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E Reaven

Publications and source records attributed to E Reaven.

53 records · Page 3Linked to original sources
PubMed1986

Uptake of low density lipoproteins by rat tissues. Special emphasis on the luteinized ovary.

The aim of this study was to determine how luteal cells of the hormone-primed (luteinized) ovary process low density lipoproteins (LDL). Ovary uptake of perfused 125I-LDL was assessed by tissue levels of radioactivity; the distribution of LDL protein in cells was assessed on autoradiograms of the fixed tissue; and the level of stimulation of steroidogenesis, as well as degradation of LDL protein, was assessed on effluent perfusion samples. Human LDL ligand used in these studies was rigorously defined biochemically and physiologically. Homologous (rat) LDL was used as a special ligand control. Other tissue controls included the use of perfused or in vivo-infused luteinized ovaries from animals pretreated to reduce circulating lipoprotein levels, perfused ovaries from a second hormone-primed model, perfused liver from estrogen-treated rats, and isolated and cultured cells from the same ovarian tissues used in the perfusion experiments. The results show that perfused LDL promptly stimulates steroidogenesis. However, the labeled protein moiety of the LDL is not interiorized by the luteal cells, nor is there evidence of LDL protein degradation in the effluent samples. In contrast, internalization of the ligand occurs when luteal cells are incubated with the ligand in vitro. We have observed also that uptake of the 125I-LDL by the ovary can be displaced equally well by excess unlabeled LDL or HDL3. Overall, these experiments suggest that in the intact luteinized ovary, LDL binds to the same sites on the cell surface where HDL "binds," and that LDL cholesterol must be obtained by these steroid hormone-producing cells by a mechanism that does not require internalization of the intact lipoprotein particle.

Animals↗
PubMed1985

Effects of antimicrotubule agents on phospholipid metabolism in rat hepatic subcellular membranes.

Treatment of animals with antimicrotubule drugs has been shown to cause a perplexing variety of cellular changes which, theoretically, could be the result of changes in endomembrane biosynthesis, composition or flow. In the current study we have focused on this possibility by identifying antimicrotubule drug-induced changes in the phospholipid metabolism of hepatic subcellular membranes. Young adult rats were pretreated with radiolabeled [32 P]orthophosphate for 12 hr, and subsequently given saline, colchicine (2.5 mg/kg body wt) or vinblastine (20 mg/kg body wt) for 4 additional hr. Afterwards, the livers were homogenized, and separate microsomal and Golgi membrane fractions were prepared and subjected to phospholipid extraction and identification using two-dimensional thin-layer chromatography. The results show that colchicine and vinblastine given in vivo caused specific, rapid and in some cases, dramatic changes in phospholipid turnover in different membrane fractions of rat liver. The drugs specifically increased labeling of phosphatidylinositol-4-monophosphate and phosphatidylinositol-4,5-biphosphate and decreased the radioactivity associated with phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol in all fractions examined. In contrast, the antimicrotubule drugs produced a differential effect on the labeling pattern of sphingomyelin and lysophosphatidylcholine, i.e. they stimulated labeling of these phospholipids in microsomes, produced no changes in heavy Golgi fractions, and markedly increased their labeling in light Golgi fractions. These data suggest that antimicrotubule drugs restrict the incorporation of certain precursor phospholipids into forming membranes but do not affect the subsequent metabolism of these phospholipids. At the same time, the drugs appear to retard the flow of membranes from one cellular compartment to another.

Animals↗
PubMed1984

Glucose-induced insulin secretion by perfused pancreas of 2- and 12-mo-old Fischer 344 rats.

Previous studies have shown that cells from older Sprague-Dawley rats secrete insulin less efficiently in response to a maximal glucose challenge than do beta-cells from young animals. In the current study we have asked whether this change in beta-cell response occurs in another strain of rat, and, if so, whether the secretory defect occurs at submaximal as well as maximal glucose stimulatory levels. Pancreas perfusions were carried out on 2- and 12-mo-old Fischer 344 rats at perfusate glucose concentrations of 150 and 300 mg/dl. The secretory data for each pancreas was subsequently corrected for differences in islet cell mass and expressed as insulin secretion per unit islet cell. The results show that 12-mo-old Fischer rats release more insulin per total pancreas than do 2-mo-old animals at both glucose concentrations. However when corrected for islet cell mass, the amount of insulin secretion per islet cell is actually reduced in the older Fischer rat. These data are comparable to those seen previously in the 12-mo-old Sprague-Dawley rat and indicate that the insulin secretory defect seen as rats grow older is not species specific.

Aging↗
PubMed1984

Morphological evidence that high density lipoproteins are not internalized by steroid-producing cells during in situ organ perfusion.

