Biomedical subjects
Publications and source records attributed to R O Scow.
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The relationship between lipoprotein lipase activity and uptake of triacylglycerol, cholesterol and phosphatidylcholine from chylomicrons was studied in mammary gland and adipose tissue of rats lactating 6--7 days. 60% of triacylglycerol [14C]oleic acid, 13% of [3H]cholesterol and 8% of [32P]phosphatidylcholine in chylomicrons injected intravenously were taken up within 11 min by mammary gland whereas negligible amounts were taken up by adipose tissue. Non-suckling for 44 h decreased markedly uptake of all lipids by mammary gland and retarded clearance of chylomicrons from blood, while it increased significantly uptake of triacylglycerol fatty acids and cholesterol by adipose tissue. Non-suckling also decreased lipoprotein lipase activity in mammary gland from 7.7 to 0.4 units/g, while it increased activity in adipose tissue from 0.1 to 2.7 units/g. These findings indicate that lipoprotein lipase is involved in uptake of chylomicron triacyglycerol and cholesterol by mammary gland and adipose tissue, and also in uptake of chylomicron phosphatidylcholine by mammary gland. They also show that reciprocal changes in lipoprotein lipase activity in mammary gland and adipose tissue, as occur during lactation, result in diversion of chylomicron lipids from one tissue to the other.
The effects of several prostaglandins on lipoprotein lipase activity of mammary gland and adipose tissue and serum triacylglycerol were studied during late pregnancy in rats. Prostaglandins were injected twice daily for 2 days before and once on the day of analysis. In rats pregnant 20 days, prostaglandin F(2alpha) (PGF(2alpha)) increased the activity of lipoprotein lipase in mammary gland fourfold, reduced the activity in adipose tissue about 60%, and decreased serum concentration of triacylglycerol 50%. PGF(2alpha) also reduced serum concentration of progesterone 90% and increased that of prolactin fivefold, but had no effect on serum concentrations of either immuno-reactive insulin or 17beta-estradiol. Injections of 13,14-dihydro-15-keto PGF(2alpha), a metabolite of PGF(2alpha), had similar effects in rats pregnant 20 days, whereas prostaglandins E(1) and E(2) did not. In rats pregnant 16 days, PGF(2alpha) did not affect lipoprotein lipase activity in the tissues or the concentration of triacylglycerol and prolactin in serum, although it decreased serum progesterone 80%.2-Br-alpha-ergocryptine prevented the increase in serum prolactin in response to PGF(2alpha), but did not alter the effect of PGF(2alpha) on lipoprotein lipase activity or serum triacylglycerol. Progesterone completely blocked the effects of PGF(2alpha) on lipoprotein lipase activity and serum triacylglycerol and prolactin concentrations. These findings indicate that the changes in lipoprotein lipase activity and serum triacylglycerol in PGF(2alpha)-treated rats are probably related to the inhibitory action of PGF(2alpha) on progesterone secretion. They also suggest that endogenous F prostaglandins may play a role in the regulation of lipoprotein lipase activity in mammary gland and adipose tissue near parturition.
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Perfused adipose and mammary tissues of the rat were used to study the role of capillary endothelium and lipoprotein lipase in the metabolism of chylomicrons. Triacylglycerol of chylomicrons was hydrolyzed stepwise during uptake by both tissues, first to di- or monoacylglycerol and fatty acid, and then to glycerol and fatty acid. Cytochemical studies in perfused adipose tissue showed that lipolysis occurred at the luminal surface of capillaries, within vacuoles and microvesicles in the endothelium, and between the endothelium and pericytes. Radioautography of perfused lactating mammary tissue showed that acyl and glyceryl moieties derived from chylomicron triacylglycerol were both incorporated into milk lipid droplets. Perfused lactating mammary tissue also took up cholesterol from chylomicrons. There was marked reduction of lipoprotein lipase activity and uptake of both triacylglycerol and cholesterol in perfused mammary tissue of hypophysectomized lactating rats. It is proposed that uptake of triacylglycerol by extrahepatic tissues requires the direct action of lipoprotein lipase, while uptake of cholesterol is dependent on reduction of the triacylglycerol core volume, resulting from action of the enzyme on the core and uptake of lipolytic products by the tissue.
The role of lipoprotein lipase in the uptake of chylomicron triacylglycerol and cholesterol from blood was studied in perfused inguinal-abdominal mammary tissue of rats lactating 10-15 days. Lipoprotein lipase activity in the tissue was reduced, from 0.47 to 0.10 units/g, by removing the anterior pituitary gland from lactating rats 2 days before the experiment. Perfused mammary tissue of normal lactating rats took up 12% of the chylomicron triacylglycerol infused, whereas the tissue of hypophysectomized lactating rats took up less than 1%. About two-thirds of the triacylglycerol taken up was retained as glyceride, and the rest was hydrolyzed and released to the perfusing fluid as fatty acids and glycerol. Autoradiographic studies of perfused tissues of normal lactating rats showed that both the acyl and glycerol moieties derived from chylomicron triacylglycerol were incorporated into milk lipid droplets. Perfused mammary tissue of normal lactating rats also took up 15% of the chylomicron cholesterol infused, whereas the tissue of hypophysectomized lactating rats took up less than 1%. The findings demonstrate that chylomicron cholesterol is taken up with triacylglycerol by lactating mammary tissue, and that uptake of both lipids is markedly suppressed when lipoprotein lipase activity is low, as in tissue of hypophysectomized rats. It is proposed that uptake of triacylglycerol from chylomicrons by mammary tissue requires the action of lipoprotein lipase, while uptake of cholesterol is dependent on reduction of the triacylglycerol core, resulting from action of the enzyme on the core and uptake of lipolytic products by the tissue.
