Effects of sodium saccharin diet on fat-cell lipolysis: evidence for increased function of the adenylyl cyclase catalyst.

OBJECTIVE : To evaluate the effect of a 14 days' sodium saccharin (NaS) diet on lipolysis and cyclic-AMP accumulation in isolated rat white epididymal adipocytes. ANIMALS : Male Wistar rats (3 weeks old) were fed, for 14-days, ad libitum with a regular diet supplemented with or without 1-5% NaS dissolved in drinking water. MEASUREMENTS : Lipolysis and cAMP accumulation were assessed on isolated adipocytes. Adenylyl cyclase activities were measured on membrane fractions prepared from isolated adipocytes. The levels of Gs and Gi proteins were determined by Western blot analysis using specific antisera. RESULTS : Only high dietary NaS (5%) affected significantly the body growth and food consumption. Feeding rats with 2.5% of NaS increased both basal and isoproterenol-stimulated lipolysis and cAMP production. A 14-days diet of rats with 2.5% aspartame did not reproduce the effects of NaS on lipolysis and cAMP production. In fat-cell membranes of 2.5% NaS-treated rats, basal and stimulated-adenylyl cyclase activities were increased by 200% whatever the agonists used : GTP, GTP[S], [AlF 4 ] - , isoproterenol or forskolin in the presence of Mg 2+ or Mn 2+ with or without GDP[S]. These effects cannot be explained by modifications of the expression of Gs and Gi proteins. The level of Gs α subunits was not affected by NaS treatment while the level of Giα 1/2 was slightly increased. The stimulatory effect of NaS on adenylyl cyclase activity appears to be specific to adipocyte when compared with thyroid, brain or heart membrane fractions. CONCLUSION : Based on these data, and on the fact that cAMP regulates the lipolytic rate, we conclude that NaS diet increases lipolysis and cAMP formation in fat-cells by modifying the activity of the adenylyl cyclase catalyst(s).