Medium-term Electrophysiologic Effects of a Cellularized Scaffold Implanted in Rats After Myocardial Infarction.

Background Cardiac repair strategies are being evaluated for myocardial infarctions, but the safety issues regarding their arrhythmogenic potential remain unresolved. By utilizing the in-vivo rat model, we have examined the medium-term electrophysiologic effects of a biomaterial scaffold that has been cellularized with spheroids of human adipose tissue, derived from mesenchymal stem cells and umbilical vein endothelial cells. Methods Mesenchymal stem cells, which exhibit adequate differentiation capacity, were co-cultured with umbilical vein endothelial cells and were seeded on an alginate based scaffold. After in-vitro characterization, the cellularized scaffold was implanted in (n=15) adult Wistar rats 15 min post ligation of the left coronary artery, with an equal number of animals serving as controls. Two weeks thereafter, monophasic action potentials were recorded and activation-mapping was performed with a multi-electrode array. An arrhythmia score for inducible ventricular tachyarrhythmias was calculated after programmed electrical stimulation. Results The arrhythmia score was comparable between the treated animals and controls. No differences were detected in the local conduction at the infarct border and in the voltage rise in monophasic action potential recordings. Treatment did not affect the duration of local repolarization, but tended to enhance its dispersion. Conclusions The fabricated bi-culture cellularized scaffold displayed favorable properties after in-vitro characterization. Medium-term electrophysiologic assessment after implantation in the infarcted rat myocardium revealed low arrhythmogenic potential, but the long-term effects on repolarization dispersion will require further investigation.

Prolonged intra-myocardial growth hormone administration ameliorates post-infarction electrophysiologic remodeling in rats.

Experimental studies indicate improved ventricular function after treatment with growth hormone (GH) post-myocardial infarction, but its effect on arrhythmogenesis is unknown. Here, we assessed the medium-term electrophysiologic remodeling after intra-myocardial GH administration in (n = 33) rats. GH was released from an alginate scaffold, injected around the ischemic myocardium after coronary ligation. Two weeks thereafter, ventricular tachyarrhythmias were induced by programmed electrical stimulation. Monophasic action potentials were recorded from the infarct border, coupled with evaluation of electrical conduction and repolarization from a multi-electrode array. The arrhythmia score was lower in GH-treated rats than in alginate-treated rats or controls. The shape and the duration of the action potential at the infarct border were preserved, and repolarization-dispersion was attenuated after GH; moreover, voltage rise was higher and activation delay was shorter. GH normalized also right ventricular parameters. Intra-myocardial GH preserved electrical conduction and repolarization-dispersion at the infarct border and decreased the incidence of induced tachyarrhythmias in rats post-ligation. The long-term antiarrhythmic potential of GH merits further study.

Intra-myocardial growth hormone administration ameliorates arrhythmogenesis during ischemia-reperfusion in rats.

Growth hormone, currently under evaluation for the prevention of left ventricular remodeling post-myocardial infarction, displays antiarrhythmic properties in the acute setting. However, it is uncertain whether these actions are retained after ischemia/reperfusion. Using implanted telemetry transmitters, we examined the effects of prolonged, intra-myocardial growth hormone administration in conscious rats. During a 24-h observation period, ventricular tachyarrhythmias and sympathetic activation were attenuated in treated rats, whereas infarct-size was unchanged. These findings call for further study on the antiarrhythmic effects of growth hormone and on the underlying mechanisms.

Attenuation of post-infarction remodeling in rats by sustained myocardial growth hormone administration.

Prevention of left ventricular remodeling is an important therapeutic target post-myocardial infarction. Experimentally, treatment with growth hormone (GH) is beneficial, but sustained local administration has not been thoroughly investigated. We studied 58 rats (322 ± 4 g). GH was administered via a biomaterial-scaffold, following in vitro and in vivo evaluation of degradation and drug-release curves. Treatment consisted of intra-myocardial injection of saline or alginate-hydrogel, with or without GH, 10 min after permanent coronary artery ligation. Echocardiographic and histologic remodeling-indices were examined 3 weeks post-ligation, followed by immunohistochemical evaluation of angiogenesis, collagen, macrophages and myofibroblasts. GH-release completed at 3 days and alginate-degradation at ∼7 days. Alginate + GH consistently improved left ventricular end-diastolic and end-systolic diameters, ventricular sphericity, wall tension index and infarct-thickness. Microvascular-density and myofibroblast-count in the infarct and peri-infarct areas were higher after alginate + GH. Macrophage-count and collagen-content did not differ between groups. Early, sustained GH-administration enhances angiogenesis and myofibroblast-activation and ameliorates post-infarction remodeling.

