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 Re s. 39: 963-968) that after the development of a mass steady state with resp ect to triacylglycerol absorption in rats, the introduction of radiolabeled trioleoylglycerol, while maintaining the input rate of trioleoylglycerol c onstant at 135 mu mol/h, was followed by a slow (60 min) achievement of a r adiolabel steady state in the intestinal endoplasmic reticulum (ER), We hyp othesized that this was due to the large input load and that the time to st eady state would be shorter at lower lipid loads. Rats were infused intradu odenally with 22.5, 45, 90, or 135 mu mol trioleoylglycerol/h for 6 h to ob tain a mass steady state in the intestine. [H-3]trioleoylglycerol was added to the infusate and the ER and Golgi were isolated from the proximal intes tine after 5-60 min of radiolabel infusion. The time required to reach a ra diolabel steady state in the ER lengthened from 10 min at the 22.5 mu mol/h infusion rate to 60 min at the 135 mu mol/h rate. Similar data were obtain ed for the Golgi, Incubation of the ER with lipase reduced the ER-triacylgl ycerol amount by 43% and increased its specific activity by 73%, The amount of [3H][H-3]TG-dpm in the ER was not reduced unless taurocholate, 10 mM, a nd colipase were added. We conclude that as the rate of triacylglycerol inf usion is increased, TG movement from the ER to the Golgi progressively leng thens until finally all the triacylglycerol infused cannot be transported, A portion of this triacylglycerol is disposed on the cytoplasmic face of th e ER and thus able to be attacked by lipase whereas another fraction is seq uestered in the ER lumen and immune to lipase attack unless the FR membrane is solubilized.