Effect of increasing lipid loads on the ability of the endoplasmic reticulum to transport lipid to the Golgi.

We have previously shown (Mansbach, C. M. and P. J. Nevin, 1998. J. Lipid Res. 39: 963;-968) that after the development of a mass steady state with respect to triacylglycerol absorption in rats, the introduction of radiolabeled trioleoylglycerol, while maintaining the input rate of trioleoylglycerol constant at 135 micromol/h, was followed by a slow (60 min) achievement of a radiolabel steady state in the intestinal endoplasmic reticulum (ER). We hypothesized that this was due to the large input load and that the time to steady state would be shorter at lower lipid loads. Rats were infused intraduodenally with 22.5, 45, 90, or 135 micromol trioleoylglycerol/h for 6 h to obtain a mass steady state in the intestine. [(3)H]trioleoylglycerol was added to the infusate and the ER and Golgi were isolated from the proximal intestine after 5;-60 min of radiolabel infusion. The time required to reach a radiolabel steady state in the ER lengthened from 10 min at the 22.5 micromol/h infusion rate to 60 min at the 135 micromol/h rate. Similar data were obtained for the Golgi. Incubation of the ER with lipase reduced the ER;-triacylglycerol amount by 43% and increased its specific activity by 73%. The amount of [(3)H]TG-dpm in the ER was not reduced unless taurocholate, 10 mm, and colipase were added. We conclude that as the rate of triacylglycerol infusion is increased, TG movement from the ER to the Golgi progressively lengthens until finally all the triacylglycerol infused cannot be transported. A portion of this triacylglycerol is disposed on the cytoplasmic face of the ER and thus able to be attacked by lipase whereas another fraction is sequestered in the ER lumen and immune to lipase attack unless the ER membrane is solubilized.