Susanne Bennett Clark

Lipolysis and Absorption of Fat in the Rat Stomach

The triglyceride lipolytic activity of gastric mucosal homogenates was studied in Wistar strain rats with exclusion of pancreatic enzyme contamination by prior prolonged pancreatic flow diversion. Considerable hydrolysis of a medium chain triglyceride, trioctanoin, occurred, but enzyme activity using triolein as substrate was barely detectable. Although the pH optimum for lipolytic activity was 6.5, significant hydrolysis occurred at pH 3.5 and the enzyme was very stable at this low pH. Taurocholate at concentration of 2 to 20 mm did not alter the rate of lipolysis. In pyloric ligated rats in vivo, considerable hydrolysis of both trioctanoin and triolein was measured when the pH was as low as 2. In addition, absorption of octanoic acid from the gastric lumen was readily detectable. Gastric lipolytic activity may be important in the digestion of fat, particularly of medium chain length, both in the stomach and in the intestinal lumen in patients with pancreatic exocrine insufficiency and steatorrhea.

Rate-limiting steps in steady-state intestinal absorption of trioctanoin-1-14C. Effect of biliary and pancreatic flow diversion.

During continuous intraduodenal infusion of emulsified fat in rats, a steady state of intestinal absorption is achieved. Maximal steady-state absorption of trioctanoin, a medium-chain triglyceride (MCT), by unanesthetized, restrained rats was found to be the same after total bile diversion as in controls (1560 mumoles of fatty acid per hr).After pancreatic and bile diversion, absorption of MCT was still one-third as rapid as in controls, and mucosal uptake apparently occurred in the form of unhydrolyzed triglyceride. Returning bile to the intestinal lumen during pancreatic diversion did not increase the absorption rate.From intestinal tissue lipid-(14)C concentrations measured during steady-state maximal absorption it was possible to calculate turnover times for labeled lipid passing through the mucosal cells. Mucosal turnover times of about 4 min for control and bile-diverted rats, and about 20 min for animals with pancreatic diversion were obtained. The rate-limiting step in octanoic acid absorption in control and bile-diverted rats was probably mucosal penetration. During absorption of unhydrolyzed triglyceride by pancreatic flow-diverted rats, both passage from the lumen into the mucosal cell and intracellular lipolysis were rate-controlling factors.

Intestinal lipid absorption: evidence for an intrinsic defect of chylomicron secretion by normal rat distal intestine.

Intracellular triglyceride accumulation and delayed chylomicron secretion were demonstrated in distal but not in proximal rat intestine following prolonged steady state fat absorption. After 1 and 4 hr of intraduodenal triolein infusion, the mucosal triglyceride content in distal intestinal segments was greatly increased compared with proximal segments. Electron microscopy at each time interval disclosed greater quantities of lipid droplets within the distal cells and these were much larger than the intracellular droplets in the proximal cells, some attaining an enormous size (9000 mμ). When proximal and distal cells were compared at intervals after cessation of a 4 hr triolein infusion, the differences in intracellular lipid accumulation were also evident because, even after 6 hr the distal cells were still filled with large droplets, whereas the proximal cells were almost devoid of fat. These results indicate a basic cellular difference between proximal and distal intestine in the processing of fat and, in contrast to current concepts, suggest that defective chylomicron secretion is not necessarily associated with limited B-apoprotein availability.

Inhibition of steady-state intestinal absorption of long-chain triglyceride by medium-chain triglyceride in the unanesthetized rat

