Remodeling of coronary vessels during aging in purebred beagles

Abstract

We compared six young (1 year) and six senescent (11 years) purebred beagles to determine the effects of aging on the coronary microvasculature. The hearts were perfusion-fixed in vitro, and myocardial specimens were subjected to microscopic image analysis. Absolute left ventricular mass increased by 55% with age, while cardiocyte cross-sectional area increased by 10% and 30% in the midmyocardium and endomyocardium, respectively. Although capillary numerical density was lower in the senescent dogs (16% in midmyocardium and 19% in endomyocardium), volume density was similar in the two groups because capillary diameter increased significantly with age in both regions. Capillary length density was reduced by 27% with age in the endomyocardium of the old beagles. Wall/lumen ratios of arterioles of three size classes (≤15, 16-25, and 26-50 μm) were found to be nearly identical for the two age groups. Mean arteriolar diameter increased within the smallest size class in both ventricular regions. In contrast, a twofold or greater increase in wall thickness with age occurred in the left anterior descending coronary artery and its first branch, which was mainly due to expansion of the medial interstitium. The connective tissue fraction of the myocardium was significantly higher in the senescent than in the young in the epimyocardium and midmyocardium but not in the endomyocardium. These data provide evidence for three conclusions regarding aging of coronary vessels in beagles. First, a decline in capillary length density is limited to the endomyocardium. Second, capillary volume density in the aged beagles is maintained by increases in capillary diameter; the latter minimizes a decrement in anatomic intercapillary distance. Third, large coronary arteries undergo substantial medial thickening but no luminal narrowing, and the arterioles are not adversely affected. Accordingly, this species appears to be a good model of coronary vessel aging because senile alterations are minimal.