| Abstract: | In adipocytes, adenylate cyclase is positively regulated by beta-adrenergic agents and negatively regulated by adenosine. Incubation of adipocytes with adenosine deaminase relieves the inhibition of adenylate cyclase by destroying the adenosine that the cells release into the medium. When adipocytes are incubated with adenosine deaminase and the beta-adrenergic agent isoproterenol, most of their ATP is converted to AMP in 5 min. Either isoproterenol or adenosine deaminase alone has little or no effect. In the additional presence of the phosphodiesterase inhibitor 4-(3-butoxy-4-methoxybenzyl)imidazolidin-2-one (Ro 20-1724) cAMP accumulates instead of AMP. Under these conditions, cAMP represents 40-50% of the total intracellular adenine nucleotides, and ATP only 5%. N6-(L-2-phenylisopropyl)adenosine, a deaminase-resistant adenosine agonist, prevents beta-adrenergic stimulation. 8-(p-Sulfophenyl)theophylline and 3-isobutyl-1-methylxanthine are both adenosine antagonists that can replace the deaminase in permitting beta-adrenergic stimulation of adenylate cyclase, but only the latter also inhibits the phosphodiesterase and causes accumulation of cAMP. When the ATP-depleted adipocytes are washed with fresh medium, the nucleoside triphosphate level can be restored within 5 min. The ATP-restored adipocytes can respond rapidly to a second dose of isoproterenol and adenosine antagonist. These findings point out the important role of adenosine in controlling adenylate cyclase activity and the possible involvement of adenylate cyclase in the control of energy flow in rat adipocytes. |
