Activation and inhibition of lipolysis in isolated fat cells by various inhibitors of cyclic AMP phosphodiesterase

Summary The effects of methylxanthines, papaverine, dipyridamole and imipramine on lipolysis and phosphodiesterase activity of rat adipose tissue were investigated. Lipolysis in isolated fat cells was stimulated by theophylline and caffeine whereas papaverine, dipyridamole and imipramine had no substantial effect on the basal lipolytic rate. Lipolysis induced by noradrenaline was potentiated by theophylline, but blocked by papaverine, dipyridamole and imipramine at concentrations between 0.02 to 0.2 mM. These agents also depressed lipolysis induced by theophylline and dibutyryl cyclic AMP and reduced the lipolytic activity of homogenates of adipose tissue. The activity of phosphodiesterase assayed over a wide range of substrate concentrations revealed two different Michaelis constants. Both types of phosphodiesterase were inhibited by theophylline, papaverine, dipyridamole and imipramine in a competitive manner, the low K_m enzyme being more sensitive for inhibition than the high K_m enzyme. On both types of phosphodiesterase papaverine and dipyridamole proved to be 10 to 100 times more potent inhibitors than theophylline and imipramine. To explain the antilipolytic effect of phosphodiesterase inhibitors it is assumed that they do not only affect substrate binding of cyclic AMP to phosphodiesterase but also displace cyclic AMP from the binding site on protein kinase, thus acting as inhibitors of the activation process within the lipolytic system.