Age-related increase of β1-adrenergic receptor gene expression in rat liver: a potential mechanism contributing to increased β-adrenergic receptor density and responsiveness during aging

Abstract

In this study we examined whether the levels of gene expressions of the three β- adrenergic receptor (βAR) subtypes, β1, β2, and β3, contribute to age-related increase in βAR density. Liver membranes and total RNA were prepared from young (4- to 6-month-old) and old (24-month-old) male Fischer 344 rats. βAR density (Bmax) in liver membranes was measured by a radioligand receptor binding assay using the receptor subtype nonselective βAR antagonist 125I-pindolol as the radioligand. Steady-state levels of β2AR mRNA in rat liver were measured by Northern blot analysis; because of the low abundance of β1AR and β3AR mRNA in rat liver, the expressions of these genes were measured by a semiquantitative RT-PCR or an RT-PCR. Scatchard analysis of saturation binding curves of the binding assay confirmed an age-related increase in Bmax (young: 7.1 ± 0.8 fmol/mg protein vs. old: 18.1 ± 4.3 fmol/mg protein). No age-related differences were found in the levels of β2AR mRNA. However, semiquantitative RT-PCR revealed an approximately twofold increase in β1AR mRNA level between young and old rats (P < 0.05). β1AR mRNA levels were also correlated with Bmax values for 125I-pindolol binding sites in individual rats (r = 0.67; P = 0.012). β3AR mRNA, which was demonstrable in rat white adipose tissue by RT-PCR, was generally not detected in livers from young or old rats, with the exception of two old rats with the highest Bmax. These results suggest that an age-related increase of β1AR gene expression contributes to increased βAR density and β adrenergic responsiveness in rat liver during aging.