Regional differences of myocardial infarct development and ischemic preconditioning

Photobiomodulation invigorating collagen deposition, proliferating cell nuclear antigen and Ki67 expression during dermal wound repair in mice

The present investigation focuses on understanding the role of photobiomodulation in enhancing tissue proliferation. Circular excision wounds of diameter 1.5 cm were created on Swiss albino mice and treated immediately with 2 J/cm² and 10 J/cm² single exposures of the Helium-Neon laser along with sham-irradiated controls. During different days of healing progression (day 5, day 10, and day 15), the tissue samples upon euthanization of the animals were taken for assessing collagen deposition by Picrosirius red staining and cell proliferation (day 10) by proliferating cell nuclear antigen (PCNA) and Ki67. The positive influence of red light on collagen synthesis was found to be statistically significant on day 10 (P < 0.01) and day 15 (P < 0.05) post-wounding when compared to sham irradiation, as evident from the image analysis of collagen birefringence. Furthermore, a significant rise in PCNA (P < 0.01) and Ki67 (P < 0.05) expression was also recorded in animals exposed to 2 J/cm² when compared to sham irradiation and (P < 0.01) compared to the 10 J/cm² treated group as evidenced by the microscopy study. The findings of the current investigation have distinctly exhibited the assenting influence of red laser light on excisional wound healing in Swiss albino mice by augmenting cell proliferation and collagen deposition.

Larger infarct size but equal protection by ischemic conditioning in septum and anterior free wall of pigs with LAD occlusion

The ischemic area at risk (AAR) is one major determinant of infarct size (IS). In patients, the largest AAR is seen with a proximal occlusion of the left anterior descending (LAD) coronary artery, which serves parts of the septum and of the anterior free wall. It is not clear, whether regional differences in the perfusion territories also impact on IS and the magnitude of cardioprotection by ischemic conditioning. We have retrospectively analyzed 132 experiments in pigs, which have a similar LAD perfusion territory as humans. The LAD was occluded for 60 min with subsequent 180 min reperfusion. Cardioprotection by either local ischemic pre- or postconditioning or remote ischemic pre- or perconditioning was induced in 93 pigs. The AAR was demarcated by blue dye staining, and IS was assessed by triphenyltetrazolium chloride (TTC) staining. Using digital planimetry, the AAR was separated into sections unequivocally located in the septum (AARS ) or the anterior free wall (AARAFW ). Relative IS was calculated for AARS or AARAFW . AARAFW was larger than AARS (51 ± 9% vs. 34 ± 8% of total AAR; mean ± SD, P < 0.001). Regional myocardial blood flow (microspheres) was not different between septum and anterior free wall. IS without ischemic conditioning tended to be larger in AARS than in AARAFW (50 ± 17% vs. 44 ± 19%; % of AARAWF or AARS , respectively; P = 0.075). Also, with robust IS reduction by ischemic conditioning, the difference in relative IS remained (AARS : 27 ± 16%; AARAFW : 21 ± 16%; P = 0.01). There is a somewhat greater susceptibility for infarction in septal than anterior free wall myocardium. However, ischemic conditioning still reduces IS in both septal and anterior free wall myocardium.

Objective assessment of endogenous collagen in vivo during tissue repair by laser induced fluorescence

Collagen, a triple helical protein with the primary role of mechanical function, provides tensile strength to the skin, and plays a pivotal task in tissue repair. During tissue regeneration, collagen level increases gradually and therefore, monitoring of such changes in vivo by laser induced fluorescence was the main objective behind the present study. In order to accomplish this, 15 mm diameter excisional wounds were created on six to eight week old Swiss albino mice. The collagen deposition accelerated upon irradiation of single exposure of 2 J/cm² He-Ne laser dose immediately after wounding was recorded by laser induced autofluorescence in vivo along with un-illuminated and un-wounded controls. Autofluorescence spectra were recorded for each animal of the experimental groups on 0, 5, 10, 30, 45 and 60 days post-wounding, by exciting the granulation tissue/skin with 325 nm He-Cd laser. The variations in the average collagen intensities from the granulation tissue/skin of mice were inspected as a function of age and gender. Further, the spectral findings of the collagen synthesis in wound granulation tissue/un-wounded skin tissues were validated by Picro-Sirius red- polarized light microscopy in a blinded manner through image analysis of the respective collagen birefringence. The in vivo autofluorescence studies have shown a significant increase in collagen synthesis in laser treated animals as compared to the un-illuminated controls. Image analysis of the collagen birefringence further authenticated the ability of autofluorescence in the objective monitoring of collagen in vivo. Our results clearly demonstrate the potential of laser induced autofluorescence in the monitoring of collegen synthesis during tissue regeneration, which may have clinical implications.

