The effects of fat feeding on apolipoprotein AI secretion from rat small intestinal epithelium.

The small intestine is known to be an important synthetic site for certain apolipoproteins, which are subsequently secreted from the enterocyte into the mesenteric lymph. We have studied apolipoprotein AI and CIII content of the enterocyte during the course of fat feeding in order to determine their relative synthetic and secretory rates. Rat intestinal enterocytes were isolated from the entire jejunal villus after fat feeding in vivo. The apo AI content fell 50% as determined by RIA one and two hours after fat feeding. By four hours, the intracellular cellular levels had returned to prefeeding levels. These changes in apolipoprotein AI levels were not seen in the terminal ileum. Apolipoprotein CIII levels remained unchanged afer fat feeding. To determine the effect of free fatty acids on apolipoprotein AI secretion, organ culture explants were incubated for four hours in the presence and absence of 360 microM oleic acid bound to albumin. Apolipoprotein AI detected in the incubation media reflected release from the lamina propria (which was not colchicine sensitive), and secretion from the enterocyte (which was inhibited by colchicine). In the absence of oleic acid, enterocyte secretion of apolipoprotein AI accounted for about half of the apo AI recovered in the medium. In the presence of oleic acid, the total apolipoprotein AI content of the tissue increased by 50 percent. A similar increase in colchicine sensitive secretion was observed. The secretion of apolipoprotein AI from explants was more rapid in the presence of oleic acid and began without the half hour lag noted when oleic acid was absent. The mid intestine was most active in the secretion of apolipoprotein AI. These data are consistent with the hypothesis that in the first few hours after feeding the rate of secretion of apolipoprotein AI exceeds the synthetic capacity of the small intestinal epithelium.