Dissociation between rate of hepatic lipoprotein secretion and hepatocyte microtubule content

Abstract

The fact that colchicine inhibits hepatic secretion of very low density lipoprotein (VLDL) particles was interpreted to mean that microtubules are involved in hepatic VLDL secretion. To further define this relationship, an attempt was made to see if changes in hepatic VLDL secretion are associated with changes in hepatocyte microtubule or tubulin content. Hepatic secretion of VLDL was increased in rats, and the hepatocyte content of both microtubules (using quantitative morphometric methods) and tubulin (using a time-decay colchicine binding assay) was determined. In acute experiments, VLDL secretion was increased by perfusion of isolated rat livers for 2 h with varying concentrations of free fatty acids (FFA). Results indicate that hepatic VLDL triglyceride (TG) secretion at perfusate FFA levels of 0.7 .mu.eq/ml is 3-fold greater (P < 0.01) than when livers are perfused without added FFA. No differences are observed in the content of microtubules in these livers: specifically, microtubules occupy 0.029% of hepatocyte cytoplasm in livers perfused without FFA and 0.030% of cytoplasm in livers perfused with FFA. In chronic experiments, rats were fed for 1 wk with either standard rat chow or a hyperlipidemic (sucrose/lard) diet. With the experimental diet, plasma TG levels increase 3-fold over controls, and liver VLDL-TG production, as determined by [3H]glycerol turnover studies, is 55% greater (P < 0.01) than controls. Microtubules occupy 0.027% of the cytoplasm of hepatocyte cytoplasm whether rats are on standard or hyperlipidemic diets. The tubulin content of isolated hepatocytes does change, and represents 1% of hepatocyte soluble protein, irrespective of diet. Increases in hepatic VLDL secretion can occur without any demonstrable change in hepatocyte assembled microtubule or tubulin content, and raise questions as to the role played by microtubules in hepatic VLDL secretion.