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Biomedical subjects

H R Knapp

Publications and source records attributed to H R Knapp.

At least 19 recordsLinked to original sources
PubMed2001

Elevation of asymmetrical dimethylarginine may mediate endothelial dysfunction during experimental hyperhomocyst(e)inaemia in humans.

Hyperhomocyst(e)inaemia is associated with endothelial dysfunction in animals and humans. Mechanisms responsible for endothelial dysfunction in hyperhomocyst(e)inaemia are poorly understood, but may involve impaired bioavailability of endothelium-derived nitric oxide (NO). We hypothesized that acute elevation of homocyst(e)ine by oral methionine loading may stimulate the formation of asymmetrical dimethylarginine (ADMA), an endogenous inhibitor of NO synthase, due to a transmethylation reaction during the formation of homocyst(e)ine from methionine. We studied nine healthy human subjects (five males, four females) aged 29+/-2 years. Flow-mediated vasodilation (FMD) in the brachial artery (endothelium-dependent) and vasodilation induced by nitroglycerine (endothelium-independent) were measured with high-resolution ultrasound before and 8 h after oral methionine (100 mg/kg in cranberry juice) or placebo (cranberry juice), on separate days and in random order. Plasma homocyst(e)ine and ADMA concentrations were measured by specific HPLC methods. After a methionine bolus, elevation of homocyst(e)ine (28.4+/-3.5 micromol/l) was associated with an increased plasma concentration of ADMA (2.03+/-0.18 micromol/l) and reduced FMD (1.54+/-0.92%). Placebo had no effect on these parameters. There was a significant inverse linear relationship between ADMA concentration and FMD (r=-0.49; P<0.05), which was stronger than the relationship between the homocyst(e)ine concentration and FMD (r=-0.36; not significant). We conclude that acute elevation of the homocyst(e)ine concentration impairs vascular endothelial function by a mechanism in which an elevated concentration of ADMA may be involved. This finding may have importance for understanding the mechanism(s) leading to homocyst(e)ine-associated vascular disease, and its potential treatment.

Acute Disease↗
PubMed1999

Role of oxidant stress in endothelial dysfunction produced by experimental hyperhomocyst(e)inemia in humans.

BACKGROUND: Moderate elevations in plasma homocyst(e)ine concentrations are associated with atherosclerosis and hypertension. We tested the hypothesis that experimental perturbation of homocysteine levels produces resistance and conduit vessel endothelial dysfunction and that this occurs through increased oxidant stress. METHODS AND RESULTS: Oral administration of L-methionine (100 mg/kg) was used to induce moderate hyperhomocyst(e)inemia ( approximately 25 micromol/L) in healthy human subjects. Endothelial function of forearm resistance vessels was assessed by use of forearm vasodilatation to brachial artery administration of the endothelium-dependent dilator acetylcholine. Conduit vessel endothelial function was assessed with flow-mediated dilatation of the brachial artery. Forearm resistance vessel dilatation to acetylcholine was significantly impaired 7 hours after methionine (methionine, 477+/-82%; placebo, 673+/-110%; P=0.016). Methionine did not alter vasodilatation to nitroprusside and verapamil. Flow-mediated dilatation was significantly impaired 8 hours after methionine loading (0.3+/-2.7%) compared with placebo (8. 2+/-1.6%, P=0.01). Oral administration of the antioxidant ascorbic acid (2 g) prevented methionine-induced endothelial dysfunction in both conduit and resistance vessels (P=0.03). CONCLUSIONS: Experimentally increasing plasma homocyst(e)ine concentrations by methionine loading rapidly impairs both conduit and resistance vessel endothelial function in healthy humans. Endothelial dysfunction in conduit and resistance vessels may underlie the reported associations between homocysteine and atherosclerosis and hypertension. Increased oxidant stress appears to play a pathophysiological role in the deleterious endothelial effects of homocysteine.

Acetylcholine↗
PubMed1998

A low temperature flotation method to rapidly isolate lipoproteins from plasma.

