Evidence for a ceiling of cardioprotection with a nitric oxide donor-induced delayed preconditioning in rabbits

Intestinal and Limb Ischemic Preconditioning Provides a Combined Protective Effect in the Late Phase, But not in the Early Phase, Against Intestinal Injury Induced by Intestinal Ischemia-Reperfusion in Rats

Intestinal ischemia/reperfusion (I/R) injury is associated with high morbidity and mortality. This study aimed to compare the protective efficacy of intestinal ischemic preconditioning (IIPC) and limb ischemic preconditioning (LIPC) against intestinal I/R injury and investigate their combined protective effect and the underlying mechanism. Male Sprague-Dawley rats were pretreated with IIPC, LIPC, or IIPC plus LIPC (combined), and intestinal I/R or sham operation was performed. The animals were sacrificed at 2 and 24 h after reperfusion and then blood and tissue samples were harvested for further analyses. In additional groups of animals, a 7-day survival study was conducted. The results showed that ischemic preconditioning (IPC) improved the survival rate and attenuated intestinal edema, injury, and apoptosis. IPC decreased the levels of tumor necrosis factor-α, interleukin -6, malondialdehyde and myeloperoxidase, and increased the activity of superoxide dismutase in serum and intestine after the I/R event. IPC downregulated the expression of Toll-like receptor-4 (TLR4) and nuclear factor-kappa B (NF-κB). The effect of combined pretreatment was better than that of single pretreatment in the late phase (24 h), but not in the early phase (2 h). The study demonstrated that IPC could significantly attenuate intestinal injury induced by intestinal I/R via inhibiting inflammation, oxidative stress, and apoptosis. IIPC and LIPC conferred no synergy in protecting I/R-induced intestinal injury in the early phase, but combined preconditioning had clearly stronger protection in the late phase, which was associated with the inhibition of the activated TLR4/NF-κB signaling pathway. It suggested that LIPC or combined preconditioning could potentially be applied in the clinical settings of surgical patient care.

Relation of the ischaemic substrate to left ventricular remodelling by cardiac magnetic resonance at 1.5 T in rabbits

OBJECTIVES

Contrast-enhanced cardiac magnetic resonance (CMR) for infarct sizing has been validated in large animals, but studies and follow-up are restricted. We sought to (1) validate CMR for assessment of myocardial area at risk (MAR) and infarct size (IS) in a rabbit model of reperfused myocardial infarction (MI); (2) analyse the relation between ischaemic substrates and subsequent left ventricular (LV) remodelling.

METHODS

Experimental reperfused acute MI was induced in 16 rabbits. Ten animals underwent cross-registered cine and contrast-enhanced CMR and histopathology at day 3 for assessment of MAR and IS (group 1). The remaining six rabbits underwent serial CMR for the study of LV remodelling (group 2).

RESULTS

In group 1, mean IS was 12.7 +/- 6.4% and 12.7 +/- 6.9% of total LV myocardial mass on CMR (late-enhancement technique) and histopathology (P = 0.52; r = 0.93). No significant difference occurred between CMR and histopathology for the calculation of MAR and IS/MAR ratio (P = 0.18 and P = 0.17), whereas correlations were strong (r = 0.92 and r = 0.95). In group 2, mean LV end-diastolic, end-systolic volumes and LV mass were significantly increased at 3 weeks compared with measurements at day 3 (P < 0.01). Significant correlations between initial IS and the increase in LV end-diastolic volume (r = 0.66) and the increase in LV mass (r = 0.48) were observed, as well as correlations between initial MAR and the increase in LV end-diastolic volume (r = 0.70) and the increase in LV mass (r = 0.37).

CONCLUSIONS

Comprehensive CMR provides accurate assessment of IS and MAR in reperfused rabbit MI. Infarct size is closely related to LV remodelling. Through the infarct size/MAR ratio, this approach has great potential for assessing interventions aimed at cardioprotection.

Nitroglycerin induces late preconditioning against arrhythmias but not stunning in conscious sheep

OBJECTIVES

Nitroglycerin, a nitric oxide donor, induces late preconditioning against stunning by short ischemia-reperfusion periods. The study purpose was to assess similar nitroglycerin protection against stunning and arrhythmias produced by prolonged reversible ischemia.

