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H G Windmueller

Publications and source records attributed to H G Windmueller.

At least 19 recordsLinked to original sources
PubMed1985

Regulated biosynthesis and divergent metabolism of three forms of hepatic apolipoprotein B in the rat.

Studies using rat livers perfused with recycled, serum-containing medium plus [3H]leucine revealed that secreted VLDL contain three forms of apolipoprotein B (apoB), B-48, B-95, and B-100, all synthesized by the liver. The B-48/(B-95 + B-100) [3H]leucine incorporation ratio ranged from 0.22 to 3.25 with livers of rats fed different diets, and the ratio was positively correlated with the triglyceride secretion rate in most of the livers. Generally, as more triglyceride was secreted, a greater proportion was packaged with B-48, which is the apoB form most rapidly cleared from the circulation. Together, these findings suggest a mechanism for regulating plasma triglyceride levels. [3H]Leucine incorporation into apoA-I also was positively correlated with the triglyceride secretion rate. Secretion of newly synthesized B-48 was delayed relative to all other apolipoproteins. There was little segregation of any of the three apoB forms into any of five subfractions of secreted VLDL separated on the basis of Sf value; only the smallest VLDL (Sf 20-100) were slightly enriched in B-95 and B-100. Less than 5% of newly synthesized apoB appeared in perfusate LDL. The B-100/B-95 [3H]leucine incorporation ratio was 3.3 with perfused livers of fed rats but only 1.6 in post-surgical, relatively fasted rats in vivo, suggesting physiologic regulation also of the relative amounts of the two large apoBs produced. With recycled serum-free perfusate, as opposed to serum-containing medium, there was hepatic reuptake of nascent VLDL, indicated by the reuptake of newly synthesized apoE and all three forms of apoB, and not other apolipoproteins. Divergent metabolism of B-100 and B-95 in the rat was evident from the following results: a) B-95 disappeared more rapidly from recycled, serum-free liver perfusate; b) B-100 disappeared more rapidly from the circulation in vivo; c) plasma lipoprotein fractions of increasing density between d less than 1.019 and d 1.072 g/ml contained increasing proportions of B-95 over B-100. In summary, these results show that hepatic VLDL production in the rat involves the biosynthesis of three forms of apoB, that the relative amounts produced are regulated by physiologic variables, and that there is divergent metabolism of the VLDL particles into which these different apoB forms, either individually or in combination, become incorporated.

Animals↗
PubMed1984

Rat liver and small intestine produce proapolipoprotein A-I which is slowly processed to apolipoprotein A-I in the circulation.

Two-dimensional electrophoretic analysis of plasma lipoproteins from male Osborne-Mendel rats consistently reveals three isoforms of apolipoprotein A-I (apo-A-I) with the following apparent pI values and quantitative distribution: isoform 3, pI = 5.68, 69%; isoform 4, pI = 5.55, 29%; isoform 5, pI = 5.44, 2%. The two major isoforms were obtained by preparative isoelectric focusing and subjected to NH2-terminal amino acid sequence analysis with the following results: isoform 3, (Asp)-Glu-Pro-Gln-Ser-Gln-Trp-Asp-Arg-Val; isoform 4, X-Glu-Phe-X-Gln-Gln-Asp-Glu-Pro-Gln-Ser. By comparison with the amino acid sequence previously reported for the primary translation product of rat intestinal apo-A-I mRNA (Gordon et al. (1982) J. Biol. Chem. 257, 971-978), isoform 3, the more basic isoform, is identified as mature apo-A-I and isoform 4 as its proform ( proapo -A-I). The proform differs from mature apo-A-I by a 6-amino acid extension at the NH2 terminus. Isoform 5 was not identified further. The plasma steady state distribution of the apo-A-I forms indicates that proapo -A-I is relatively stable in the circulation. Virtually all plasma proapo -A-I is lipoprotein-associated. No significant differences in the steady state proportions of plasma apo-A-I forms were observed between male and female rats, or among various subfractions of plasma high density lipoproteins obtained by heparin-Sepharose affinity chromatography or by density gradient ultracentrifugation. Rats fed a high fat, high cholesterol diet, however, showed an increase in the proportion of circulating proapo -A-I. The relative increase in proform was even more pronounced in rats fed a fat-free diet containing orotic acid. The biosynthesis, secretion, and metabolism of the various apo-A-I forms were also studied. In liver and intestine, the only known sites of apo-A-I synthesis in the rat, approximately 85% of the newly synthesized intracellular apo-A-I, was the proform . Proapo -A-I was also the predominant form (approximately 80%) released into the circulation by isolated, perfused livers and by autoperfused intestinal segments in vivo. Gradual processing of circulating proapo -A-I to mature apo-A-I was observed in vivo following pulse-labeling of apo-A-I with [3H]leucine. Processing in vivo was approximately 80% complete in 10 h.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1981

Variant forms of plasma apolipoprotein B. Hepatic and intestinal biosynthesis and heterogeneous metabolism in the rat.

Recent studies of circulating lipoproteins in man and the rat have shown that apolipoprotein B in plasma exists in two immunologically distinct forms with apparent Mr, in the rat, of approximately 335,000 (apoB335K) and 240,000 (apoB240K). With isolated, perfused organs and single and double isotope labeling methods in vivo (1979) J. Biol. Chem. 254, 7316), we have now found that the rat liver produces both apoB variants in abundance while the small intestine produces apoB240K but only trace amounts of apoB335K. The hepatic apoB335K/apoB240Kleucine incorporation ratio ranged from 0.5-1.7 under different conditions, suggesting physiological regulation of the relative amounts produced. Disappearance of both newly synthesized hepatic apoB variants from plasma followed first order kinetics. The circulating half-life of hepatic apoB335K was 2.4 times that of hepatic apoB240K, indicating independent catabolism of the two variants and consistent with the hepatic production of multiple apoB-containing lipoproteins with different metabolic properties. The half-life of apoB240K from the intestine was even shorter than that of apoB240K from the liver.