The effects of lipoprotein lipase, phospholipase A2 and phospholipase C on chylomicron phosphatidylcholine and triacylglycerol were studied with rat lymph chylomicrons containing phosphatidylcholine labeled with [14C]oleic acid. Lipoprotein lipase purified from bovine milk readily hydrolyzed chylomicron phosphatidylcholine to lysophosphatidylcholine and fatty acid, and triacylglycerol to monoacylglycerol, fatty acid and glycerol. The rates of hydrolysis of phosphatidylcholine and triacylglycerol increased with enzyme concentration, and both decreased when fatty-acid binding sites on albumin in the incubation medium were limited. The proportion and amount of phosphatidylcholine hydrolyzed was always less than that of triacylglycerol. Analyses of hydrolytic products showed that lipoprotein lipase cleaved the 1-acyl ester bond of phosphatidylcholine. The findings indicate that lipoprotein lipase can account for some of the phospholipase A1 activity found in postheparin plasma. Phospholipase A2 and phospholipase C hydrolyzed chylomicron phosphatidylcholine, greater than 92% in 10 min, but not triacylglycerol. The resultant phosphatidylcholine-deficient chylomicrons, which could be concentrated by ultra-centrifugation and resuspended in incubation medium, were readily depleted of triacylglycerol when incubated with lipoprotein lipase. The findings indicate that phosphatidylcholine can be removed from the surface film of chylomicrons without disrupting the particles or blocking the action of lipoprotein lipase on the core triacylglycerol.
The effects of lipolysis on the structure of chylomicrons were studied with the scanning electron microscope using rat chylomicrons incubated with purified bovine milk lipoprotein lipase for 20 minutes at pH 8.1. Since the amount of albumin added to the medium was limited, some of the free fatty acids and partial glycerides formed by lipolysis accumulated in the chylomicrons. Lipolyzed chylomicrons fixed with OSO4 at pH 7.4 appeared in scanning electron micrographs as spheres with multiply idented irregular surfaces, while those fixed at pH 5.5, AS WELL AS CONTROL CHYLOMICRONS FIXED AT BOTH PHs, appeared as spheres with smooth surfaces. Sections of OSO4-fixed specimens, viewed with the transmission electron microscope, showed that the core of lipolyzed chylomicrons fixed at pH 7.4 contained numerous circular electron-lucent areas at the periphery, thus accounting for the indented surfaces observed above, while the core surfaces of the other specimens were circular and smooth. These findings confirm an earlier report that aqueous spaces form in chylomicrons during lipolysis when albumin in the medium is limited, and that the aqueous spaces disappear when specimens are prepared at pH 5.5 for microscopy. Thin sections of specimens that had been prepared for scanning electron microscopy showed that the gold-palladium coating was desposited directly on the indented surface of the lipid core of lipolyzed chylomicrons fixed at pH 7.4. It is concluded that vacuum dehydration during specimen preparation ruptures the outer wall of the aqueous spaces in lipolyzed chylomicrons and thereby exposes the interior of the spaces to gold-palladium coating and viewing with the scanning electron microscope.
Uptake of chylomicron triglyceride and lipoprotein lipase was studied in red and white skeletal muscles, heart, and adipose tissue of rats. Retention of triglyceride fatty acids 10 min after injection was 1.6%/g in heart and adipose tissue, 0.2-0.4%/g in red (soleus and diaphragm) muscles, and 0.1%/g in white (psoas minor) muscles of fed rats. Fasting (24 h) increased retention two- to fourfold in red skeletal muscles and heart, had no effect in white muscles, and decreased retention greater than 75% in adipose tissue. Lipoprotein lipase activity in fed rats was lowest in white muscles and in certain red (posterior belly of digastric and medial head of triceps brachii) musclws, intermediate in soleus and diaphragm muscles and adipose tissue, and highest in heart. Fasting doubled lipoprotein lipase activity in all red skeletal muscles and heart, had no effect in white muscles, and decreased activity 60% in adipose tissue. The findings indicate that triglyceride uptake is related to lipoprotein lipase activity in skeletal muscle and the changes in enzyme activity during fasting divert blood triglyceride to red skeletal muscles.
