Impaired beta-adrenergic signaling pathway in white adipocytes of sucklingfa/fa Zucker rats: a defect in receptor coupling

BACKGROUND: In fa/fa Zucker rats, leptin receptor deficiency is responsible for both a deficit of energy expenditure and hyperphagia which lead to mas sive obesity and insulin resistance in adulthood. This obesity is also char acterised by alterations of the beta -adrenergic signaling pathway. OBJECTIVE: To determine whether alterations in beta -adrenergic pathway cou ld occur at the onset of obesity when fa/fa rats are not yet hyperinsulinem ic. ANIMALS: Fourteen-day-old suckling fa/fa and Fa/fa littermates (from hetero zygous lean (Fa/fa) female and homozygous obese (fa/fa) male mating). MEASUREMENTS: Membranes were prepared from isolated adipocytes after collag enase treatment of inguinal adipose tissue. The response of adenylyl-cyclas e activity to stimulation by isoprenaline, GTP gamma -S or forskolin was st udied. B-max and K-d of (beta (1) + beta (2)) and of beta (3) adrenoceptors were measured using H-3-CGP saturation binding experiments. mRNA concentra tion of beta (1) and beta (3)-AR was determined by semi-quantitative RT-PCR . G(s)alpha protein was quantified by Western blotting and Gi protein by AD P-ribosylation. RESULTS: Despite an almost normal body weight, inguinal fat pad weight was increased two-fold by the expression of fa mutation. This increase was enti rely accounted for by fat cell hypertrophy (x2.5 in volume). In fa/fa compa red to Falfa pups, response of adenylyl cyclase to isoprenaline was decreas ed two-fold but responses to GTP(gamma)S or forskolin were unchanged. Densi ty Of (beta (1) + beta (2)) and beta (3)-AR was not affected by the fa/fa g enotype, as well as G(s)alpha and Gi concentration. CONCLUSION: Response of inguinal fat cells to catecholamines was decreased without any quantitative modifications of the different elements of the ade nylyl cyclase cascade. This suggests an alteration in the coupling between beta -AR and G proteins. Due to the important increase in fat cell volume w e hypothesize that changes in the physical properties of plasma membranes a nd/or changes in cytoskeleton - extracellular-matrix interactions could dis turb the beta -adrenergic pathway responsiveness. In addition to the excess of lipid storage, which occurs very early at the onset of obesity, the imp airment of the responsiveness to catecholamines reported in this study migh t worsen the obesity syndrome.