To minimize oxidative modification, a low temperature, sequential flotation method was developed to isolate plasma lipoproteins in 18 h using a benchtop ultracentrifuge. The protein distributions were characterized using agarose and SDS-polyacrylamide gel electrophoresis, and an SDS-Lowry protein assay. The lipid distributions were assessed using a gas chromatography-mass spectrometric assay for cholesterol and an enzymatic assay for triglycerides. To validate the rapid flotation method, lipoproteins were also isolated from the same plasma samples using a modified Havel et al. flotation method (J. Clin. Invest. 34: 1345-1353, 1955). The same lipoproteins and apolipoproteins were present in fractions of comparable density, and the summed recoveries of protein, cholesterol, and triglyceride were also identical for the Havel et al. and rapid flotation procedures. Likewise, the amount of cholesterol and triglyceride in corresponding very low, intermediate, and low density lipoprotein (VLDL/IDL and LDL) fractions was the same for the two flotation procedures. The triglyceride and cholesterol levels in high density lipoprotein (HDL) isolated by rapid flotations, however, were 9-12% higher than in the HDL as isolated by Havel et al. Because a 9-12% increase in the HDL fraction reflects only 1-4% of the total triglyceride and cholesterol in plasma, we conclude that, while maintained at 4 degrees C, lipoproteins were quantitatively isolated from human plasma in 1 day.

Adult↗
PubMed1997

Longitudinal follow-up comparison of educational interventions: multimedia textbook, traditional lecture, and printed textbook.

RATIONALE AND OBJECTIVES: The goal of this prospective, interinstitutional study was to compare the long-term instructional effectiveness of a pediatric multimedia textbook (MMTB) to that of a standard lecture and a printed textbook. MATERIALS AND METHODS: A randomized cohort of 89 3rd-year medical students from two institutions were initially evaluated from June 1992 to June 1993 and reevaluated in May 1994. Students were randomly assigned to one of four instructional groups: computer-aided instruction by means of MMTBs (n = 21), traditional lecture (n = 23), printed textbook (n = 19), and a control group (n = 26). After instruction, all groups were tested by means of a multiple choice test at the end of their pediatric clerkship; they were given this same test 11-22 months later. RESULTS: The long-term instructional effectiveness of the MMTB, printed textbook, and lecture were the same as that in the control group, as determined by analysis of variance of mean test scores. CONCLUSION: The educational advantage of MMTBs observed immediately after instruction was not detected 1 year later. Because attrition reduced statistical power, further research is necessary to determine how educational fading affects these instructional formats.

Analysis of Variance↗
PubMed1997

Dietary fatty acids in human thrombosis and hemostasis.

The effects of fatty acids on hemostasis are controversial. It has been difficult to show convincing effects of saturated or monounsaturated fatty acids that are clearly related to hemostatic variables in humans. Unsaturated fatty acids alter platelet aggregation and processes related to coagulation and fibrinolysis. Indirect evidence exists that n-6 polyunsaturated fatty acids may exert favorable effects on thrombotic processes in vivo, but large clinical trials have failed to show benefits of 5-6 g linoleic acid (18:2n-6) or linolenic acid (18:3n-3)/d. Only long-chain n-3 fatty acids prolong the template bleeding time, and they may exert some beneficial effect on erythrocyte flexibility. It appears unlikely that n-3 fatty acids lower fibrinogen or interact with the fibrinolytic system directly. One prospective secondary prevention trial showed benefits that may have resulted from either an improved hemostatic profile or an antiarrhythmic effect. A similar time course of clinical improvement was noted with reduced rates of cardiac mortality and postoperative thrombosis in Norway during World War II, and this was associated with a drastic dietary alteration involving increased consumption of n-3 fatty acids and reduced consumption of saturated fatty acids. Further work is needed to develop better tools to examine in vivo hemostasis so that the mechanisms and eventual clinical utility of n-3 fatty acids can be elucidated in well-designed clinical trials.

Blood Coagulation↗
PubMed1997

Pentoxifylline does not alter the response to inhaled grain dust.

Pentoxifylline (PTX) has been shown to reduce sepsis-induced neutrophil sequestration in the lung and inhibit endotoxin-mediated release of tumor necrosis factor-alpha (TNF-alpha). Previously, we have shown that endotoxin appears to be the principal agent in grain dust causing airway inflammation and airflow obstruction following grain dust inhalation. To determine whether PTX affects the physiologic and inflammatory events following acute grain dust inhalation, 10 healthy, nonsmoking subjects with normal airway reactivity were treated with PTX or placebo (PL) followed by corn dust extract (CDE) inhalation (0.08 mL/kg), using a single-blinded, crossover design. Subjects received PTX (1,200 mg/d) or PL for 4 days prior to CDE inhalation and 400 mg PTX or PL on the exposure day. Both respiratory symptoms and declines in FEV1 and FVC occurred following CDE exposure in both groups, but there were no significant differences in the frequency of symptoms or percent declines from baseline in the FEV1 and FVC at any of the time points measured in the study. Elevations in peripheral blood leukocyte and neutrophil concentrations and BAL total cell, neutrophil, TNF-alpha, and interleukin-8 concentrations were measured 4 h following exposure to CDE in both the PTX- and PL-treated subjects, but no significant differences were found between treatment groups. These results suggest that pretreatment with PTX prior to inhalation of CDE, in the doses used in this study, does not alter the acute physiologic or inflammatory events following exposure to inhaled CDE.

