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Biomedical subjects

M L Overturf

Publications and source records attributed to M L Overturf.

At least 19 recordsLinked to original sources
PubMed1994

Resistance to dietary-induced hypercholesterolemia exhibited by a unique strain of New Zealand white rabbits.

The search for a precise metabolic explanation for the capacity of some individuals to resist the development of dietary-induced hypercholesterolemia, thus avoiding attendant cardiovascular atherosclerotic complications, has long been the focus of our research. From 1 New Zealand white rabbit that failed to show any cholesterolemic response, we have, over the course of 10 years, established a partially inbred strain of strongly cholesterol-resistant rabbits. This achievement has resulted in the production of a large number of cholesterol-resistant animals for study; more importantly, it has shown that a strong genetic factor operates in dietary regulation of plasma cholesterol levels. We have focused our research on the different possibilities associated with this genetic predisposition. Since the cholesterol-resistant rabbits do not accumulate cholesterol or its esters in plasma or in any tissue compartments, we investigated several biochemical pathways involved in cholesterol metabolism. We have recently concentrated on the enzyme cholesterol 7 alpha-hydroxylase and liver bile acid metabolism. We have cloned the complete gene and partial cDNAs for cholesterol 7 alpha-hydroxylase from both normal and cholesterol-resistant rabbits. This has allowed the discovery of changes in the transcription of this gene in the cholesterol-resistant rabbits compared with normal littermates. These cholesterol-resistant rabbits have provided a model demonstrating that there are biological means to prevent large dietary loads of cholesterol from accumulating in plasma or tissues. Our hypothesis is that cholesterol-resistant animals increase cholesterol turnover by increasing bile acid excretion, thus providing a way to reduce plasma cholesterol of either dietary or endogenous origin. The methods and observations of our research are presented chronologically in this review.

Animals↗
PubMed1993

Bile acid excretion and cholesterol 7 alpha-hydroxylase expression in hypercholesterolemia-resistant rabbits.

We have developed a partially inbred substrain of New Zealand white rabbits (CRT/mlo) that are resistant to the hypercholesterolemia that accompanies cholesterol feeding to normal rabbits. The plasma cholesterol concentration of normal rabbits increases dramatically from about 30 mg/dl to > 300 mg/dl after they are fed a 0.1% cholesterol-enriched diet for 3-4 months. Cholesterol-fed CRT/mlo animals, however, maintain a cholesterol level of about 30 mg/dl during the entire cholesterol feeding period. In addition to the low plasma cholesterol level, measurements of cellular cholesterol indicate that the hepatic cholesterol content of the cholesterol-fed resistant rabbit remains markedly lower than it does in normal animals fed the same diet. The only mechanism for removal of significant quantities of cholesterol carbon from the body is via the fecal excretion of cholesterol, neutral sterol metabolites, and bile acids. In comparison to the basal, low-cholesterol diet, we observed that cholesterol-fed resistant rabbits had increased excretion of lithocholic acid, while excretion of this bile acid by cholesterol-fed normal rabbit remained similar to basal diet levels. Deoxycholic acid excretion, the other main bile acid excreted in the feces of rabbits, was decreased in response to cholesterol challenge in animals with either resistant or normal phenotypes, but the decrease was significantly less in the resistant rabbits. Thus, the resistant rabbits excreted relatively more lithocholic and deoxycholic acid than did the cholesterol-fed normal rabbit. The difference in bile acid excretion was also manifest by a higher than normal level of cholesterol 7 alpha-hydroxylase activity and cholesterol 7 alpha-hydroxylase mRNA in the livers from resistant versus normal rabbits. As cholesterol 7 alpha-hydroxylase is the putative rate-limiting step of bile acid synthesis, we believe that the increased excretion of bile acids by resistant animals is due, at least in part, to increased levels of cholesterol 7 alpha-hydroxylase expression.

Animals↗
PubMed1991

Cholesterol metabolism in hypercholesterolemia-resistant rabbits.

