Effects of Adenosine on Lipolysis in Human Subcutaneous Fat Cells*

Adenosine inhibited isoproterenol-stimulated lipolysis in isolated human adipocytes. At a 1-microM stimulator concentration, half-maximal inhibition was observed at about 0.3 microM adenosine. The addition of adenosine deaminase to the incubation medium enhanced the stimulation of lipolysis by 1 microM isoproterenol; the enzyme was also lipolytic by itself. The combined effect of adenosine deaminase and isoproterenol was strongly inhibited by 250 nM N6-(phenylisopropyl)adenosine; the latter compound cannot be deaminated by adenosine deaminase. This inhibition was almost totally overcome by increasing the concentration of the catecholamine to 10 microM. These results suggest that human fat cells excrete adenosine, and this nucleoside reversibly inhibits the stimulation of lipolysis by catecholamines.