Maximal steady-state intestinal absorption rates in unanesthetized rats for triolein, a long-chain triglyceride, and for trioctanoin, a medium-chain triglyceride, are known to differ. Both these lipids are hydrolyzed in the intestinal lumen but the products of hydrolysis are metabolized differently by the mucosal cell. Intraduodenal infusion of trioctanoin was found to reduce steady-state triolein absorption. Luminal lipolysis was shown not to be rate-controlling. High rates of trioctanoin infusion significantly lowered the pH of the luminal aqueous phase and altered the partition of oleic acid between aqueous and oil phases. Two possible mechanisms for the inhibition of triolein uptake are considered. In the intestinal lumen medium chain lipids might have lowered the activity of oleic acid monomers in the aqueous phase and reduced passive diffusion into mucosal cells. Alternatively, competition between long and medium chain fatty acids for some common receptor during transport into the intestinal mucosal cell may have occurred. Despite significant inhibition of triolein absorption by high levels of trioctanoin, the maximum number of calories absorbed from mixtures of triglycerides exceeded the maxima from either glyceride alone. The optimum proportion of triolein to trioctanoin in lipid infusion mixtures was about 3:4 by weight and the optimum dosages about half maximal for each triglyceride, which represented a caloric intake of 4 kcal/rat per 2 hr. The absorption coefficient for this lipid mixture was about 90%. It is suggested that in patients who have a limited intestinal absorptive capacity dietary fat intake might be doubled with a caloric supplement of medium-chain triglycerides without increase in steatorrhea of long-chain fat.

Ultrastructural features of regional differences in chylomicron secretion by rat intestine

Abstract An electron microscopic study of the effects of prolonged intraduodenal triolein perfusion on intestinal fat absorption was performed to investigate the ultrastructural features of regional differences in chylomicron secretory capacity in the rat small intestine. Utilizing conditions in which intraluminal triolein concentrations were high enough to saturate the mechanism for fat uptake it was demonstrated that the proximal segments reach a steady state of uptake and chylomicron secretion within one hour whereas triglycerides accumulate progressively in the distal epithelial cells. After one or four hours perfusion with [1- 14 C]triolein the mucosal triglyceride content was much greater in distal than in proximal segments and electron microscopy disclosed many more and larger intracellular fat droplets in the distal absorptive cells. Six hours after stopping the 4 hr perfusion many distal cells were still filled with large fat droplets whereas the proximal cells were devoid of fat. In comparison to proximal intestine, triglycerides accumulated in distal segments following direct segmental triolein perfusion and electron microscopy disclosed large intracellular fat droplets similar to those observed after intraduodenal triolein perfusion. The ultrastructural observations suggest that fat accumulation, in the distal segments, is due to a relatively decreased capacity for chylomicron release. These studies suggest an intrinsic difference between proximal and distal intestine in fat absorption, which may reflect regional differences in cellular function such as membrane availability for the final assembly and secretion of chylomicrons.

Intestinal lymph chylomicron cholesteryl ester during duodenal triolein infusion at increasing rate.

Chylomicron lipid composition in mesenteric lymph fistula rats was followed during duodenal triolein infusion at three rates. Esterified cholesterol decreased progressively in all groups as triglyceride transport continued, and the rate of decrease was reciprocally related to the triolein infusion rate. Suppression of cholesterol ester synthesis may spare unesterified cholesterol for chylomicron membrane formation. Ratios of triglyceride to phospholipid and to unesterified cholesterol increased with the duration of triglyceride secretion, while the chylomicron surface lipid composition remained constant over time. The rate of increase in the core:surface lipid ratios was the same irrespective of the triolein transport rate.

Regulation of rat proximal intestinal glycolytic enzyme activity by ileal perfusion with glucose.

Specific activities of the glycolytic enzymes fructose-1-phosphate aldolase, fructose-1,6-diphophate aldolase, and pyruvate kinase, are higher in rat duodenum and jejunum than in ileum. Whether this gradient reflects the failure of dietary sugars to reach the ileum in high concentrations is unknown. Rats were first fed a carbohydrate-free diet for 3 days, which virtually removed the proximal-distal gradient of enzyme specific activities. Twenty percent glucose was then perfused directly into either the duodenum or the ileum for 3 days. Both proximal and distal glucose perfusion restored the normal gradient of all three enzymes. Ileal pyruvate kinase was also increased by ileal glucose perfusion, but ileal aldolases were no higher after distal perfusion than after duodenal perfusion. The low ileal aldolase levels normally found in fed rats therefore are an intrinsic property of distal intestine and are not due to failure of dietary sugar to reach this portion of the gut. Furthermore, adaptation of duodenal and jejunal glycolytic enzymes to ileal glucose perfusion suggests a humoral and/or neural mechanism rather than a direct local luminal effect of the sugar itself.