Normal rabbits typically respond to a diet high in cholesterol with a large increase in the concentration of plasma cholesterol. We have previously described the breeding and partial characterization of a variant rabbit which does not respond to a high cholesterol diet with changes in plasma cholesterol concentration. In the present report we have characterized three components involved in cholesterol homeostasis: the B/E (LDL) receptor, 3-hydroxy-3-methylglutaryl coenzyme A reductase activity (HMG-CoA reductase, EC 1.1.1.34) and acyl-coenzyme A: cholesterol acyltransferase activity (ACAT, EC 2.3.1.26) in the livers of the hypercholesterolemia-resistant rabbits. Using normal cholesterol-fed rabbit [125I] beta-VLDL as a ligand, liver membranes prepared from resistant rabbits fed a low-cholesterol diet had 70% higher binding capacity than membranes from normal rabbits fed the same diet. Similar experiments demonstrated that the resistant rabbits had a 240% higher B/E receptor binding capacity compared to normal animals when liver membranes were prepared from animals fed a 0.25% cholesterol-enriched diet. No difference in the binding affinity of [125I]beta-VLDL was detected in membranes prepared from normal or resistant animals. When fed a low-cholesterol diet, the resistant rabbits had approximately 2-fold higher hepatic HMG-CoA reductase activity (97.4 +/- 3.5 pmol product/mg/min in resistant animals compared to 45 +/- 1.1 pmol product/min/mg in normal animals). The difference was exaggerated in animals fed the 0.25% cholesterol-enriched diet, 73.3 +/- 5.5 vs 2.4 +/- 0.56 pmol product/min/mg for resistant and normal membranes respectively. The basal activity of ACAT in hepatic membranes was significantly lower in the resistant rabbits compared to normal rabbits (138 +/- 11 vs 268 +/- 19 pmol cholesteryl ester/min/mg in resistant and normal rabbits respectively); when fed a 0.25% cholesterol-enriched diet, the enzyme was induced 6-fold in normal animals but was increased only 2-fold in the resistant animal. These biochemical data suggested that the resistant rabbit maintained low intracellular cholesterol even when fed a cholesterol-enriched diet. Direct measurement of cellular cholesterol and cholesteryl esters demonstrated that the concentration of these lipids was significantly lower in the resistant animal than in normal animals with the largest differences found in the cytoplasmic rather than the membrane compartment. These studies demonstrate that the resistant rabbit manifests several quantitative differences in cholesterol metabolism and in the regulation of cholesterol metabolism; but these studies do not directly explain the underlying cause of the resistance to hypercholesterolemia in the resistant rabbit.

Animals↗
PubMed1990

Dietary cholesterol absorption, and sterol and bile acid excretion in hypercholesterolemia-resistant white rabbits.

The New Zealand white (NZW) rabbit fed a 0.1% cholesterol-enriched diet (CD) typically responds (normoresponsive, NR) by quickly developing hypercholesterolemia. To study the underlying mechanisms responsible for the widespread phenomenon of inter-individual variability of response to dietary cholesterol, a unique hypercholesterolemia-resistant (RT) rabbit model was developed. These animals were utilized to investigate selected potential mechanisms that might enable the RT animal to compensate for dietary cholesterol overload. When rabbits were fed the low-cholesterol stock diet, there was no significant difference in the plasma cholesterol concentrations of the NR and the RT animals. However, a significant rise was observed in the NR rabbits within 1 month of their being placed on the cholesterol-enriched diet; the plasma cholesterol concentration of the RT animals was not affected. During consumption of the cholesterol diet the cholesterol absorption rate was somewhat greater in the NR rabbits (P less than 0.05), whereas intestinal transit times and the fecal excretion of neutral steroids were substantially the same in both groups. In contrast, the fecal bile acid excretion of the RT animals was more than twice as great (P less than 0.0001) as that of the NR animals. We conclude that the response to dietary cholesterol is a heritable trait in these rabbits and that, although less dietary cholesterol was absorbed by the RT animals, it appears that a major mechanism controlling plasma cholesterol levels involves the rate of conversion of cholesterol to bile acids and their subsequent excretion.

Animals↗
PubMed1990

Cholesterol metabolism in fibroblasts from rabbits resistant to diet-induced hypercholesterolemia.

We have previously described a colony of New Zealand White rabbits that are resistant to hypercholesterolemia when fed a cholesterol-enriched diet. The present studies used skin fibroblasts obtained from normal and hypercholesterolemia-resistant rabbits to investigate cholesterol metabolism and lipid composition in vitro. The lipid compositions of the two cell lines after incubation in either fetal calf serum or lipoprotein-deficient serum were similar. The conversion of radiolabeled acetate into sterol and phospholipids was higher in resistant fibroblasts than in normal fibroblasts. In contrast, incorporation of radiolabeled oleic acid into cholesteryl ester was significantly lower in resistant fibroblasts than in normal cells. In parallel experiments, the 3-hydroxy-3-methylglutaryl coenzyme A reductase activity was higher and acyl-coenzyme A:cholesterol acyltransferase activity was lower in resistant cells compared to normal cells. Furthermore, binding, uptake, and degradation of normal rabbit 125I-labeled LDL (low density lipoproteins) were 30% higher in resistant than in normal fibroblasts. These observations are consistent with results from previous studies of cholesterol metabolism in the liver membranes of these rabbits. The results indicate that extrahepatic cells (such as fibroblasts) from the resistant rabbit exhibit the same altered cholesterol metabolism as that found in the hepatic tissues of these rabbits. These studies suggest that the resistant rabbit may provide an in vivo and in vitro system for studying the mechanisms by which some individuals of a species can minimize the effect of dietary cholesterol on the development of hypercholesterolemia and atherosclerosis.

