Rat adipocyte lipoprotein lipase activity is inhibited by theophylline and stimulated by inosine through adenosine-independent mechanisms

Incubation of rat adipose tissue or isolated rat adipocytes with high (50 mM) but not with low concentrations (0.5 mM) of theophylline results in a decrease of lipoprotein lipase (LPL) activity. This effect is not altered by the addition of adenosine deaminase, indicating that the decrease of adipose LPL activity by theophylline is not due to the competition of theophylline with adenosine. On the contrary, incubation of isolated fat cells with adenosine (0.1 – 100 μM) results in an increase of the intracellular form of LPL activity. As this effect is also observed in cells incubated with adenosine deaminase (40 mU/ml) or with inosine (0.1 – 100 μM) but not in cells incubated with the adenosine analog N⁶-phenylisopropyladenosine, it is concluded that the increase in the intracellular form of LPL found after incubation with adenosine is not due to adenosine $\text{per se}$ but to inosine generated from the breakdown of endogenous adenosine by adenosine deaminase.