AJP - Endocrinology and Metabolism, Vol 243, Issue 4 305-E309, Copyright © 1982 by American Physiological Society
Catecholamine-thyroid hormone interaction on myocardial ornithine decarboxylase
E. W. Chideckel, S. J. Rosovski and E. B. Belur
Myocardial phosphorylase alpha activity responds to stimulation by
catecholamines and thyroid hormone. In hyperthyroidism this enzyme is
supersensitive to beta-adrenergic stimulation and blockade, indicating that
its increased activity is an indirect effect of thyroid hormone. Myocardial
ornithine decarboxylase (ODC) activity also responds to catecholamine and
thyroid hormone stimulation. In the present studies, we sought to determine
whether ODC shares the responses of phosphorylase alpha in hyperthyroidism.
As opposed to euthyroid rats, isoproterenol acutely inhibited myocardial
OCD activity in hyperthyroid rats. Timolol (60 mg/kg) injected immediately
before the isoproterenol blocked this paradoxical inhibitory effect,
defining it as beta-adrenergic. When timolol (100 mg/kg), distributed over
a 24-h period, was administered during the 3 days of triiodothyronine (T3)
administration, it blocked the T3 stimulation of myocardial OCD activity by
35%. However, timolol affected weight gain of the hyperthyroid rats. When
fasted rats were used, timolol was without effect on T3-induced myocardial
ODC stimulation. Timolol was also without effect on T3-induced stimulation
of hepatic ODC or on T3-induced cardiomegaly. Timolol did decrease the
T3-induced tachycardia. In summary, in the hyperthyroid heart, 1)
isoproterenol paradoxically inhibits myocardial ODC activity and 2)
timolol, when food intake is not a variable, is without effect. We conclude
that the effect of thyroid hormone on myocardial ODC is not mediated by
change in catecholamine sensitivity. Thus the behavior of phosphorylase
alpha does not represent a general enzymatic phenomenon.
