Endogenously synthesized liver triglyceride (TG) is secreted into the blood in the form of TG-rich very low density lipoprotein (VLDL) containing lipids and lipoproteins such as phospholipids, cholesterol ester, and apolipoprotein B (apoB). It is known that microsomal TG transfer protein (MTP) in the liver catalyzes the assembly of TG, the conversion of apoB into VLDL, and the secretion of VLDL into the blood.1-3) Recently, it was reported that calcium antagonists inhibit the secretion of VLDL from rat hepatocytes into the blood. [4][5][6] From these results, [4][5][6] it could be speculated that MTP activity might be stimulated by the increase of intracellular Ca 2ϩ . A few studies have reported that the calcium ionophore (A23187) and thapsigargin, an inhibitor of Ca 2ϩ -ATPase in the endoplasmic reticulum (ER), inhibit VLDL secretion by hepatocytes. 7,8) At present, despite many studies on MTP activity, the role of Ca 2ϩ on MTP activity in hepatocytes is poorly understood, and the inhibition of MTP activity is associated with the suppression of hypertriglyceridemia and atherosclerosis. In recent, in another experiment, we found that MTP activity was relatively higher under high Ca 2ϩ level (10.1Ϯ1.8 mM/protein-mg) than under low Ca 2ϩ level (6.2Ϯ1.9 mM/protein-mg) in liver tissues, and its activity was about 2-fold higher than under low Ca 2ϩ level, and had a relative association with hyperlipoproteinemia. It is inferred that MTP activity may have a correlation with Ca 2ϩ level in the liver. Accordingly, we investigated the effects of Ca 2ϩ on MTP activity in hepatocytes, our data demonstrate that MTP activity may be activated by Ca 2ϩ and calmodulin, and may inhibit hypertriglyceridemia by decreasing the level of Ca 2ϩ in hepatocytes. MATERIALS AND METHODS MaterialsHanks' balanced salts (HBSS) buffer (without calcium chloride, magnesium sulfate, and sodium bicarbonate) for collagenase perfusion and Williams' medium E (with L-glutamine but without sodium bicarbonate) for hepatocytes culture were purchased from Sigma (St. Louis, MO, U.S.A.). MTP assay kits were obtained from Calbiochem ® (an affiliate of Merck KGaA, Darmstadt, Germany). Lipid assay kits were obtained from Nissui Pharmaceutical (Tokyo, Japan). Collagenase (type IV) and other chemical reagents were obtained from Sigma (St. Louis, MO, U.S.A.).Preparation of Hepatocytes and Cell Culture Freshly isolated hepatocytes were prepared from adult male Sprague-Dawley rats weighing between 200 and 250 g. The hepatocytes were isolated by the collagenase-liver perfusion method of Berry and Friend. 9) Hepatocytes viability was assessed by trypan blue exclusion. Cell viability assessed by trypan blue exclusion averaged 95%. Hepatocytes with 95% viability were washed three times with washing buffer [Williams' medium E, 23.8 M NaHCO 3 , 0.5 mM dexamethasone, and 15 mM HEPES, pH 7.4 supplemented with 1000 U/l penicillin, 1 mg/l streptomycin, 1 mM insulin]. Hepatocytes (10 6 cells/well) suspended in culture medium (washing buffer supplemented with 10% fetal bovine ...