Characterization of adrenergic receptor binding in rat lung: physiological regulation

The ontogeny, circadian rhythm and sex differences of adrenergic receptors were studied in a crude particulate fraction prepared from rat lung. The density of alpha-2 adrenergic receptors ([3H]yohimbine binding sites) decreased rapidly with age from 304 fmol/mg of protein on the 1st day to 123 fmol/mg after the 1st week and then to an undetectable level at 5 weeks of age. Alpha-1 adrenergic receptors ([3H]prazosin binding) showed a moderate variation in density, with the greatest density at the 3rd week of age (311 fmol/mg). The density at birth (194 fmol/mg) was higher than the adult (126 fmol/mg). Beta adrenergic receptors ([3H]dihydroalprenolol binding) demonstrated changes in density with a sharp rise in the 3rd week (to 813 fmol/mg from 315 fmol/mg at birth), which was maintained through the 10th week. Unlike receptor densities, the affinity constants were not significantly altered during postnatal development. Alpha-1 and beta receptors in the adult rat lung did not show any significant changes in their densities or their affinities in the two other physiological variables studied (circadian rhythm and sex). During postnatal development, the highest concentration of norepinephrine in the rat lung is reported to be at about 3 weeks of age, which correlates with the changes in both alpha-1 and beta adrenergic receptors, but not alpha-2 receptors. The presence of alpha-2 receptors in the lungs of young, but not mature animals, may suggest an important developmental role, which is not yet understood.