AbstractBeta‐adrenergic receptor number and receptor affinity for isoproterenol were assessed at various in vitro ages of the human diploid fibroblast cell line IMR‐90. From population doubling level (PDL) 33 to 44, there was a positive correlation between beta‐adrenergic receptor density and PDL (r = 0.709). Beta‐adrenergic receptors, assessed by Scatchard analysis of [125I]‐iodocyano‐pindolol (ICYP) binding, increased from 15 fmol/mg protein at PDL 33 to 36 fmol/mg protein at PDL 44. In contrast, from PDL 44 to 59, there was a negative correlation between beta‐adrenergic receptor density and PDL (r = −0.768). Receptor density declined to 12 fmol/mg protein at PDL 59. When the density of beta‐adrenergic receptors was expressed as receptor per cell, the findings were similar. Receptor agonist affinity for isoproterenol was determined from Hill plots of [125I]‐ICYP competition with isoproterenol. There was no change in the dissociation constant for isoproterenol with in vitro age. In humans, serum norepinephrine concentrations increase with age. This increase in serum norepinephrine may be partially responsible for the decreased betaadrenergic receptor‐agonist affinity observed with age in human lymphocytes and rat heart and lung. Similar changes in receptor‐agonist affinity are observed in rat heart and human lymphocytes following exposure to betaagonists and are part of the desensitization process. The present findings are consistent with the hypothesis that the decreases in receptor agonist affinity in rat and man with age are secondary to increases in catec
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