Gene transfer as a strategy to achieve permanent cardioprotection I: rAAV-mediated gene therapy with inducible nitric oxide synthase limits infarct size 1 year later without adverse functional consequences

The ultimate goal of prophylactic gene therapy is to confer permanent protection against ischemia. Although gene therapy with inducible nitric oxide synthase (iNOS) is known to protect against myocardial infarction at 3 days and up to 2 months, the long-term effects on myocardial ischemic injury and function are unknown. To address this issue, we created a recombinant adeno-associated viral vector carrying the iNOS gene (rAAV/iNOS), which enables long-lasting transgene expression. The ability of rAAV/iNOS to direct the expression of functional iNOS protein was confirmed in COS-7 cells before in vivo gene transfer. Mice received injections in the anterior LV wall of rAAV/LacZ or rAAV/iNOS; 1 year later, they underwent a 30-min coronary occlusion (O) and 4 h of reperfusion (R). iNOS gene transfer resulted in elevated iNOS protein expression (+3-fold vs. the LacZ group, n = 6; P < 0.05) and iNOS activity (+4.4-fold vs. the LacZ group, n = 6; P < 0.05) 1 year later. Infarct size (% of risk region) was dramatically reduced at 1 year after iNOS gene transfer (13.5 ± 2.2%, n = 12, vs. 41.7 ± 2.9%, n = 10, in the LacZ group; P < 0.05). The infarct-sparing effect of iNOS gene therapy at 1 year was as powerful as that observed 24 h after ischemic preconditioning (six 4-min O/4-min R cycles) (19.3 ± 2.3%, n = 11; P < 0.05). Importantly, compared with the LacZ group (n = 11), iNOS gene transfer (n = 10) had no effect on LV dimensions or function for up to 1 year (at 1 year: FS 34.5 ± 2.0 vs. 34.6 ± 2.6%, EF 57.0 ± 2.0 vs. 59.7 ± 2.9%, LVEDD 4.3 ± 0.1 vs. 4.2 ± 0.2 mm, LVESD 2.8 ± 0.1 vs. 2.9 ± 0.2 mm) (echocardiography). These data demonstrate, for the first time, that rAAV-mediated iNOS gene transfer affords long-term, probably permanent (1 year), cardioprotection without adverse functional consequences, providing a strong rationale for further preclinical testing of prophylactic gene therapy.

TNFα in myocardial ischemia/reperfusion, remodeling and heart failure

TNFα is crucially involved in the pathogenesis and progression of myocardial ischemia/reperfusion injury and heart failure. The formation and release of TNFα and its downstream signal transduction cascade following activation of its two receptor subtypes are characterized. Myocardial TNFα and TNF receptor activation have an ambivalent role in myocardial ischemia/reperfusion injury and protection from it. Excessive TNFα expression and subsequent cardiomyocyte TNF receptor type 1 stimulation induce contractile dysfunction, hypertrophy, fibrosis and cell death, while a lower TNFα concentration and subsequent cardiomyocyte TNF receptor type 2 stimulation are protective. Apart from its concentration and receptor subtype, the myocardial action of TNFα depends on the duration of its exposure and its localization. While detrimental during sustained ischemia, TNFα contributes to ischemic preconditioning protection, no matter whether it is the first, second or third window of protection, and both TNF receptors are involved in the protective signal transduction cascade. Finally, the available clinical attempts to antagonize TNFα in cardiovascular disease, notably heart failure, are critically discussed.

