Increased concentrations of proteins Gi1 and Gi2 in adipocytes from aged rats alter the sensitivity of adenylyl cyclase to inhibitory and stimulatory agonists

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We used a series of antipeptide antisera to estimate the relative amounts of G proteins in adipocytes from young lean versus aged obese Sprague-Dawley rats. Western blots were analyzed using antisera that recognize (1) the alpha-subunits of Gi1 and Gi2 (serum SG1), (2) the alpha-subunit of Gi3 (serum I3B) (3) two forms of the alpha-subunit of Gs (serum CS1), and (4) forms of the beta-subunits common to all the G proteins (serum BN2). Adipocyte membranes from aged rats contained approximately fivefold to sixfold more alpha i1 and alpha i2 than those from young rats, but almost equal amounts of alpha i3. Membranes from aged rats had a modestly higher (50%) amount of a 43-kd and normal amounts of a 47-kd form of alpha s. Membranes from old rats also had approximately a threefold higher amount of beta-subunits, consistent with increased concentrations of some of the G proteins but not others. Finally, the functional consequences of these differences in G proteins was investigated by measuring the effect of N6-phenylisopropyl adenosine ([PIA] an A1-adenosine receptor agonist) and isoproterenol on adenylyl cyclase activity. Adenylyl cyclase was more sensitive to inhibition by PIA in membranes from old rats than from young rats, but was less sensitive to stimulation by isoproterenol, suggesting that the differences we observed are functionally active G proteins. These findings may account for the altered sensitivity of adipocytes from old rats to antilipolytic and lipolytic hormones.