Calcium-antagonists inhibit secretion of very-low-density lipoprotein from cultured rat hepatocytes

Abstract

The effects of different calcium-antagonists on secretion of very-low-density lipoprotein (VLDL) from cultured rat hepatocytes were examined. Verapamil (an inhibitor of voltage-dependent calcium channels) and EGTA (a calcium chelator) decreased VLDL-triacylglycerol secretion in a concentration-dependent manner, with maximum inhibition (about 90%) at 0.2 mM-verapamil and 5 mM-EGTA. Inorganic calcium-antagonists such as lanthanum, nickel, cobalt and manganese decreased secretion of VLDL-triacylglycerol by 55-95%, whereas the calcium-agonist barium did not affect secretion. Inhibition of VLDL-triacylglycerol secretion appeared within 30 min, without inhibition of triacylglycerol synthesis. Pulse-chase experiments revealed that verapamil and cobalt inhibited the secretory pathway itself. Cobalt showed a concentration-dependent inhibition of VLDL-triacylglycerol secretion, with maximal effect at 8 mM. Although inhibition by cobalt was not completely reversible, Trypan Blue exclusion and lactate dehydrogenase leakage indicated that the hepatocytes were not injured by cobalt or any of the other calcium-antagonists tested. Inhibition of protein synthesis by cycloheximide did not affect triacylglycerol secretion (up to 2 h), and the observed effects were therefore probably not due to impaired production of apolipoproteins. Taken together, these results suggest that calcium is important for secretion of VLDL particles.

Augmentation of Ca2+-Induced Microsomal Triglyceride Transfer Protein Activity by Glucose Supply Enhances Hypertriglyceridemia in Vivo

“…The normal blood glucose was 188.7Ϯ6.2 mg/dl, and this was higher than the reported glucose concentration (128.0Ϯ4.7 mg/dl) 29) observed in SPD rats fasted for 16 h. This might be not due to the gluconeogenesis or glycogenolysis causing as a general response of fasting but the result of continuous feeding, which was to avoid any changes on MTP activity and TG level in the liver. 4) If so, it could be anticipated that the glucose (245.3Ϯ11.5 mg/dl) increased by continuous glucose administration until the post-mortem could exert on the release of insulin than that by glucose (188.7Ϯ6.2 mg/dl) of normal (Fig. 4B).…”

“…13) Animals of both group were not fasted until the post-mortem to avoid any changes on MTP activity and TG concentration. 4) Animals were anaesthetizes with 20% urethane before the postmortem.…”

“…[1][2][3] However, it has recently been reported that intracellular Ca 2ϩ antagonists inhibit the secretion of TG-rich VLDL from the liver into the blood stream. [4][5][6] These findings suggested that intracellular Ca 2ϩ was closely related to the TG-rich VLDL secretion into the blood stream. A number of studies 7,8) have reported that calcium ionophore A23187 (A23187) increased the concentration of cytosolic free Ca 2ϩ while thapsigargin, an inhibitor of Ca 2ϩ -ATPase in the endoplasmic reticulum, inhibited VLDL secretion by hepatocytes.…”

The Possible Role of Ca2+ on the Activation of Microsomal Triglyceride Transfer Protein in Rat Hepatocytes

“…This reflects that Ca 2ϩ antagonists inhibit the secretion of TG-rich VLDL from rat hepatocytes. [4][5][6] Because the secretion of TG was increased from hepatocytes to culture medium by A23187 that increased MTP activity (Figs. 2A, 4A), it is thought that the activated MTP results in a decrease in TG level in hepatocytes.…”

“…[1][2][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.…”