ANTICALMODULIN DRUGS DUE TO THE NET EFFECTS CANNOT ANTAGONIZE DIBUTYRYL-CAMP-MEDIATED SUPPRESSION OF DE NOVO SYNTHESIZED LIPID SECRETION IN BOTH CULTURED MCARDLE CELLS AND RAT HEPATOCYTES

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 McArdle-RH7777 cells and normal rat hepatocytes. Dibutyryl cAMP caused a significant decrease in the secretion of de novo synthesized triacyl [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 µM. Dibutyryl-cAMP at a concentration of 50 11M 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 phosphatidylcholine, except at the highest concentration tested, 500 µM, where both triacylgJycerol 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 µM suppressed triacylglycerol secretion by about 37% (p<0.05), while the secretion of phosphatidylcholine and synthesis o f triacylglycerol and phosphatidylcholine were not affected, unless at more than 20 µM concentration, at which both triacylglycerol and phosphatidylcholine synthesis were decreased significantly. The inhibitory effect elicited by dibutylyl-cAMP (100 µM) was not abolished in the presence of calmodulin antagonists, W-7 (20 µM), trifluo perazine (20 µM) and chlorpromazine (20 µM). The simultaneous effects of dibutyryl-cAMP and e ither calmodulin antagonists were not additive or synergistic. None of the calmodulin antagonists affected the cellular content of de novo synthesized triacylglycerol and p hosphatidy1choline significantly. Neither dibutyryl-cAMP nor any calmodulin antagonist or their combination did affect the overall rate of de novo synthesis of triacylglycerol and phosphatidylcholine. All calmodulin antagonists examined alone also 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.