Endothelium-dependent coronary response is impaired in the myocardium at an early phase of post-infarct remodeling

Doppler echocardiography and tissue Doppler imaging in the healthy rabbit: differences of cardiac function during awake and anaesthetised examination

OBJECTIVE

In the past years, Doppler echocardiography has evolved into a commonly used technique. More recent sophisticated advances in imaging quality have substantially improved spatial and temporal resolution allowing the adaptation of this technique to small animal models, particularly in rabbits but even in mice. Recently, parameters obtained by Tissue Doppler Imaging (TDI) have been shown to be more independent of pre- and afterload than classic hemodynamic Doppler measurements. Exploration of animal models may require anaesthesia but there is only very little information on the effect of anaesthesia on echocardiographic parameters in rabbits.

METHODS

We therefore performed Doppler-echocardiographic examinations of 20 wild-type New Zealand White rabbits in awake state and under light ketamine-xylazine anaesthesia. Special focus was put on the evaluation of global and regional left ventricular systolic and diastolic function using TDI and the myocardial performance index (Tei-index).

RESULTS

Doppler-echocardiographic measurements including TDI in rabbits were feasible to assess cardiac morphology and function within a short examination time. There were some distinct changes of functional parameters during anaesthesia. Exemplary for systolic function, fractional shortening, cardiac output and systolic TDI velocity of the lateral wall decreased distinctly. Global left ventricular function measured by the Tei-index deteriorated.

CONCLUSIONS

Doppler echocardiography and TDI can be performed easily, quickly and safely in the rabbit. Anaesthesia with the cardiodepressive ketamine-xylazine shows some distinct Doppler-echocardiographically measurable negative effects on cardiac function. Thus, echocardiography with less cardiodepressive anaesthetic regimes or even without anaesthesia after training of the animals should be considered as alternatives whenever possible.

Role of nitric oxide during coronary endothelial dysfunction after myocardial infarction

This study aimed to investigate whether permanent ischaemia influences subacute vasodilatation responses of non-infarcted rat coronary vasculature, and to characterise these coronary changes. Ischaemia led to a significant impairment of the endothelium-dependent vasodilator response, while coronary vasodilatory capacity remained unaltered. In normal coronary circulation, nitric oxide (NO) and prostanoids contributed to vasodilatation, while basal involvement of endothelium-derived hyperpolarising factor was limited. Vasodilatory impairment following myocardial infarction did not originate from alterations in the prostanoid pathway, and only a slightly increased influence of K+ channels was observed. However, NO-mediated vasodilatation was significantly increased after ischaemia, as also confirmed by higher mRNA and protein levels of iNOS and eNOS. Additionally, the amount of superoxide was enhanced following infarction. We conclude that subacute postinfarction remodeling is accompanied by endothelial dysfunction in non-infarcted coronary arteries. Although the NO-mediated response is increased after ischaemia, its final action is restricted due to the presence of superoxide.

Does enhanced expression of the Na+-Ca2+ exchanger increase myocardial vulnerability to ischemia/reperfusion injury in rabbit hearts?

Reverse-mode activation of the Na+-Ca2+ exchanger (NCX) at the time of reperfusion following ischemia contributes to Ca2+ overload and cardiomyocyte injury. The aim of the present study was to determine whether increased NCX in the myocardium that survived after infarction enhances its vulnerability to ischemia/reperfusion injury. Rabbits were divided into post-MI and sham groups and underwent ligation of the left circumflex coronary artery and sham operation, respectively. Two weeks later, hearts were isolated and perfused with crystalloid in the Langendorff mode with monitoring of left ventricular (LV) pressure. NCX level in the myocardium was determined by Western blotting. Myocardial stunning was induced by 5 episodes of 5-min global ischemia/5-min reperfusion. Using separate groups of hearts, myocardial infarction was induced by 30-min global ischemia/2-h reperfusion with or without treatment with 0.3 microM KB-R7943, a reverse-mode selective blocker of NCX. Heart weight-to-body weight ratio was 20% larger and NCX protein level was 60% higher in the post-MI group than in the sham group. However, there were no significant differences between severities of myocardial stunning after the repetitive ischemia/ reperfusion (18 +/- 7 vs. 25 +/- 2% reduction in LV developed pressure) and between infarct sizes after 30-min ischemia (59.1 +/- 4.1 vs. 63.0 +/- 4.5% of risk area) in the post-MI and sham groups. KB-R7943 limited infarct size in the post-MI group by 53%, and the extent of this protection was not different from that we have reported for hearts without previous infarcts (i.e. 45% reduction of infarct size). These results suggest that enhanced NCX expression does not necessarily increase myocardial vulnerability to myocardial stunning and infarction.

Blockade of angiotensin II type 1 receptors suppressed free radical production and preserved coronary endothelial function in the rabbit heart after myocardial infarction

The hypothesis that blockade of angiotensin II type 1 (AT1) receptors after myocardial infarction prevents coronary endothelial vasomotor dysfunction by suppressing oxygen free radical production was examined. Rabbits underwent coronary ligation or a sham operation with or without infusion of valsartan, an AT 1 receptor blocker. Two weeks after the operation, the heart was isolated from each rabbit and perfused with buffer in the Langendorff mode, and coronary flow responses to acetylcholine and sodium nitroprusside were assessed. The ratio of heart weight to body weight and the lipid peroxide level in the myocardium were increased by 30 and 50%, respectively, 2 weeks after infarction. The coronary flow response to acetylcholine (10(-8) to 10(-5) M) was reduced by 50% in the hearts with infarction compared with the sham controls, although coronary flow responses to sodium nitroprusside were similar. The coronary flow response to acetylcholine in the hearts with infarction was restored by concurrent infusion of N -2-mercaptopropionyl-glycine, a free radical scavenger. Valsartan (10 mg/kg/d) infused after infarction prevented both ventricular remodeling and elevation of the tissue lipid peroxide level and preserved coronary flow response to acetylcholine. In conclusion, long-term AT1 receptor blockade after infarction protects the coronary arteries from endothelial vasomotor dysfunction through suppression of free radical production.