DESIGN

Four groups of conscious sheep were studied, control: 12 minutes ischemia and 2 hour reperfusion; late preconditioning: six periods of 5 min ischemia-5 min reperfusion 24 h before 12 min ischemia and late preconditioning with 120 microg/kg and 600 microg/kg nitroglycerin administered instead of the ischemia-reperfusion periods.

RESULTS

Although late preconditioning protected against stunning (mean postischemic recovery of wall thickening fraction, control (n=10): 54.8+/-3.2, late preconditioning (n=9): 74.4+/-3.0, p<0.01), nitroglycerin 120 microg/kg (n=6) did not reproduce mechanical protection (50.1+/-3.8), even with a higher concentration of 600 microg/kg (59.1+/-3.7, n=4). However, nitroglycerin decreased arrhythmia severity index (control: 2.3+/-0.6, late preconditioning: 0.5+/-0.4, nitroglycerin 120 microg/kg: 1+/-0.4 and 600 microg/kg: 0.1+/-0.1 (p<0.05 vs. control).

CONCLUSIONS

Nitroglycerin only has a limited late preconditioning protective effect in conscious animals submitted to a reversible prolonged ischemia since it protects against arrhythmias but not against stunning.

Pharmacological postconditioning with the phytoestrogen genistein

Estrogens are known to activate the phosphatidyl-inosityl 3-kinase (PI3K)/Akt pathway, which is central in the cardioprotection afforded by ischemic postconditioning. Therefore, our goal was to investigate whether a phytoestrogen, genistein, could induce a pharmacological postconditioning and to investigate potential mechanisms. We used low doses of genistein in order to avoid tyrosine kinases inhibition. Thus, pentobarbital-anesthetized rabbits underwent a coronary artery occlusion followed by 4 h of reperfusion. Prior to reperfusion, they randomly received an i.v. injection of either saline (Control), 100 or 1000 microg/kg of genistein (Geni(100) and Geni(1000), respectively), and 10 or 100 microg/kg of 17beta-estradiol (17beta(10) and 17beta(100), respectively). Infarct size (IS, % area at risk) was significantly reduced in Gen(100), Gen(1000) and 17beta(100) but not in 17beta(10) (6+/-2, 16+/-5, 12+/-3 and 29+/-7%, respectively) vs. Control (35+/-4%). A significant decrease in the percentage of TUNEL-positive nuclei within infarcted area was observed in Gen(100) and 17beta(100) vs. Controls. The estrogen receptor antagonist fulvestrant (1 mg/kg i.v.) and the PI3K inhibitor wortmaninn (0.6 mg/kg) abolished the cardioprotective effect of genistein. Western blots also demonstrated an increase in Akt posphorylation in Gen(100). In the same group, in vitro mitochondrial swelling studies demonstrated a significant inhibition of calcium-induced opening of mitochondrial transition pore vs. Controls. In conclusion, genistein exerts pharmacological postconditioning with a similar potency as 17beta-estradiol through a pathway involving activation of the estrogen receptor, of PI3K/Akt and mitochondrial preservation. Therefore, genistein should not be only considered as an inhibitor of tyrosine kinase but also as a cardioprotective estrogen.

Differential effects of postconditioning on myocardial stunning and infarction: a study in conscious dogs and anesthetized rabbits

Postconditioning, i.e., brief intermittent episodes of myocardial ischemia-reperfusion performed at the onset of reperfusion, reduces infarct size after prolonged ischemia. Our goal was to determine whether postconditioning is protective against myocardial stunning. Accordingly, conscious chronically instrumented dogs (sonomicrometry, coronary balloon occluder) were subjected to a control sequence (10 min coronary artery occlusion, CAO, followed by coronary artery reperfusion, CAR) and a week apart to postconditioning with four cycles of brief CAR and CAO performed at completion of the 10 min CAO. Three postconditioning protocols were investigated, i.e., 15 s CAR/15 s CAO ( n = 5), 30 s CAR/30 s CAO ( n = 7), and 1 min CAR/1 min CAO ( n = 6). Left ventricular wall thickening was abolished during CAO and similarly reduced during subsequent stunning in control and postconditioning sequences (e.g., at 1 h CAR, 33 ± 4 vs. 34 ± 4%, 30 ± 4 vs. 30 ± 4%, and 33 ± 4 vs. 32 ± 4% for 15 s postconditioning, 30 s postconditioning, and 1 min postconditioning vs. corresponding control, respectively). We confirmed this result in anesthetized rabbits by demonstrating that shortening of left ventricular segment length was similarly depressed after 10 min CAO in control and postconditioning sequences (4 cycles of 30 s CAR/30 s CAO). In additional rabbits, the same postconditioning protocol significantly reduced infarct size after 30 min CAO and 3 h CAR (39 ± 7%, n = 6 vs. 56 ± 4%, n = 7 of the area at risk in postconditioning vs. control, respectively). Thus, contrasting to its beneficial effects on myocardial infarction, postconditioning does not protect against myocardial stunning in dogs and rabbits. Conversely, additional episodes of ischemia-reperfusion with postconditioning do not worsen myocardial stunning.

