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.
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