Arrhythmogenesis after acute myocardial necrosis with and without preceding ischemia in rats.

BACKGROUND: The relative role of acute myocardial ischemia and infarction in ventricular arrhythmogenesis is incompletely understood. We compared the arrhythmia pattern after ischemia/infarction to that observed after direct myocardial necrosis without preceding ischemia in rats. METHODS: Coagulation necrosis was induced in Wistar rats (n=20, 280±3 g) by radiofrequency current application (for 15 s) from a 4-mm-tip ablation catheter. Myocardial infarction was induced by coronary artery ligation with (n=10) or without (n=10) reperfusion. Using 24-h telemetry recording, we examined ventricular arrhythmias, voluntary motor activity and indices of sympathetic activation. RESULTS: The coagulation-necrosis volume was 24.4%±0.6%, comparable to the infarct size in the absence of reperfusion. Acute left ventricular failure and sympathetic activation were similar in the three groups. Coagulation necrosis induced ventricular fibrillation immediately, followed by a second peak after ∼1 h. Reperfusion decreased ventricular arrhythmias, whereas a second arrhythmogenic period (between the third and the eight hour) was noted in non-reperfused infarcts (mainly monomorphic ventricular tachycardia). CONCLUSIONS: Distinct arrhythmia patterns occur after myocardial infarction (with or without reperfusion) and after direct necrosis. They are not produced by differences in sympathetic activation and are likely related to the evolution of myocardial injury. The necrosis rat model may be useful in studies of arrhythmogenesis.

Effects of pre- and postconditioning on arrhythmogenesis in the in vivo rat model.

The antiarrhythmic potential of postconditioning in in vivo models remains poorly defined. We compared the effects of pre- and postconditioning on ventricular arrhythmogenesis against controls with and without reperfusion. Wistar rats (n = 40, 269 ± 3 g) subjected to ischemia (30 minutes)--reperfusion (24 hours) were assigned to the following groups: (1) preconditioning (2 cycles), (2) postconditioning (6 cycles), or (3) no intervention and were compared with (4) nonreperfused infarcts and (5) sham-operated animals. Infarct size was measured, and arrhythmogenesis was evaluated with continuous telemetric electrocardiographic recording, heart rate variability indices, and monophasic action potentials (MAPs). During a 24-hour observation period, no differences in mortality were observed. Reperfusion decreased infarct size and ameliorated sympathetic activation during the late reperfusion phase. Preconditioning decreased infarct size by a further 35% (P = .0017), but only a marginal decrease (by 18%, P = .075) was noted after postconditioning. Preconditioning decreased arrhythmias during ischemia and early reperfusion, whereas postconditioning almost abolished them during the entire reperfusion period. No differences were noted in MAPs or in the magnitude of sympathetic activation between the 2 interventions. Compared to postconditioning, preconditioning affords more powerful cytoprotection, but both interventions exert antiarrhythmic actions. In the latter, these are mainly evident during the ischemic phase and continue during early reperfusion. Postconditioning markedly decreases reperfusion arrhythmias during a prolonged observation period. The mechanisms underlying the antiarrhythmic effects of pre- and postconditioning are likely different but remain elusive.

Short-term ventricular restraint attenuates post-infarction remodeling in rats.

