Time dependent changes in myocardial norepinephrine concentration and adrenergic receptor density following X-irradiation of the rat heart.

The hearts of 9 to 12-weeks-old Sprague-Dawley rats were locally irradiated with a single dose of 20 Gy. The effects on myocardial norepinephrine concentrations and on alpha-adrenergic and beta-adrenergic receptor densities was examined up to 16 months post-treatment. Myocardial norepinephrine concentrations were reduced (to 50% of control values between 8 and 16 months) after irradiation. Receptor binding studies using radioactive ligands demonstrated that alpha-adrenergic receptor density was increased to maximally 210% of control values and that beta-adrenergic receptor density was increased to maximally 150% of control values, both measured at 8 months posttreatment. The affinities of both receptor types were not changed after irradiation. An inverse correlation was found between the myocardial norepinephrine concentration and the alpha-adrenergic receptor density. Myocardial norepinephrine concentration was not correlated to the beta-adrenergic receptor density. The changes in myocardial norepinephrine concentration and receptor density observed after irradiation suggest that even 16 months after irradiation overt cardiac failure was not occurring as the radiation-induced alterations differ considerably from those reported for failing hearts.