Administration, Inhalation↗
PubMed1996

Arachidonic acid diols produced by cytochrome P-450 monooxygenases are incorporated into phospholipids of vascular endothelial cells.

Epoxyeicosatrienoic acids (EETs) are synthesized by cytochrome P-450 monooxygenases and released into the blood. When taken up by vascular endothelial and smooth muscle cells, the EETs are primarily esterified to phospholipids or converted to dihydroxyeicosatetraenoic acids (DHETs) and released. In the present studies, radiolabeled 8,9-, 11,12-, and 14,15-DHETs released into the medium from vascular smooth muscle cells were isolated and incubated for 4-16 h with cultured bovine aortic endothelial cells. The uptake ranged from 2 to 50% for the three regioisomers. Hydrolysis of the endothelial lipids and gas chromatographic-mass spectral analyses of the products indicated that all three DHET regioisomers were incorporated intact into phosphatidylcholine and phosphatidylinositol. Similar incubations with EETs confirmed that small amounts of DHETs were also esterified to endothelial phospholipids. These studies indicate that DHETs are incorporated into phospholipids either at the time of EET conversion to DHET or upon release and re-uptake of DHETs. Beside demonstrating for the first time that fatty acid diols are incorporated intact into endothelial lipids, these studies raise the possibility that both EETs and DHETs remain long enough in the vascular wall to produce chronic vasoactive effects.

Animals↗
PubMed1996

Fatty acid-related functions.

The first recommendations for specific nutrient quantities that must be obtained to support health were made by the US Department of Agriculture before 1939. Hazel Stiebeling was the leader of this effort and the scientific background was published in the Yearbook of Agriculture. The recommendations clearly stated that food must be available to provide the nutrients to support health. The science of nutrition in the United States is engaged in the most thorough review and reexamination of the recommended dietary allowances in at least a generation of nutrition scientists. There is a new awareness of nutrition complexity and the likelihood of identification of new essential nutrients. This meeting was devoted to the search for functional endpoints to reach quantitative estimates of dietary substances needed to support a function. Included in that concept is determining a range of individual needs and identifying factors that alter these needs. We give the rationale for endpoints of fatty acid metabolism related to platelets and the risk of thrombosis, give the rationale for the recommendation for a new nutrient, and show the necessity for including nutrient interaction in the determination of needs for two nutrients.

Blood Platelets↗
PubMed1996

n-3 fatty acids and human hypertension.

Clinical studies of the antihypertensive effects of n-3 fatty acids in different groups of patients are summarized, and recent information of possible mechanisms of antihypertensive effects is discussed. Because of marked differences between fatty acid metabolism and blood pressure control of humans and laboratory animals, the focus is on human studies.

Antihypertensive Agents↗
PubMed1995

Omega-3 fatty acids in respiratory diseases: a review.

The health benefits of dietary omega-3 fatty acids have been investigated in a variety of conditions but there have been few studies of their effects in human respiratory diseases. Although many of the physiological changes associated with omega-3 polyunsaturate ingestion have been attributed to alterations in endogenous eicosanoid production, effects on blood rheology, host-microbial interactions and lung surfactant production have also been described. In reviewing the literature, there is little evidence that these polyunsaturates have beneficial effects in allergic disorders, but they may have potential as modulators of respiratory diseases involving chronic inflammatory and infectious processes or impaired pulmonary blood flow. Further work on the effects of omega-3 fatty acids in several chronic pulmonary syndromes, for which there are currently no effective therapies, appears to be warranted.

Asthma↗
PubMed1995

Essential fatty acid deficiency impairs the responsiveness of renal pelvic sensory receptors.