Although it is clear that high density lipoproteins (HDL) can support steroidogenesis in several rat cell systems, questions still arise as to how HDL are processed by cells. In particular, it is not yet clear whether HDL are internalized by a pathway similar to that used for low density lipoproteins. This issue was examined in the present study using the luteinized ovaries of hormone-primed immature rats in an in situ perfusion system. Ovaries were perfused for 2-120 min with 125I-labeled human or rat HDL and processed for autoradiographic studies at the light and electron microscopic level, or homogenized and used for isolation of subcellular membranes. The results show that the luteal cells of this tissue bind both human and rat HDL with great specificity. Moreover, the intact HDL particle does not appear to be internalized by the luteal cell during the period of perfusion: i.e., the protein moiety of the labeled HDL remains associated with the plasma membrane at all times. Evidence from the autoradiographs suggest, however, that with time, an increasing proportion of the plasma membrane-bound protein is associated with inverted microvilli, which are embedded within the cytoplasm and make close contact with structures of the interior of the cell. We speculate that HDL-cholesterol may be transferred at such sites.

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PubMed1983

Unique microtubules in luteal cells from superovulated rats.

Luteal cells of immature female rats treated with gonadotropins contain microtubules with a number of interesting features. Many of the microtubules of these cells are arranged in bundles in which they are separated one from another by strands of material (i-MT bands) of unknown composition. The microtubules within the bundles assume a hexagonal packing pattern with i-MT bands between any two microtubules. The bundle microtubules and their i-MT bands are further connected via crosslinking filaments: pattern obtained from densitometer scans (measuring the arrangement of the crosslinking filaments) suggest that the filaments may represent microtubule-associated proteins. The complex arrangement of the microtubules within a bundle does not appear to extend for the entire length of the individual microtubules, and occasionally one sees profiles of single microtubules fanning out from the ends of the bundle: whether the same microtubules are regrouped at some other point in the cell is not known. Structures similar to the i-MT band and the crosslinking filaments have also been observed connecting microtubules to segments of the luteal cell plasma membrane: in these instances the i-MT-like band is found between the longitudinally sectioned microtubule and the membrane, with filaments connecting the two structures via the intermediate band. It is of interest that the microtubules of these luteal cells are not sensitive to treatment with antimicrotubule drugs and we suggest that the complex bundling arrangement provides their unusual stability.

Animals↗
PubMed1983

Effect of age and environmental factors on insulin release from the perfused pancreas of the rat.

In this study we examined the effect of age and various age-related environmental factors on maximal glucose-stimulated insulin release by the intact perfused pancreas. Male Sprague-Dawley rats were maintained from 40 d to 12 mo of age on standard chow, or on a sucrose-rich or calorie-restricted diet. At 12 mo, studies were carried out on the isolated pancreas of each animal to determine maximal (300 mg/ml) glucose-stimulated insulin secretion. After these studies were completed, each pancreas was perfused with formalin fixative and processed for morphometric estimation of the mass of the endocrine pancreas. Data from these older animals were compared with data from 2-mo-old control rats. The results indicate that maximal glucose-stimulated insulin secretion per unit endocrine pancreas was markedly reduced in all three groups of 12-mo-old rats, and was only 25-33% of that of 2-mo-old rats. Thus, aging led to a decline in insulin secretion per beta cell that was not modifiable by environmental manipulation. On the other hand, environmental factors can influence the development of endocrine tissue within the pancreas, and in so doing, modify total pancreatic insulin secretion. The mass of the endocrine pancreas of 12-mo-old rats fed either sucrose or chow was between three and four times that of 2-mo-old control rats, and these older rats were able to maximally secrete as much insulin per total pancreas as the young rats. In contrast, the endocrine cell mass of the calorie-restricted rats had not enlarged to this extent, and the maximally stimulated perfused pancreas from these rats secreted less insulin. These data suggest that the aging animal, challenged in vivo to secrete insulin, can overcome the loss of the beta cell response by expanding its pancreatic pool of beta cells. Although this compensation is successful in the 12-mo-old, obese, middle-aged rat, it is not yet clear what effect further aging would have on these events.

Aging↗
PubMed1983

Effect of age and diet on insulin secretion and insulin action in the rat.

