Liraglutide increased phosphorylation of Akt and GSK3? in both wild-type (Fig. 3B) and Glp1r+/+ littermate controls (Fig. 3C), but not in Glp1r-/- mice (Fig. 3D).

To assess whether the effects of liraglutide remained detectable after cessation of liraglutide therapy and before the peak incidence of mortality post-MI (which occurred on day 5; Fig. 1A), we examined the hearts of liraglutide- and PBS-treated controls 4 days post-MI (i.e., 4 days after the final dose of liraglutide or PBS). The HW/BW ratio was reduced in liraglutide-treated mice (5.53 ? 0.14 versus 5.83 ? 0.15 mg/g; P = 0.03) at day 4, and levels of phosphorylated Akt were significantly higher in the infarct area of hearts from liraglutide-treated mice (Fig. 4). Similarly, liraglutide pretreatment was associated with increased GSK phosphorylation at Ser 9 and a significant reduction in levels of cleaved caspase-3 and ANP (Fig. 4). Quantitative gelatin zymography showed a significant decrease in MMP-9 activity in hearts from liraglutide-treated mice (Fig. 4). These data demonstrate a persistent effect of liraglutide pretreatment on cardiac prosurvival and remodeling pathways detectable both before (Fig. 3) and after (Fig. 4) induction of ischemic injury.

The results of cardiac ultrasound biomicroscopy performed 4 weeks after LAD ligation or sham surgery in nondiabetic mice are shown in Table 1. After experimental MI, measures of systolic function such as cardiac output and stroke volume were significantly increased in liraglutide-treated mice. Measures of diastolic function such as mitral inflow velocities (E/A ratio) were also improved in liraglutide-treated mice and LV dilatation in the liraglutide-treated group was less severe than in PBS-treated control mice (Table 1). In contrast, although liraglutide-treated diabetic mice exhibited a significant increase in cardiac output in the absence of experimental MI, no consistent differences in echocardiographic parameters were detected in diabetic mice treated with liraglutide versus metformin at day 28 post-MI (data not shown).

To determine whether liraglutide exhibits rapid cardioprotective actions in the isolated mouse heart ex vivo, we examined the effects of acute liraglutide administration in the isolated perfused mouse heart. Direct infusions of liraglutide immediately before and postischemia did not improve functional recovery after ischemia-reperfusion (I/R) injury in the isolated mouse heart (Fig. 5A). In contrast, pretreatment of normal healthy mice with liraglutide (200 ?g/kg b.i.d.) in vivo for 1 or 7 days before heart isolation enhanced recovery of LVDP after I/R relative to untreated controls (1 day: 41.54 ? 1.5, n = 4; 7 days: 38.31 ? 0.6 mmHg, n = 5; untreated: 28.24 ? 1.8 mmHg, n = 5; P < 0.01, Fig. 5B). These results imply that liraglutide-induced cardioprotection may require a minimum dose or pretreatment period for the activation of a cardiac gene and protein expression profile and/or may be indirect, reflecting the consequences of GLP-1R activation in noncardiac tissues.

We next examined whether liraglutide exerts direct actions on mouse cardiomyocytes. Liraglutide (100 nmol/l) significantly increased cAMP formation in mouse cardiomyocytes (P < 0.0001) in a GLP-1R?dependent manner, because cAMP stimulation was abolished by the GLP-1R antagonist Ex^9?39 (Fig. 6A). To determine whether liraglutide directly modulates cardiomyocyte survival, we examined the effects of liraglutide in TNF-??treated neonatal cardiomyocytes. Liraglutide dose-dependently reduced TNF-??induced activation of caspase 3 in cardiomyocytes in vitro. Furthermore, the protective effect of liraglutide was completely abolished after coincubation of cells with Ex^9?39 (Fig. 6B).