Animals↗
PubMed1989

Development and partial metabolic characterization of a dietary cholesterol-resistant colony of rabbits.

A colony of New Zealand white rabbits has been developed which, when fed a cholesterol-supplemented diet, exhibit unusual resistance to hypercholesterolemia and atherosclerosis, disorders usually observed in normal cholesterol-fed rabbits. When resistant rabbits (RT) were fed a normal low cholesterol diet (ND), their plasma lipoprotein patterns were significantly different from those of normal rabbits (NR) fed the same diet. The low density lipoprotein cholesterol (LDL-c)/high density lipoprotein cholesterol (HDL-c) ratio and LDL-c/very low density lipoprotein cholesterol (VLDL-c) ratio were lower in the resistant rabbits. The hydrated density of HDL of the normal-responsive rabbits was greater than that of the resistant rabbits. LDL from resistant rabbits contained a lower proportion of esterified cholesterol and protein than LDL from normal rabbits. Peripheral mononuclear cells from resistant rabbits bound about 30% more 125I-labeled rabbit LDL than mononuclear cells from normal rabbits. These results demonstrate that the plasma cholesterol levels of these animals is at least partly under genetic control and that compositional differences exist between the major plasma lipoprotein classes of normal and resistant rabbits even during the ingestion of low-cholesterol diet. The results indicate that at least a part of the difference in the cholesterolemic responses between the two rabbit groups is due to an enhanced LDL uptake by the mononuclear cells, and presumably by other somatic cells of the resistant group.

Animals↗
PubMed1988

Effects of moderate hypertension on cardiac function and metabolism in the rabbit.

To study the early effects of hypertension on the heart, we examined isolated hearts from rabbits with slowly developing hypertension of up to 64 weeks in duration after unilateral nephrectomy and renal artery stenosis. Normotensive animals kept under identical conditions served as controls. Mean arterial blood pressure rose from 83 to 155 mm Hg in the hypertensive group of longest duration, but the ratio of left ventricular weight to body weight was not different between the experimental and control groups. Although left ventricular hypertrophy was not present, left ventricular peak systolic pressure of perfused hearts was significantly higher in hypertensive than in normotensive hearts. Furthermore, while in hypertensive hearts the left ventricular end-diastolic volume was increased, the peak systolic pressure did not respond to an increase in left ventricular end-diastolic volume. Functional changes were accompanied by metabolic changes in the left ventricle. Rates of glucose utilization were increased and rates of ketone body utilization were decreased in hypertensive hearts. Activities of key enzymes of carbohydrate metabolism (phosphorylase, hexokinase, phosphofructokinase, and lactate dehydrogenase) were increased, while those of ketone body metabolism (3-oxoacid-CoA transferase, acetoacetyl-CoA synthase) were decreased and those of the citric acid cycle (citrate synthase, 2-oxoglutarate dehydrogenase) were not different between groups. In summary, moderate hypertension for a period of more than 1 year resulted in functional and metabolic changes of the left ventricle in hypertensive animals that were already manifest at 8 weeks of hypertension.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1986

Failure of nifedipine to reduce atherogenesis in cholesterol-fed rabbits.

The purpose of this study was to determine if therapeutic dosages of nifedipine, a drug that reduces intracellular calcium concentrations, suppresses atherogenesis in normotensive, cholesterol-fed rabbits. Control groups were fed either normal chow (Group I), or normal chow supplemented with 0.10 and 0.25% cholesterol (wt/wt) (Group II). Group III animals were fed the same diets as Group II animals and received nifedipine (0.625 and 1.250 mg/kg/d). During the course of the study (3 months) no systematic quantitative difference (P greater than 0.05) was observed between Groups II and III with regard to mean arterial blood pressure, serum cholesterol, serum electrolytes (sodium and potassium), plasma renin activity, or serum lipoprotein classes. At necropsy, there likewise was no difference in affected aortic surface area, aortic cholesterol content, or renal renin content. We conclude that nifedipine, at human therapeutic dosages, has no effect on either atherogenesis or the renin-angiotensin system in normotensive rabbits fed a moderately cholesterol-rich diet.

Animals↗
PubMed1985

Captopril-induced hyperreninemia in cholesterol-fed rabbits.

