Effects of age and endurance training on β-adrenergic receptor characteristics in Fischer 344 rats

The purpose of this investigation was to examine changes in β-adrenergic receptor characteristics in various tissues with age and endurance training. Forty-eight young (6 months), middle-aged (15 months), and old (25 months) male Fischer 344 rats were assigned to either a trained or sedentary running group. Animals were endurance trained by 10 weeks of treadmill running at 75% maximal capacity, 1 h/day, 5 days/week. Animals were sacrificed at rest and the heart, liver, and soleus were removed for analysis. Percent of high and low affinity binding sites were determined by competitive binding experiments. Competition curves were generated using 12 concentrations of ICI-89406 (β₁ antagonist) and ICI-118551 (β₂ antagonist) to inhibit the total binding of (-) [¹²⁵I] pindolol (IPIN). Maximal binding site number (B_max) and affinity (K_d) were determined by Scatchard analysis. Heart B_max did not differ with age or training. An aging effect was observed in liver such that middle-aged and old animals had greater B_max compared to young animals. In soleus, B_max was not altered with training but decreased with age. While training had no affect on affinity in the liver and soleus, heart affinity increased with training in both the middle-aged (21%) and old (27%) animals. In soleus, affinity increased but remained unaltered in heart and liver with age. The ratio of β₁:β₂ receptors in the heart and liver did not differ with age or training. The influence of age and training on β-adrenergic receptor characteristics appear to be tissue specific.