INSERM U 465, Centre Biomédical des Cordeliers, Paris, France

Correspondence to: R Bazin, INSERM U 465, Centre Biomédical des Cordeliers, 15 rue de I^'Ecole de Médecine, 75270 Paris, Cedex 06, France. E-mail: rbazin@bhdc.jussieu.fr

BACKGROUND: In fa/fa Zucker rats, leptin receptor deficiency is responsible for both a deficit of energy expenditure and hyperphagia which lead to massive obesity and insulin resistance in adulthood. This obesity is also characterised by alterations of the -adrenergic signaling pathway.

OBJECTIVE: To determine whether alterations in -adrenergic pathway could occur at the onset of obesity when fa/fa rats are not yet hyperinsulinemic.

ANIMALS: Fourteen-day-old suckling fa/fa and Fa/fa littermates (from heterozygous lean (Fa/fa) female and homozygous obese (fa/fa) male mating).

MEASUREMENTS: Membranes were prepared from isolated adipocytes after collagenase treatment of inguinal adipose tissue. The response of adenylyl-cyclase activity to stimulation by isoprenaline, GTP-S or forskolin was studied. B_max and K_d of (₁+₂) and of ₃ adrenoceptors were measured using ³H-CGP saturation binding experiments. mRNA concentration of ₁- and ₃-AR was determined by semi-quantitative RT-PCR. G_s protein was quantified by Western blotting and Gi protein by ADP-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 entirely accounted for by fat cell hypertrophy (´2.5 in volume). In fa/fa compared to Fa/fa pups, response of adenylyl cyclase to isoprenaline was decreased two-fold but responses to GTPS or forskolin were unchanged. Density of (₁+₂) and ₃-AR was not affected by the fa/fa genotype, as well as G_s and Gi concentration.

CONCLUSION: Response of inguinal fat cells to catecholamines was decreased without any quantitative modifications of the different elements of the adenylyl cyclase cascade. This suggests an alteration in the coupling between -AR and G proteins. Due to the important increase in fat cell volume we hypothesize that changes in the physical properties of plasma membranes and/or changes in cytoskeleton-extracellular-matrix interactions could disturb the -adrenergic pathway responsiveness. In addition to the excess of lipid storage, which occurs very early at the onset of obesity, the impairment of the responsiveness to catecholamines reported in this study might worsen the obesity syndrome.

International Journal of Obesity (2001) 25, 1592-1598

₁-adrenoceptors; ₃-adrenoceptors; Gs protein; Gi protein; adenylyl cyclase; lipolysis