Cyclic AMP activation of a triglyceride lipase in broken cell preparations of rat heart

The effect of CAM [cyclic AMP, Mg-ATP, and 3-isobutyl, 1-methylxanthine (MIX)] on triacylglycerol (TG) lipase activity in extracts from heparin-perfused rat heart was determined. TG lipase activity in homogenate, 10,000g supernatant, 105,000g supernatant, ammonium sulfate supernatant, and the eluate from heparin-Sepharose was increased between 62 and 151% when incubated with a combination of 0.3 mM cyclic AMP, 5 mM MgCl2, and 2 mM ATP. The addition of Mg-ATP + cyclic AMP caused a greater activation of TG lipase in the various fractions than did Mg-ATP + MIX or cyclic AMP + MIX. These results suggest that activation may be mediated by the classical cyclic AMP-protein kinase cascade. Control and CAM-stimulated activities were increased by heparin and inhibited by NaCl and protamine sulfate. In the absence of serum in the assay, the CAM system caused a relatively greater stimulation of lipolytic activity in each fraction compared to when serum was present in the assay. However, the absolute values were 6.1 to 16.3-fold greater with serum in the assay than without serum. In a similar manner, TG lipase activity was stimulated by CAM between 1.75 and 4.26-fold at pH 7.4, and only between 1.62 and 2.51-fold at pH 8.1. However, the absolute values at pH 8.1 were 6.77 to 31.83-fold greater than those seen at pH 7.4. These data demonstrate, for the first time, the cyclic AMP activation of a TG lipase above basal levels in cell-free fractions of rat heart. It is intriguing to speculate that the intracellular fraction of lipoprotein lipase may play a role in the hormonal regulation of cardiac TG lipolysis.