Initiation of cardiac hypertrophy in response to thyroxine is not limited by age

We examined the role of age in the initiation of thyroxine-induced (T4) cardiac hypertrophy. T4 (0.4 mg/kg sc) was administered to prepubescent (2 mo), young adult (6 mo), and senescent (24 mo) Fischer 344 rats for 4 days. While significant increases in left ventricular (LV) mass and RNA/LV were evident at 4 days in all T4-treated groups, the elevation in RNA/LV occurred earlier (2 days) in the senescent group. A 0.2-mg/kg dose of T4 elevated RNA values within 24 h in senescent, but not in prepubescent, rats. LV norepinephrine levels were measured to determine whether it plays a role in this model of cardiac hypertrophy. When synthesis of this catecholamine was blocked with alpha-methyl-p-tyrosine, tissue levels fell significantly in all groups, and the decrement was similar in T4-treated and control rats in the two younger groups. We conclude that: 1) the initiation of T4-induced cardiac hypertrophy is not compromised in senescent rats, 2) hearts of senescent rats respond earlier and to a lower dose of T4 than young rats, and 3) the cardiac hypertrophy that occurs in hyperthyroidism is not due to enhanced levels of available cardiac norepinephrine.