The effects of aging on various aspects of insulin secretion and action were studied in male Sprague-Dawley rats, maintained from 1 1/2 to 12 mo of age on conventional rat chow, sucrose-rich, or calorie-restricted diets. In chow-fed rats, islet volume increased as the animals grew from 1 1/2 to 12 mo of age, but glucose-stimulated insulin secretion (per volume islet) declined over the same interval. In addition, in vivo insulin-stimulated glucose utilization fell in these rats. However, the plasma insulin response to an oral glucose challenge was sufficient to prevent frank decompensation of glucose tolerance (presumably due to an increase in total pancreatic endocrine cell mass). All these changes, with the exception of the decline in glucose-stimulated insulin secretion per volume islet, were accentuated by feeding sucrose. Thus, 12-mo-old sucrose-fed rats had larger islets and higher plasma insulin levels in response to an oral glucose challenge, and the rats were more insulin-resistant than chow-fed rats. However, glucose-stimulated insulin release per volume islet was similar in 12-mo-old chow-fed and sucrose-fed rats. In contrast, calorie restriction led to an amelioration in all but one of the age-related changes, i.e., islets from calorie-restricted rats were comparable in size to those of 2-mo-old rats, the animals had lower plasma insulin levels in response to an oral glucose load, and they were less insulin resistant than the other two groups of 12-mo-old rats. On the other hand, glucose-stimulated insulin secretion per volume islet was similar to that of the other 12-mo-old rats. These results suggest that aging leads to marked changes in both insulin secretion and insulin action. The decline in glucose-stimulated insulin secretion per unit endocrine pancreas appears to be an inevitable consequence of the aging process. In contrast, the age-related changes in islet size, insulin response to a glucose load, and in vivo insulin-stimulated glucose uptake are extremely responsive to variations in amount and kind of calories. DIABETES 32:175-180, February 1983.

Aging↗
PubMed1982

Functional homogeneity of pancreatic islets of aging rats.

Islest from different regions of pancreases of aging rats were compared for size and variations of response to glucose stimulation. The results show that pancreatic islets from the ventral-duodenal and splenic regions of 12-mo-old retired breeder. Spraque dawley rats are comparable in all respects measured: thus, pancreatic regional differences cannot explain the age-associated reduction in beta cell secretory response noted in previous studies.

Aging↗
PubMed1982

Effect of antimicrotubule agents on microtubules and steroidogenesis in luteal cells.

This report represents an effort to reinvestigate the relationship of steroid hormone processing with microtubule protein, using the highly active progesterone-producing cells of superovulated immature rats as a model steroid hormone system. For the most part, gonadotropin-primed rats were used at 6 days and were reinjected with saline or various doses of colchicine and vinblastine: after 3 h the animals were injected with human chorionic gonadotropin (hCG, 10 IU) or saline for an additional hour. Subsequently, the ovaries were perfused with fixative for morphological studies or luteal cells were isolated for in vitro incubation with various additives. The results are as follows: 1) luteal cell progesterone synthesis is reduced in a dose- and time-dependent manner after treatment with antimicrotubule drugs; 2) additional stimulating agents given in vitro (hCG, Bt2cAMP, epinephrine) do not overcome the reduction produced by treatment with antimicrotubule drugs; 3) luteal cells show normal protein synthesis after treatment with antimicrotubule drugs; 4) luteal cells have microtubules of unusual appearance that are insensitive to the action of antimicrotubule agents. These results show that in the luteinized ovary, both colchicine and vinblastine interfere with hormone-stimulated steroidogenesis. Although the mechanism for the effect is not yet understood, it does not appear to be related to the content of the intact microtubules in luteal cells. Whether the drugs exert their action by binding to the unassembled form of microtubule protein in the cells remains to be determined.

Animals↗
PubMed1981

Effect of various hepatic membrane fractions on microtubule assembly-with special emphasis on the role of membrane phospholipids.

This report describes an interaction between rat brain microtubule protein and various hepatic fractions in vitro. Purified preparations of Golgi membranes, plasma membrane, rough and smooth endoplasmic reticulum, nuclear membranes, and mitochondria were obtained from the livers of 200-g rats. Several concentrations of fresh or sonicated frozen membranes were incubated with twice-cycled rat brain microtubule protein in a microtubule assembly buffer for 60 min at 30 degrees C. Changes in microtubule assembly were assessed either by quantitative electron microscopy on negatively stained samples or by spectrophotometric methods. The results show that all the tested membranes "bound" microtubule protein, preventing assembly: Golgi and plasma membranes, as well as mitochondria, were especially potent in this regard. To identify the membrane-associated components responsible for microtubule protein binding, the membranes were extracted with methanol-chloroform, and liposomes were prepared from the resulting lipids. Microtubule protein incubated with these liposomes showed a differential ability to assemble that was similar to the effect obtained with intact membranes. Membrane-extracted phospholipids were identified as the lipid component responsible for these changes, with the negatively charged phospholipids (cardiolipin and phosphatidylserine) being uniquely active. These findings indicate that hepatic membranes differentially interact with brain microtubule protein; this interaction may be dependent on membrane phospholipids.

