Chronic Heart Rate Reduction by Ivabradine Does Not Modify the Elastic Fiber Content in Post‐Myocardial Infarction Scar of Middle‐Aged Rats

Abstract

A large myocardial infarction (MI) triggers progressive cardiac remodeling which results in left ventricular (LV) chamber dilatation and, as a consequence, in greater wall tension, particularly in the area of scar formation. Chronically elevated mechanical stress applied to fibroblasts and myofibroblasts within the scar has been known to stimulate the production of extracellular matrix proteins, predominantly collagen and elastin, that improves the tensile strength of the thinning LV wall. Considering that during a prolonged diastole the end‐diastolic volume and, hence, the force and duration of mechanical signals transduced into the scar become even more augmented it is feasible to assume that any heart rate‐lowering agent would promote further deposition of extracellular proteins by activated fibrogenic cells. Nevertheless, we have previously shown that chronic heart rate reduction (HRR) by ivabradine (IVA), a selective inhibitor of a pacemaker If current, did not alter the content of fibrillar collagen in post‐MI scar of middle‐aged rats. However, whether IVA‐induced HRR affects the content of elastic fibers in the scar remains undetermined. To test this hypothesis a large transmural MI (greater than 50% of the LV free wall) was induced in 12‐month‐old male Sprague‐Dawley rats by ligation of the left coronary artery. Post‐MI rats were then assigned in IVA‐treated (MI+IVA) and corresponding placebo (MI) groups. In MI+IVA groups, rats were treated with IVA i.p. via osmotic pumps in a dose of 10.5 mg/kg/d for 8 and 12 weeks. At the end of each treatment period, the hearts were collected and processed into paraffin‐embedded sections for histological evaluations. Eight‐micron‐thick sections of the left ventricle were stained with picrosirius red and Verhoeff's elastic tissue stains. The entire scar of each heart stained for collagen and elastic fibers were digitally captured on a computer using an Olympus BX53 microscope and the fractional volumes of collagen and elastic fibers were determined by using Image‐Pro Analyzer v.7.0 software. We found that the mean scar size and thickness were comparable among all groups of rats. Furthermore, the spatial distribution of the elastic fibers within the scars demonstrated almost identical pattern with an evident accumulation along the subendocardial and subepicardial regions, in the remnants of coronary vessels, and in isolated areas near the edge or throughout the middle portion of the scar. In addition, IVA‐treated and untreated rats showed a similar tendency in reduction of elastic component of the scars that was directly correlated with scar maturation. The quantitative analysis has revealed that the content of elastic fibers remained relatively analogous in scars of MI and MI+IVA rats 8 weeks (1.88±0.36% vs. 1.21±0.31%, respectively) and 12 weeks (0.99±0.45% vs. 0.89±0.18%, respectively) after MI. Moreover, there was no significant difference in ratio between elastic and collagenous components of the scar in corresponding IVA‐treated and untreated groups of rats. Our data are the first to demonstrate that chronic IVA‐induced HRR does not modify the elastic fiber content in post‐MI scar.