EARLY ALTERATIONS IN THE BROWN ADIPOSE-TISSUE ADENYLATE-CYCLASE SYSTEM OF PREOBESE ZUCKER RAT FA FA PUPS - DECREASED G-PROTEINS AND BETA-3-ADRENOCEPTOR ACTIVITIES/

This study was undertaken to determine whether receptor and non-recept or components of the adenylate cyclase (AC) cascade were altered in br own adipose tissue (BAT) of 14-day-old preobese (fa/fa) rats, before e ndocrine status is strongly modified by fa gene expression. Activity o f the AC catalytic subunit did not differ between the two genotypes. I n fa/fa rats compared with control Fa/fa rats, there was a 50 % decrea se in the activity of alpha G(s) (stimulated by NaF or guanosine 5'-[g amma-thio]triphosphate) but no change in protein content (Western blot ting). alpha G(i) function, assessed by the inhibitory action of low c oncentrations of guanosine 5'-[beta gamma-imido]triphosphate upon 10(- 4) M forskol-instimulated AC activity, was equally low in both genotyp es. Analysis of dose-response curves for different beta-agonists revea led that (i) both the basal and the maximally stimulated activity of A C were 2-fold lower in fa/fa rats than in Fa/fa rats; (ii) BRL37344 an d CGP12177 (beta 3 agonists) were less potent in fa/fa than in Fa/fa r ats (K-aet. multiplied by 2); (iii) noradrenaline and isoprenaline (Is o), at the low-affinity site (beta 3-AR), were less potent in fa/fa th an in Fa/fa pups (K-act. increased by 30 and 20 % respectively). At th e high-affinity site (mainly beta(1)) these two agonists were more pot ent in fa/fa than in Fa/fa rats (K-act. decreased by 40 and 80 % respe ctively). In good agreement with the latter result, the beta 1-adrener gic receptor (beta 1-AR)-selective antagonist CGP20712A had more effec t on the Iso-stimulated AC activity in pre-obese than in lean pups (2- fold decreased in IC50). Binding experiments with [H-3]CGP12177 show t hat in BAT of suckling rats, beta 3-ARs represent 80 % of the total be ta-ARs. B-max. values for the two sites were not affected by the genot ype, although the beta 3-AR mRNA concentration in BAT (quantitative re verse-transcriptase PCR) was 3-fold lower in fa/fa rats than in Fa/fa pups. In conclusion, these results provide evidence for alterations in beta 1- and beta 3-AR signalling in BAT of 14-day-old suckling pre-ob ese Zucker rats with a decreased activity of alpha G(s). The impaired AC responsiveness to catecholamines might be a primary contributor to the development of this genetic obesity.