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.
Tomanek
R J
RJ
Department of Anatomy, University of Iowa, Iowa City 52242.
Butters
C A
CA
Zimmerman
M B
MB
D000375:Aging / Q000502:physiology*; D000818:Animals; D004247:DNA / Q000032:analysis; D004195:Disease Models, Animal; D004305:Dose-Response Relationship, Drug; D006321:Heart / Q000502:physiology; D006439:Hemodynamics / Q000502:physiology; D017379:Hypertrophy, Left Ventricular / Q000139:chemically induced* / Q000503:physiopathology; D008297:Male; D009206:Myocardium / Q000737:chemistry / Q000473:pathology; D009638:Norepinephrine / Q000032:analysis / Q000502:physiology; D051381:Rats; D011916:Rats, Inbred F344; D013974:Thyroxine / Q000009:adverse effects*
D004247:DNA; D013974:Thyroxine; D009638:Norepinephrine