A highlight of myoglobin diversity: the nitrite reductase activity during myocardial ischemia-reperfusion

Myoglobin, famous as an important intracellular oxygen binding hemeprotein, displays a variety of functions. The first pioneering review on myoglobin was published as early as 1939, in which Millikan concluded that "muscle hemoglobin" acts primarily as a short-term dioxygen store, tiding the muscle over from one contraction to the next. Since that time, myoglobin has become one of the most widely studied proteins in a variety of research fields ranging from chemistry to medicine. Recently it was discovered that in the heart myoglobin changes its function in dependence of oxygen tension, acting as an oxygen sensor. Under normoxic conditions myoglobin plays the role of a nitric oxide (NO(*)) scavenger, protecting the heart from the deleterious effects of excessive NO(*). During hypoxia however, myoglobin changes its role from an NO(*) scavenger to an NO(*) producer. Deoxygenated myoglobin reduces nitrite to bioactive NO(*). The produced NO(*) downregulates the cardiac energy status and reduces myocardial oxygen consumption, thus protecting the heart. Myoglobin also exhibits a nitrite reductase function under further pathophysiological conditions. During myocardial reperfusion after ischemia, myoglobin - via nitrite - regulates respiration and cellular viability. This leads to a dramatic reduction of myocardial infarct size and to an improvement of myocardial function. The reaction between myoglobin and nitrite thus seems to play an imminent role in the regulation of cardiac function in physiology and pathophysiology.

The cardioprotective effect of dexmedetomidine on global ischaemia in isolated rat hearts

AIM

Dexmedetomidine is a highly specific and selective alpha-2 adrenergic agonist that is now widely used in the intensive care setting. Many intensive care unit (ICU) patients are at risk of respiratory or cardiac arrest. This study was conducted to determine whether dexmedetomidine exhibits a cardioprotective effect on global ischaemia and subsequent myocardial infarction.

METHODS

Isolated rat hearts were subjected to 30 min of global ischaemia followed by 120 min reperfusion, with administration of 0, 1 and 10nM dexmedetomidine during the pre-ischaemic period (n=7 each group). Secondly, 1 microM yohimbine, an alpha-2 antagonist, was given during the pre-ischaemic period, alone or in combination with 10 nM dexmedetomidine (n=7 each group).

RESULTS

Dexmedetomidine administration reduced coronary flow significantly (103.6+/-4.7%, 77.9+/-3.7, 63.7+/-6.1%, of the baseline values for 0, 1 and 10 nM dexmedetomidine, respectively), and yohimbine administration reversed this effect (88.0+/-2.2%). Dexmedetomidine improved the infarct size at each concentration (45.3+/-3.6, 30.2+/-3.3, and 21.2+/-2.3% of the total left ventricular mass for 0, 1, and 10nM dexmedetomidine, respectively), which was also reversed by yohimbine (43.6+/-1.4%).

CONCLUSION

Dexmedetomidine exhibited a cardioprotective effect on global ischaemia in the isolated rat heart model, which was mediated by alpha-2 adrenergic stimulation.

Hypothermia during reperfusion does not reduce myocardial infarct size in pigs

We previously described a method for regional myocardial cooling that reaches the target temperature within 4 min. The present study evaluated whether this method for regional myocardial cooling during reperfusion reduces myocardial infarct size (IS) in 75-kg pigs. Myocardial infarction was induced by inflation of an angioplasty balloon in the left anterior descendent artery for 45 min followed by 3 h reperfusion. First, 15 pigs were randomized to regional myocardial cooling during reperfusion (n = 8) or control (n = 7). As further control experiments, systemic hypothermia was induced prior to ischemia (n = 3) and during reperfusion (n = 3). IS and area at risk (AAR) were evaluated in vivo by single photon emission cardiac tomography (SPECT) and by standard histochemical staining. Regional cooling during reperfusion did not reduce IS/AAR as assessed by histochemistry (cooling: 0.71 +/- 0.8; control: 0.68 +/- 0.10; p = ns) and SPECT (cooling: 0.90 +/- 0.20; control: 0.88 +/- 0.32; p = ns). Systemic hypothermia during ischemia reduced IS/AAR (histochemistry: 0.09 +/- 0.11; SPECT: 0.25 +/- 0.22; p < 0.001 and p = 0.01 vs control, respectively). Induction of systemic hypothermia during reperfusion had no significant effect on IS/AAR (histochemistry: 0.63 +/- 0.07; SPECT: 0.74 +/- 0.09; p = ns vs control for both comparisons). In conclusion, hypothermia during ischemia is strongly myocardioprotective while hypothermia during reperfusion does not reduce myocardial infarct size in human-sized pigs.