BACKGROUND/OBJECTIVES: Left ventricular restraint attenuates post-infarction remodeling, but may be associated with unfavorable long-term histological response. We hypothesized that beneficial effects can be obtained with short-term restraint during the early post-infarction period; for this purpose, we evaluated a biodegradable scaffold in the in vivo rat model and compared it with epicardial hydrogel application. METHODS: A total of 230 Wistar rats (358 ± 7 g) were studied. Implantation was performed with and without prior myocardial infarction, induced by permanent coronary artery ligation. Diastolic filling was evaluated by left ventricular pressure recordings after scaffold implantation. Degradation rates and inflammatory/foreign body response were studied at 3, 7 and 15 days post-ligation. Remodeling indices were evaluated by echocardiography 15 days post-ligation. RESULTS: No differences were found in diastolic pressure. Biodegradability was ~50% by 7 days and 100% by 15 days for both materials. Likewise, inflammatory/foreign body response peaked at 3 days post-implant, with subsequent remission, but fibroblastic reaction was more pronounced after scaffold than after hydrogel implantation. Post-ligation, ejection fraction was higher in the scaffold (40.0 ± 1.5%) or hydrogel groups (37.0 ± 1.3%), compared to controls (30.6 ± 1.9%). Wall tension index was lower with either biomaterial, but left ventricular end-diastolic diameter was shorter (p=0.044) and sphericity was attenuated (p=0.029) after scaffold, compared to hydrogel implantation. CONCLUSIONS: Both biomaterials showed a favorable histological response and attenuated remodeling, but epicardial restraint produced better results compared to hydrogel alone. The latter approach merits further investigation due to the ease of implantation.

Endothelin-B Receptors and Left Ventricular Dysfunction after Regional versus Global Ischaemia-Reperfusion in Rat Hearts.

Background. Endothelin-1 (ET-1) is implicated in left ventricular dysfunction after ischaemia-reperfusion. ETA and ETB receptors mediate diverse actions, but it is unknown whether these actions depend on ischaemia type and duration. We investigated the role of ETB receptors after four ischaemia-reperfusion protocols in isolated rat hearts. Methods. Left ventricular haemodynamic variables were measured in the Langendorff-perfused model after 40- and 20-minute regional or global ischaemia, followed by 30-minute reperfusion. Wild-type (n = 39) and ETB-deficient (n = 41) rats were compared. Infarct size was measured using fluorescent microspheres after regional ischaemia-reperfusion. Results. Left ventricular dysfunction was more prominent in ETB-deficient rats, particularly after regional ischaemia. Infarct size was smaller (P = 0.006) in wild-type (31.5 ± 4.4%) than ETB-deficient (45.0 ± 7.3%) rats after 40 minutes of regional ischaemia-reperfusion. Although the recovery of left ventricular function was poorer after 40-minute ischaemia-reperfusion, end-diastolic pressure in ETB-deficient rats was higher after 20 than after 40 minutes of regional ischaemia-reperfusion. Conclusion. ETB receptors exert cytoprotective effects in the rat heart, mainly after regional ischaemia-reperfusion. Longer periods of ischaemia suppress the recovery of left ventricular function after reperfusion, but the role of ETB receptors may be more important during the early phases.

Transforming growth factor-ß inhibition and endothelin receptor blockade in rats with monocrotaline-induced pulmonary hypertension.

Transforming growth factor-ß (TGF-ß) inhibition is an investigational therapy for pulmonary arterial hypertension with promising results in experimental studies. The present work compared this approach with endothelin-receptor blockade and evaluated the effects of combined administration. Pulmonary arterial hypertension was induced by single monocrotaline injection (60 mg/kg) in 75 Wistar rats and 15 rats served as controls. Intervention groups consisted of treatment with an antibody against TGF-ß-ligand, bosentan, both or none, initiated four weeks after monocrotaline injection. Right ventricular systolic pressure, pulmonary vascular remodeling, and exercise tolerance were evaluated eight weeks after monocrotaline injection. Either treatment, alone or in combination, lowered mortality. Comparable efficacy was found in the three treatment groups in terms of right ventricular systolic pressure (~45% decrease) and hypertrophy (~30% decrease), as well as exercise capacity. The three treatment groups equally ameliorated pulmonary vascular remodeling, evidenced by decreased vessel-wall thickness (in vessels 50-200 µm) and a smaller number of pre-capillary arterioles (< 50 µm) with a muscularized media. Treatment either with an antibody against TGF-ß or with endothelin receptor blockade are equally effective in experimental pulmonary hypertension. Their combination provides no added benefit, indicating common mechanisms of action.