Characterization of Cyclic AMP Efflux from Swine Adipocytes In Vitro

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Abstract
Objective: A variety of cell types transport cyclic AMP (cAMP) to the extracellular fluid; the purpose of this study was to determine if and how this process occurs in adipocytes. Research Methods and Procedures: Adipocytes were isolated from 3‐month‐old swine and incubated with stimulators of adenylate cyclase for 2 to 120 minutes to promote cAMP synthesis and efflux. Efflux was characterized in the presence of agents that inhibit ATP production, anion transport, intracellular cAMP metabolism, and extracellular cAMP metabolism. Extracellular cAMP was measured by enzyme immunoassay, then corrected for cell lysis by measuring lactate dehydrogenase release. Results: cAMP efflux averaged 24.7 fmol/min/cm2 adipocyte surface area, was linear for 2 hours, and was proportional to adipocyte surface area (r = 0.94, pp(p. Extracellular cAMP levels more than doubled by the addition of 1 μM 1,3‐dipropyl‐8‐p‐sulfophenylxanthine, a purported inhibitor of extracellular phosphodiesterase. Discussion: Our data demonstrate that cAMP is transported from swine adipocytes by an energy‐dependent anion transporter and can be metabolized extracellularly. Future studies will evaluate extracellular cAMP as a potential source of extracellular adenosine, a potent inhibitor of adipocyte lipolysis.