Increased beta-adrenergic receptors in the light-density membrane fraction in lungs from senescent rats

There is a dynamic balance between cell surface and intracellular beta-adrenergic receptors. To investigate whether this balance is altered with aging, we determined the number of beta-adrenergic receptors in the heavy- (48,000 x g) and light-density (200,000 x g) membrane fractions from lungs and hearts of 4-, 13-, and 25-month-old rats. There were no differences in the density of receptors with age in the heavy membrane fraction. However, the density of receptors in the light-density fraction from lungs increased with age (109 +/- 7, 230 +/- 43, and 225 +/- 36, F(2,12) = 4.4, p less than .05). Receptors in the heavy membrane fraction expressed both high and low affinity binding for isoproterenol, whereas receptors in light-density fraction expressed only low affinity binding. In contrast to the lung, there were no changes in the number of receptors with age in the light-density membrane fraction from hearts. These data indicate there are increased numbers of beta-adrenergic receptors in the light membrane fraction in lungs from aged rats, which are separated from the other components of the adenylate cyclase complex.