Isoflurane inhaled at the onset of reperfusion potentiates the cardioprotective effect of ischemic postconditioning through a NO-dependent mechanism

Postconditioning (PCD) is known to reduce infarct size (IS). Here, we investigated whether isoflurane, which is known to potentiate preconditioning, also potentiates PCD and whether NO is involved. Accordingly, open-chest rabbits underwent 30-minute coronary artery occlusion (CAO) followed by 3-hour coronary artery reperfusion (CAR). In control and postconditioned (4 cycles of 30s-CAR/30s-CAO after the 30-min CAO), rabbits were anesthetized with pentobarbital alone or in combination with isoflurane inhaled (i) throughout the experiment or (ii) only during CAR. With pentobarbital alone, PCD significantly reduced IS versus control (39 +/- 7% vs. 55 +/- 4% of the risk zone, respectively, P < 0.05). Isoflurane--0.5% throughout the experiment did not alter IS in both control and PCD groups. Isoflurane--2% throughout the experiment reduced IS in control (37 +/- 8%, P < 0.05 vs. pentobarbital alone) and enhanced the protective effect of PCD (IS = 21 +/- 3%, P < 0.05 vs. both control and PCD under pentobarbital alone). When isoflurane--2% was administered only during reperfusion, IS was not changed in control (53 +/- 3%) but combination with PCD reduced IS to 23 +/- 4% (P < 0.05 vs. both control and PCD under pentobarbital alone). L-arginine analog N-nitro-L-arginine methyl ester administered before reperfusion did not properly alter IS (53 +/- 6%) but abolished the effect of PCD alone (IS = 47 +/- 7%) or in combination with isoflurane--2% during reperfusion (55 +/- 3%). Thus, isoflurane potentiates PCD at reperfusion through a NO-dependent mechanism.

Hypercholesterolemia blunts NO donor-induced late preconditioning against myocardial infarction in conscious rabbits

Although NO donors have been shown to confer late preconditioning (PC) against myocardial ischemia/reperfusion injury in healthy rabbits, it is unknown whether concurrent systemic disorders affect NO donor-induced cardioprotection. Since many patients with coronary artery disease have hypercholesterolemia (HC), we examined the effect of this condition on late PC induced by the NO donor diethylenetriamine/nitric oxide (DETA/ NO). Chronically instrumented rabbits were fed a normal diet (normocholesterolemia, NC) or a diet enriched with 1% cholesterol (HC) for 4 weeks. Plasma cholesterol levels were significantly elevated and the arterial pressure response to the endothelium-dependent vasodilator bradykinin was blunted in cholesterol diet-fed rabbits. Conscious rabbits underwent a 30-minute coronary occlusion followed by 3 days of reperfusion. When NC rabbits were pretreated with DETA/NO (0.1 mg/kg, i. v. x 4, group II, n = 7) 24 hours before the 30-minute occlusion, infarct size was reduced by 52% (29.7 +/- 3.4% versus 62.4 +/- 4.0% of the region at risk in NC controls [group I, n = 5], P < 0.05), indicating that DETA/NO induced a late PC effect against myocardial infarction. In contrast, when HC rabbits were pretreated with the same dose of DETA/NO (group IV, n = 6), infarct size was not significantly reduced (61.0 +/- 5.7% versus 68.1 +/- 4.5% of the region at risk in HC [group III, n = 5], P = NS), suggesting that DETA/NO failed to induce a delayed cardioprotective effect. These data demonstrate, for the first time, that HC blunts NO donor-induced late PC against myocardial infarction, implying that the inhibitory effects of HC on ischemia-induced and NO donor-induced late PC are caused by disruption of biochemical pathways distal to the generation of NO that triggers these adaptations.