EFFECTS OF HYDRATION STATE ON THE SYNTHESIS AND SECRETION OF TRIACYLGLYCEROL BY ISOLATED RAT HEPATOCYTES - IMPLICATIONS FOR THE ACTIONS OF INSULIN AND GLUCAGON ON HEPATIC SECRETION

The effects of hepatocyte volume on the secretion of triacyl-glycerol were studied in order to test the suggestion that increases in the por tal concentrations of osmolyte amino acids and metal ions during the p randial/early-absorptive phase may be involved in mediating the acute changes in glycerolipid metabolism observed in vivo [Zammit (1995) Bio chem Sec. Trans. 23, 506-511]. Incubation of isolated rat hepatocytes with hypo-osmotic medium or in the presence of glutamine (in the prese nce or absence of leucine), conditions which gave an increase in cell water content of between 8 and 27%, resulted in a decrease in the rate of [C-14]triacylglycerol (TAG) secretion when [C-14]palmitate was use d as substrate. The inhibition was proportional to the increase in cel l water content. At low exogenous palmitate concentration (0.05 mM), t he inhibition of [C-14]TAG secretion was accompanied by a marked shift in the incorporation of label from TAG to phospholipid. In the presen ce of 0.5 mM palmitate this effect was attenuated, and in the presence of 1 mM palmitate it was abolished. Increased cell volume associated with incubation of hepatocytes with glutamine (in the presence or abse nce of leucine) also resulted in a decrease in the fraction of newly l abelled TAG that was secreted into the medium. Decreased cell volume, achieved by incubation of hepatocytes with hyperosmotic medium (suffic ient to decrease cell water content by approx. 9%) decreased overall [ C-14]TAG secretion, but did not affect the amount of label that was in corporated into phospholipid as a fraction of that incorporated into t otal glycerolipids. Cell shrinkage, however, diminished the fraction o f newly labelled [C-14]TAG that was secreted. When intracellular TAG w as prelabelled with [H-3]glycerol, it was found that cell shrinkage ma rkedly inhibited (preformed) [H-3]TAG secretion, whereas cell swelling did not affect this route of TAG secretion. The data are discussed, i n terms of the possible-action of changes in cell hydration at the dif ferent loci at which hepatocyte TAG secretion is controlled, with refe rence to previous observations that both insulin and glucagon are able to inhibit TAG secretion in cultured rat hepatocytes and HepG2 cells.