The aims of this study were to investigate the potential effects of captopril (CPT)-induced chronic hyperreninemia on atherogenesis, and to describe and quantitate the morphological changes which occur in the juxtaglomerular (JG) apparatus of drug-treated rabbits. Four groups of normotensive New Zealand rabbits were used. Drug control groups were fed regular rabbit chow (Group I), or regular chow supplemented with cholesterol (Group III). Group II animals were fed regular chow and treated with captopril, and Group IV animals were fed the cholesterol-diet and treated with captopril. Daily captopril administration for a period of six months resulted in significantly (p less than 0.001) increased levels of plasma renin activity and blood urea nitrogen. Mean systemic arterial pressure, plasma aldosterone levels, and hematocrits were significantly reduced in the CPT-treated animals as compared to untreated control groups. No effect on atherogenesis was found. Morphometric analysis showed no difference in the size of the glomeruli between the untreated and the CPT-treated, normal-diet groups, however, significant (p less than 0.001) hypertrophy and hyperplasia of the JG complex were observed in all CPT-treated animals. It is concluded that captopril-induced reductions in systemic arterial blood pressure and perfusion pressure, in concert with a blocked renin-angiotensin system which interferes with the normal autoregulation of renal blood flow and glomerular filtration, leads to significant morphologic and functional alterations in the kidney of normotensive animals.

Aldosterone↗
PubMed1982

Capoten-induced juxtaglomerular hyperplasia in rabbits.

The effects of the converting enzyme (CE) inhibitor, captopril, on blood pressure, plasma aldosterone, plasma renin activity (PRA), and kidney morphology were studied. Captopril, at a near maximum daily recommended human dose of approximately 5.0 mg/kg, was administered to rabbits over a period of six months. Mean arterial pressure, CE activity, and aldosterone levels were significantly reduced; PRA and renal renin activity were increased. Microscopic examination of the kidney showed marked hyperplasia of the juxtaglomerular apparatus in all of the treated animals.

Aldosterone↗
PubMed1981

Is renin a risk factor?

1. These results show that elevated blood pressure and a hyperlipidemic diet exacerbate atherogenesis in the two kidney, one-clip hypertensive rabbit. Elevated PRA activity was not essential for the hypertension and did not exacerbate atherogenesis. 2. In addition, the experimentally induced, low renin state following DOC-saline did not result in a protective effect on cardiovascular lesions in rabbits fed an atherogenic diet when compared to normal renin controls. 3. Thus, in these experiments neither an increase in plasma renin accelerated atherogenesis, nor did a decrease in PRA slow the rate of production of atherosclerosis in the rabbit. 4. These observations lend no support to the thesis that renin is an independent risk factor when it is generated within the body in response to these stimuli. Indeed, it suggests that in this setting other factors, not PRA, are responsible for both hypertension in the two-kidney, one-clip rabbit and the arterial damage which occurs in this hypercholesterolemic model.

Aldosterone↗
PubMed1980

Lability of HDL-cholesterol during serum storage.

In many clinical and research situations it would be desirable to store, for extended periods, specimens on which measurements of high density lipoprotein cholesterol (HDL-C) levels were desired. To test the effects of storage on subsequent HDL-C levels, divided serum specimens from 46 subjects were frozen at -80 degrees C and analyzed at intervals throughout a 12-month period. A statistically significant increase in the HDL-C levels was found (P < 0.01). Thus, caution must be exercised in interpreting the results of HDL-C analysis done on specimens stored for prolonged periods even at -80 degrees C.

Blood Preservation↗
PubMed1979

The effects of kallikrein, plasmin, and thrombin on hog kidney renin.

Pretreatment of hog high molecular weight renin for 30 min at 37 degrees C with 0.12 unit of either kallikrein or thrombin significantly increased (p less than 0.001) the amount of angiotensin I formed during subsequent incubations with homologous angiotensinogen. However, the thrombin-treated hog renin had 13 times more activity than the kallikrein-treated enzyme. Aprotinin did not inhibit the kallikrein-mediated activation of renin; the results indicated that aprotinin inhibited renin preferentially. Plasmin (0.25 unit) had little effect on the activity of high molecular weight renin. The molecular weight of hog renin on sodium dodecyl sulfate-polyacrylamide gel electrophoresis was not altered after exposure to either kallikrein, thrombin, or plasmin. These results do not exclude the occurrence of a limited proteolytic event or a conformational change beyond the detection of the current method. The data show that the activation of hog high molecular weight renin by thrombin and kallikrein was not associated with the conversion of renin to Mr = 43,000.

Animals↗