COMPENSATED CORONARY MICROVASCULAR GROWTH IN SENESCENT RATS WITH THYROXINE-INDUCED CARDIAC-HYPERTROPHY

We tested the hypothesis that coronary angiogenesis in response to chr onic thyroxine (T-4) treatment is not limited by age. Male Fischer 344 rats aged either 8 (young adult) or 24 (senescent) mo were studied af ter receiving either L-thyroxine (0.2 mg/kg sc) or vehicle for 2 mo. H eart weight-to-body weight ratio, compared with age-matched controls, increased by 47 and 44% in 8- and 24-mo T-4 groups, respectively. Maxi mal myocardial perfusion per unit mass, measured in diastole-arrested, maximally dilated, isolated hearts, was similar in T-4 rats and their age group controls; however, flow tended to be lower in senescent tha n in young adult rats. Thus the cross-sectional area of the coronary v essels grew in proportion to the increase in cardiac mass. Morphometri c analyses, based on image analysis, showed that capillary length dens ity was slightly lower in the midmyocardium but not the epimyocardium of the 24-mo T-4 group compared with their age group controls. However , volume density, surface density, and intercapillary distance were no t influenced by T-4 treatment and the presence of cardiac hypertrophy. We conclude that in this model of cardiac hypertrophy 1) coronary ves sel growth parallels the increase in ventricular mass, 2) capillaries grow by proliferation and an increase in diameter, and 3) vascular gro wth is not notably compromised during senescence.