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Early effects of lipolysis on the structure of chylomicrons in vitro were studied in rat chylomicrons incubated with purified bovine mild lipoprotein lipase at pH 8.1. The amount of the albumin added to the incubation medium was limited so that free fatty acids (FFA) and partial glycerides formed during lipolysis would accumulate in the chylomicrons. The structures visualized in lipolyzed chylomicrons was found to be affected by pH during preparation of specimens for microscopy, whether fixed with OsO4 and sectioned, or stained with sodium phosphotungstate and examined as whole mounts. Circular aqueous spaces were present in the triglyceride core of lipolyzed chylomicrons processed at pH 8.1 and 7.4. Sometimes the spaces contained aggregates of osmiophilic material and whorls of bilayered lamellae. The spaces were replaced by lamellar structures having a periodicity of 40 A, in chylomicrons processed at pH 5.5, and the spaces and lamellae were both absent at pH 3.0. The findings indicate that these spaces were lined by a lipid monolayer which formed bilayered lamellae under certain conditions. It is concluded that the monolayer lining the aqueous spaces is an inward extension of the chylomicron surface film produced by the accumulation and movement of lipolytic products, FFA and partial glycerides, in the interfacial plane between core triglyceride and water.
The effect of prolactin on lipoprotein lipase activity of crop sac, omental adipose tissue, and esophagus was studied in adult female pigeons. Prolactin injected for 4 days, 1 mg/day, increased lipoprotein lipase activity from 17 to 177 U/g in crop sac and from 68 to 118 U/g in adipose tissue, but had no effect on the activity in esophagus, 4 U/g. (10 = 1 mumol of chylomicron triglyceride hydrolyzed to free fatty acid and glycerol per hour.? Prolactin increased the weight of crop sac from 1.4 to 7.2 G. The effect of prolactin on lipoprotein lipase activity and weight of crop sac occurred mostly during the 3rd and 4th days of treatment, whereas the effect on the activity of adipose tissue occurred later, during the 4th day of treatment. Crop "milk" collected from pigeons injected with 2 mg of prolactin daily for 4 days contained a small amount of lipoprotein lipase activity, 12 U/g, is smaller than 10% of that found in crop sac. The finding of markedly increased lipoprotein lipase activity in crop sac of prolactin-treated pigeons suggests that blood triglyceride may be used by crop sac for the formation of crop milk lipid.
Lipolytic activity was studied in esophageal and gastric aspirates obtained with a nasogastric tube from 14 healthy adult subjects. Samples were collected from esophagus, first at 30-35 cm and then at 40-45 cm from the nose, as the subject, after drinking 15-30 ml of a cream-milk mixture, swallowed small amounts of water. The samples from stomach were taken last and usually contained a small amount of cream-milk mixture. Lipolytic activity was assayed using chylomicron, milk, and corn oil triglyceride as substrate. Esophageal and gastric samples both contained lipolytic activity which hydrolyzed long-chain triglyceride to diglyceride, monoglyceride, and FFTA, had a pH optimum of 5.4, and was not affected by either had a pH optimum of 5.4, and was not affected by either 0.5 M NaCl or 4 mM sodium taurodexycholate. The activity, expressed as nanomoles of chylomicron triglyceride hydrolyzed per milliter per minute, ranged from 0 to 145 in upper esophageal, 5 to 303 in lower esophageal, and 50 to 357 in gastric samples. Only a trace of lipolytic activity was found at pH 5.4 in saliva collected from the parotid, submandibular, and sublingual glands, thus excluding those tissues as a source of the activity found in esophageal and gastric aspirates. The findings suggest that in man glands in or near the pharynx secrete a lipase that acts in the stomach to hydrolyze long-chain triglyceride to partial glycerides and FFA. It is proposed this reaction is the first step in the digestion of dietary fat and that the amphiphilic lipids formed by lipolysis facilitate the emulsification of triglyceride in the stomach.
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Chylomicrons isolated from rat lymph were complexed with lipoprotein lipase of post-heparin plasma (chylomicrons-LPL) in order to study the effects of lipolysis on the structure of chylomicrons. Triglyceride in the chylomicron core was readily hydrolyzed to free fatty acids (FFA) and glycerol when chylomicrons-LPL were incubated at pH 8.3 in medium containing albumin. Although most of the FFA were immediately released to the medium, some were retained within chylomicrons when FFA-binding sites on albumin were not available. These observations suggest that albumin may have a specific role in the transfer of FFA across the chylomicron surface film. Chylomicrons-LPL assumed many different shapes as they were depleted of triglyceride by the lipolytic action of the enzyme, and total removal of core triglyceride resulted in empty sacks of surface film. The surface film was visualized in sections of OsO(4)-fixed chylomicrons-LPL as a thin electron-opaque line, 25-30 A wide, in areas where the underlying electron-opaque core had been replaced by zones of decreased electron opacity, and in folds of surface film extending outward from chylomicrons partially depleted of core lipid. The findings demonstrate that chylomicrons consist of a core of liquid triglyceride enveloped by a pliable and durable monolayer surface film, and that lipoprotein lipase reduces the triglyceride core without disrupting the surface film.