Adipocytes contain adenosine receptors, termed A1 receptors, which inhibit lipolysis by decreasing adenylate cyclase activity. The inhibition of lipolysis by adenosine agonists in vivo acutely suppresses the plasma concentrations of free fatty acids (FFA) and triglycerides. We have found that infusions of the adenosine receptor agonist phenylisopropyladenosine (PIA) initially decreases plasma FFA concentrations; however, with prolonged exposure (6 d), rats become very tolerant to the effects of the drug. Adipocytes isolated from epididymal fat pads from PIA-infused rats have altered lipolytic responses. When lipolysis is stimulated with a relatively high concentration of isoproterenol (10(-7) M), PIA does not inhibit lipolysis in adipocytes from the infused animals. However, PIA inhibits isoproterenol-stimulated cyclic AMP (cAMP) accumulation in adipocytes from the infused rats although with decreased sensitivity compared with controls. The explanation for the impaired antilipolytic effect appears to be due to the fact that isoproterenol-stimulated cAMP accumulation is markedly increased in cells from infused rats. Indeed, basal lipolysis and lipolysis stimulated with lower concentrations of isoproterenol (10(-9), 10(-8) M) are effectively inhibited by PIA. cAMP accumulation is greatly increased in adipocytes from infused rats when stimulated by isoproterenol, ACTH, and forskolin. The results have some striking analogies to changes induced in nerve cells by prolonged exposure to narcotics. These data suggest that tolerance to PIA develops in adipocytes as a consequence of enhanced cAMP accumulation.
B B Hoffman, H Chang, E Dall'Aglio, G M Reaven
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