Animals↗
PubMed1980

Effect of age on leucine-induced insulin secretion by the beta-cell.

We have previously shown that aging is associated with a marked decrease in glucose-stimulated insulin release by the beta-cell. To determine whether this age-related defect occurs with other insulin secretagogues, we have investigated leucine-stimulated insulin release. Islets of Langerhans were isolated from 2- and 12-month-old rats, and incubated with 5-20 mM 5-20 mM leucine or with 16.7 mM glucose. Islets from 12-month-old rats showed a decline in rate of leucine-induced secretion as compared to islets of 2-month-old donors (i.e., mean (+/- SEM) insulin secretion at 20 mM leucine was 0.79 (+/- 0.06) and 1.17 (+/- 0.09) microU/min/islet), respectively for 12- and 2-month-old rats. beta-cell number is increased in islets from 12-month-old donors, and when this increase is taken into account, the average beta-cell from 12-month-old donors secretes only one-half the rate of beta-cells from 2-month-old donors. These findings are comparable to those seen after glucose-stimulation, and emphasize the potential global nature of the age-related defect in beta-cell function.

Aging↗
PubMed1973

Vasopressin: possible role of microtubules and microfilaments in its action.

Colchicine, vinblastine, podophyllotoxin, and cytochalasin B inhibit the action of vasopressin and cyclic adenosine monophosphate on osmotic water movement across the toad bladder. The findings suggest that microtubules, and possibly microfilaments, play a role in the action of vasopressin, perhaps through involvement in the mechanism of release of secretory material from the bladder epithelial cells.

Animals↗
PubMedDate not supplied

Diabetic hypertriglyceridemia.

The conference opened with the clinical presentation of a 50-year-old male with fasting hyperglycemia (296 mg per cent) and hypertriglyceridemia (2736 mg per cent). The discussion began with a summary of current concepts regarding the manner in which chylomicra (intestine) and very low density lipoproteins (intestine and liver) are formed, transported into the plasma, and removed from the circulation. This was followed by a consideration of diabetic hypertriglyceridemia in which this syndrome was subdivided into two categories. The first form is seen in patients with severe fasting hyperglycemia, and is characterized by marked insulin deficiency, decreased very low density lipoprotein production, a fall in the activity of lipoprotein lipase, and hypertriglyceridemia secondary to a defect in removal of lipoproteins from the plasma. In contrast the other form of diabetic hypertriglyceridemia is seen in patients with minimal abnormalities of carbohydrate tolerance, and in this instance insulin resistance, not insulin lack, seems to play the pivotal role. In these patients, the rise in plasma triglyceride levels seems to be secondary to increased production of very low density lipoproteins, presumably as a result of the hyperinsulinemia associated with the insulin resistance. The conference ended with an attempt to relate the patient presented to the models of diabetic hypertriglyceridemia that had been defined.

Animals↗
PubMedDate not supplied

Evidence for surface entrapment of cholesterol-rich lipoproteins in luteinized ovary.

Previous studies showed that perfused 125I-labeled low and high density lipoproteins (LDL, HDL) have affinity for specialized microvillar regions of luteal cells in hormone-primed, luteinized rat ovaries. In the current report, we re-examined the interaction of cholesterol-rich lipoproteins with these specialized plasma membrane regions using native lipoproteins visualized as discrete particles by standard electron microscopic techniques. In ovaries perfused with the various lipoproteins, spherical particles (varying in size from 12 to 28 nm depending on the particle used) were found over the surfaces of all luteal cells and filling up extensive "channel" space formed by the apposed plasma membranes of adjacent microvilli or cytoplasmic surfaces. Only 30% of these tissue-associated particles were removable after prolonged washing with perfused media or heparin. Few intact particles were found inside the cells, despite the fact that the lipoproteins induced a substantial hormone response by the ovary. To determine the total protein internalized by cells during the course of the experiments, parallel biochemical experiments were carried out with nonreleasable (14C-sucrose-coupled) human LDL. Of the total bound 14C-sucrose LDL, only 8.5% was degraded (trichloroacetic acid-soluble) and presumed internalized by the cells. Thus, while large numbers of cholesterol-rich lipoprotein particles interact with the luteal cell surface in specialized microvillar channels and elicit a progesterone response, relatively few intact lipoprotein particles appear to enter the cells to be degraded. We speculate that in the luteinized ovary, a large majority of the lipoprotein-cholesterol transfer occurs at the surface of the luteal cells, and that the membranes of the microvillar channels are involved in this process.

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