Animals↗
PubMed1981

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.

Animals↗
PubMed1981

Source and fate of circulating citrulline.

Previous work has demonstrated a continuous release of citrulline from the small intestine into the circulation. To evaluate the physiologic significance of this process, we have now measured citrulline uptake and release by isolated, perfused livers and, through surgical means and arteriovenous difference measurements, by various organs of the rat in vivo. Intestinally derived citrulline, an end product of glutamine nitrogen metabolism, passes through the liver without appreciable uptake. No significant extraintestinal source of circulating citrulline was found. Contrary to earlier suggestions, the liver releases no citrulline normally, but only when supplied with unphysiological high levels of ornithine and ammonia. Renal citrulline uptake was equivalent to approximately 83% the rate of intestinal release; kidneys, in turn, released arginine equivalent to approximately 75% of the citrulline taken up. Acute experiments in which the intestine, intestine plus liver, or kidneys were excluded from the circulation indicate that additional organs may also participate in citrulline exchange, at least when the circulating citrulline level is abnormal. The intestinal-renal pathway seems to account for a large though still unmeasured portion of the citrulline turnover in the circulation.

Amino Acids↗
PubMed1979

Accelerated clearance of low-density and high-density lipoproteins and retarded clearance of E apoprotein-containing lipoproteins from the plasma of rats after modification of lysine residues.

Selective chemical modification of lysine residues of lipoproteins by acetoacetylation dramatically altered the metabolism of the lipoproteins without significantly altering other physical or chemical properties. Modification of 30-60% of the total lysine residues of iodinated rat or human low-density lipoproteins ((125)I-LDL) resulted in a rapid removal of these acetoacetylated lipoproteins from the plasma of rats. Within minutes after intravenous injection into intact rats, greater than 80% of the total injected dose disappeared from the plasma. The rapidly cleared acetoacetylated LDL appeared in the liver, and within 6-30 min as much as 50-80% of the total injected dose of modified LDL could be accounted for in the liver. Furthermore, it was possible to demonstrate in the isolated perfused rat liver that the Kupffer cells were responsible for the lipoprotein uptake. Human high-density lipoproteins (HDL(3)) were also rapidly removed from the plasma after acetoacetylation. In striking contrast, acetoacetylation (30-60%) of two E apoprotein-containing lipoproteins (rat HDL(1) and dog HDL(c)) retarded their removal from the plasma. The accelerated removal of modified LDL and HDL(3), in contrast to the retarded removal of modified HDL(1) and HDL(c), suggests that the recognition and removal process is specific for a property acquired by only certain lipoproteins after acetoacetylation. Moreover, these results suggest that lysine residues of the E apoprotein may play a functional role in the recognition process for the normal clearance of HDL(1) and HDL(c), a process that is interfered with after acetoacetylation.

Acetylation↗
PubMed1978

Identification of circulating apolipoproteins synthesized by rat small intestine in vivo.

Apolipoproteins synthesized by intestine were identified in mesenteric lymph and plasma collected from an animal preparation in which exposure to [3H]leucine was restricted to the mucosal epithelial cells of a fat-transporting small intestinal segment in situ. The intestine released labeled chylomicrons and very low density lipoproteins only into lymph but labeled high density lipoproteins into both plasma and lymph. Of the 3H incorporated into individual apolipoproteins, separated by polyacrylamide gel electrophoresis, 25% was recovered in apolipoprotein B (apoB), 50% in apoA-I, 21% in apoA-IV, and 4% in C apolipoproteins, nearly all in apoC-II and apoC-III-0. Only traces of radioactivity were associated with the arginine-rich protein and none was detected in apoA-II. Labeled apoB was found in chylomicrons and very low density lipoproteins and labeled apoA-I and apoA-IV in all lipoproteins. Evidence suggested that substantial amounts of labeled apoA-IV were also present in the lipoprotein-free lymph fraction (d greater than 1.21 g/ml), further indicating that apoA-IV is a major apolipoprotein product in intestine. Thus, the pattern of apolipoprotein synthesis by intestine is notably different from that reported for liver.

Animals↗
PubMed1977

Vascular perfusion of rat small intestine: metabolic studies with isolated and in situ preparations.

Two different preparations of vascularly-perfused rat small intestine are described and their application to metabolic studies in this tissue illustrated. The first, a totally isolated, blood-perfused intestinal preparation, has an apparent requirement for glucocorticoid and norepinephrine. Lymph is produced and collected, permitting studies of fat transport and lipoprotein biosynthesis. Results indicate that intestine synthesizes some, but not all, of the proteins associated with chylomicrons and other lipoproteins of intestinal lymph. The isolated intestine from 260-g rats metabolizes 75 mumoles of circulating glutamine per hour, mostly to CO2, lactate, citrulline, proline, alanine, and ammonia. In a second preparation, all venous blood is collected from a short, isolated intestinal segment in situ with intact arterial blood supply. Adaptations allow the quantitative determination of rates of uptake and metabolism of substances from both blood and lumen. Results indicate that over 36% of CO2 produced by intestine of fasted rats is derived from plasma glutamine and only 8% from glucose. Virtually all glutamate and aspartate and significant amounts of glutamine and arginine absorbed from the lumen are also metabolized. The resulting metabolic products were quantitated and include CO2, lactate, proline, citrulline, alanine, and glucose.

Amino Acids↗