RESUMO
The effects and interaction between cAMP-analogue dibutyryl-cAMP and calmodulin antagonists were investigated on de novo synthesis and secretion of lipids in cultures of hepatoma MeArdle-RH7777 cells and normal rat hepatocytes. Dibutyryl-cAMP caused a significant decrease in the secretion of de novo synthesized triacy1[3H]glycerol in both cultures of McArdle cells and rat hepatocytes. The inhibitory effect of dibutyryl-cAMP was concentration-dependent and appeared at the lowest concentration examined, 5 mirco M. Dibutyryl-cAMP at a concentration of 50 mirco M suppressed secretion of triacylglycerol by approximately 38% [p<0.05] and secretion of phosphatidylcholine by 30% [p<0.05]. Dibutyryl-cAMP did not affect the synthesis of triacylglycerol and phosophatidylcholine, except at the highest concentration tested, 500 mirco M, where both triacylglycerol and phosphatidylcholine synthesis were suppressed significantly. Anticalmodulin W-7 also inhibited secretion of newly made triacylglycerol in a concentration-dependent manner in both cultures of McArdle cells and rat hepatocytes. W-7 at a concentration of 20 mirco M suppressed triacylglycerol secretion by about 37% [p<0.05], while the secretion of phosphatidylcholine and synthesis of triacylglycerol and phosphatidylcholine were not affected, unless at more than 20 mirco M concentration, at which both triacylglycerol and phosphatidylcholine synthesis were decreased significantly. The inhibitory effect elicited by dibutyryl-cAMP [100 mirco M] was not abolished in the presence of calmodulin antagonists, W-7 [20 mirco M], trifluoperazine [20 mirco M] and chlorpromazine [20 mirco M]. The simultaneous effects of dibutyryl-cAMP and either calmodulin antagonists were not additive or synergistic. None of the the calmodulin antagonists affected the cellular content of de novo synthesized triacylglycerol and phosphatidylcholine significantly. Neither dibutyryl-cAMP nor any calmodulin antagonist or their combination did affect the overall rate of de nove synthesis of triacylglycerol and phosphatidylcholine. All calmodulin antagonists examined alone had a net significant inhibitory effect on the secretion of newly made triacylglycerol. The results presented here suggest that calmodulin antagonists have net direct effects and hence could not overcome dibutyryl-cAMP-induced suppressive effects on the secretion of newly made triacylglycerol. The cell types, normal hepatocytes relative to hepatomas, did not influence the results