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 prepubesc ent (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 eleva tion 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 no t in prepubescent, rats. LV norepinephrine levels were measured to det ermine whether it plays a role in this model of cardiac hypertrophy. W hen synthesis of this catecholamine was blocked with alpha-methy-p-tyr osine, tissue levels fell significantly in all groups, and the decreme nt was similar in T4-treated and control rats in the two younger group s. We conclude that 1) the initiation of T4-induced cardiac hypertroph y is not compromised in senescent rats, 2) hearts of senescent rats re spond earlier and to a lower dose of T4 than young rats, and 3) the ca rdiac hypertrophy that occurs in hyperthyroidism is not due to enhance d levels of available cardiac norepinephrine.