Effect of age on hypertrophy of the rat heart following infarction.

To separate the physiologic process of growth from the pathologic process of hypertrophy in rat hearts with myocardial infarction, we compared 32 young, actively-growing rats (initial age 7 weeks) with 45 mature, slowly-growing rats (initial age 18 weeks). Animals were sacrificed five weeks after induction of infarction. During this period, young animals had a 58% increase in body weight and mature animals an 8% increase (p less than 0.001). Cardiac hypertrophy was directly demonstrated by an increase in cell diameter, in both young and mature animals with more than 15% of the left ventricle infarcted (8.5 +/- 0.8 micron in controls vs. 10.2 +/- 1.4 micron in infarcted rats, p less than 0.01). Hypertrophy was further indicated by constant left ventricular weight despite myocardial loss due to infarction, and by constant viable tissue volume estimate ("remaining myocardial area"). Because of hypertrophy the total amount of remaining myocardial area was constant across subgroups with and without infarction (p greater than 0.2). Therefore, cardiac hypertrophy following infarction in rats is not dependent upon age.