Lack of an effect of age on β-adrenoceptor-mediated lipolysis in isolated human adipocytes

Abstract

We examined the effect of age on the β-adrenoceptor response in adipocytes that contain both β1- and β2-adrenoceptors. Twelve healthy young and 12 old subjects on a 150 mEq/24 h sodium diet underwent gluteal fat biopsies. Isolated adipocytes from all the subjects were stimulated with increasing concentrations of isoproterenol for glycerol release. In 13 of the subjects (7 young and 6 old), we also performed β-adrenoceptor binding studies on adipocyte membranes. In addition in eight subjects (four young and four old), we also utilized a competitive binding assay to calculate the percent of β-adrenoceptors that were of the β1 subtype. Our data showed that old subjects, when treated under identical conditions as the young subjects prior to fat biopsy, did not demonstrate any differences in the β- adrenoceptor stimulated tipolysis. The V(max) of lipolysis was 10.6 ± 1.4 nmoles glycerol/mg lipid/2 h in the young group and 9.9 β 1.1 in the old group. The concentrations of isoproterenol that resulted in V(max) and 1/2 V(max) were also the same in the two age groups. The addition of 8- cyclopentyl-1,3-dimethylxanthine (CPT), a specific A1-adenosine receptor antagonist, resulted in mild but equivalent enhancement of glycerol release in both age groups. The β-adrenoceptor numbers and affinities from adipocyte membranes did not demonstrate age-related differences either (B(max) 106 ± 17 fmol/mg of protein in the young, and 137 ± 27 in the old; K(d) 79.6 ± 21.3 pM in the young and 81.9 ± 16.6 in the old). The percent of β1- adrenoceptors on the adipocyte membranes was also similar in the two age groups (35.2 ± 2.6% in the young; 37.1 ± 4.5% in the old). In conclusion, we could not demonstrate any differences in the β-adrenoceptor responses from peripheral adipocytes that contain both β1 and β2-adrenoceptors, in a group of healthy elderly and young subjects who were subjected to identical dietary and orthostatic conditions prior to the biopsy. These data suggest that neither β1- nor β2-adrenoceptor responses in human adipocytes show significant changes due to aging.