Biosynthesis of plasma apolipoproteins by rat small intestine without dietary or biliary fat

Using two complementary methods developed and applied earlier in rats absorbing triacylglycerols, we have now determined the intestinal contribution of individual plasma apolipoproteins when the rate of intestinal lipid absorption is very low. After all dietary fat was withheld for 16-20 h, the intestine produced approximately 50% of the plasma apolipoprotein A-I (apo-A-I) and apo-A-IV, similar to its contribution during fat absorption. When rats were fed a fat-free diet for 12 days and, in addition, biliary fat was diverted from the intestine for 16 h, the intestine still produced approximately 50% of the apo-A-I and only slightly less of the apo-A-IV. Under both conditions of reduced fat absorption, a larger proportion of the intestinally-derived apo-A-I and apo-A-IV bypassed the mesenteric lymph and was released directly into intestinal venous blood. The intestinal apo-B contribution, 16% of the total in fat-fed rats, was reduced to approximately 5% when dietary fat was withheld. Intestinal apo-B was released entirely into the lymph. Intestine produced only small amounts of apo-C and little or no apo-E under all conditions. The results indicate that production of apo-A-I and apo-A-IV by the small intestine is not regulated by the rate of intestinal triacylglycerol transport.