The role of prostaglandins in renal sensory receptor activation was examined in rats fed an essential fatty acid-deficient (EFAD) diet to cause tissue arachidonate depletion. Littermates fed a standard diet were used as controls. In anesthetized rats, the increases in afferent renal nerve activity due to increasing ureteral pressure 2.5, 5, 7.5, 10, 12.5, and 15 mmHg were significantly reduced by the EFAD diet (P < 0.02): 3 +/- 5, 3 +/- 5, 11 +/- 5, 9 +/- 5, 19 +/- 3, and 17 +/- 5%, respectively, in EFAD rats and 23 +/- 11, 36 +/- 15, 50 +/- 15, 52 +/- 8, 72 +/- 17, and 90 +/- 19%, respectively, in control rats. In EFAD rats, addition of prostaglandin E2 (PGE2) to the renal pelvic perfusate restored the afferent renal nerve activity response to increased ureteral pressure toward that in control rats. PGE2 had no effect in control rats. Also the afferent renal nerve activity responses to renal pelvic perfusion with bradykinin at 4, 20, 100, and 500 micrograms/ml were significantly suppressed by the EFAD diet (P < 0.01): 13 +/- 15, 5 +/- 7, 60 +/- 19, and 63 +/- 20%, respectively, in EFAD rats and 122 +/- 23, 142 +/- 31, 172 +/- 19, and 190 +/- 39%, respectively, in control rats. These results demonstrate an important role for arachidonate metabolites, particularly PGE2, in renal sensory receptor activation. Together with our previous studies showing that indomethacin blocks the afferent renal nerve activity responses to increased ureteral pressure or bradykinin, the present studies provide strong evidence for an essential role of prostaglandins in renal sensory receptor activation.

Afferent Pathways↗
PubMed1995

Identification of arachidonate epoxides/diols by capillary chromatography-mass spectrometry.

The identification of epoxide regioisomers of arachidonic acid (EETs) as methyl esters is difficult because they coelute during gas chromatography and possess similar mass spectra. In the present study, EETs and their hydrolysis products, dihydroxyeicosatrienoic acids (DHETs), were analyzed as pentafluorobenzyl ester derivatives and their properties were compared to other esters. The four EET regioisomers were not resolved by gas chromatography as pentafluorobenzyl, trimethylsilyl, t-butyldimethylsilyl, or methyl esters. However, after being hydrolyzed to DHETs, three of the four regioisomers were resolved as (bis)-t-butyldimethylsilyl ether, pentafluorobenzyl esters. The fourth regioisomer (5,6-DHET) was resolved after being converted to a delta-lactone. Thus, the EETs could be resolved by capillary gas chromatography once converted to DHETs. Pentafluorobenzyl esters of both EETs and DHETs (15-40 ng) provided diagnostic spectra when analyzed by electron ionization mass spectrometry. The mass spectral interpretations that indicated epoxide and diol positions were validated using synthesized EET/DHET [17,17,18,18-d4, 5,6,8,9,11,12,14,15 d8] standards. Lesser amounts of DHETs (5-150 fg) also indicated molecular weights when analyzed in the negative-ion chemical-ionization mode. In summary, EETs in nanogram quantities were identified as pentafluorobenzyl esters using electron ionization mass spectrometry. EETs in femtogram-to-picogram quantities were also identified after conversion to DHETs and analysis by gas chromatography-mass spectrometry in the negative ion-chemical ionization mode.

Arachidonic Acid↗
PubMed1994

The effect of linoleic, arachidonic and eicosapentaenoic acid supplementation on prostacyclin production in rats.

We examined the effect of dietary supplementation of linoleic acid (LA), arachidonic acid (AA) or eicosapentaenoic acid (EPA) to rats fed a diet low in linoleic acid on in vitro and in vivo production of prostacyclin. Male Sprague Dawley rats were fed a high-fat diet (50% energy as fat, 1.5% linoleic acid) for two weeks. Three of the groups were then supplemented orally with either 90 mg/d of LA, AA or EPA, all as the ethyl esters, for a further two weeks while remaining on the high-fat diet. Forty-eight hour urine samples were collected at the end of the second and fourth weeks. In vivo prostacyclin production was determined by a stable isotope dilution, gas chromatography/mass spectrometry assay for the major urinary metabolite of prostacyclins (2,3-dinor-6-keto-PGF1 alpha or PGI2-M and delta 17-2,3-dinor-6-keto-PGF1 alpha or PGI3-M). In vitro prostacyclin production was determined by radioimmunoassay of the stable metabolite (6-keto-PGF 1 alpha) following incubation of arterial tissue. Oral supplementation with AA resulted in a rise in plasma and aorta 20:4n-6, and increased in vitro prostacyclin and urinary PGI2-M production. EPA supplementation resulted in a rise in plasma and aorta 20:5n-3 and 22:5n-3, and a decline in plasma 20:4n-6, but not in the aorta. In the EPA-supplemented group, the in vitro prostacyclin and the urinary PGI3-M increased, but urinary PGI2-M decreased. The increase in in vitro prostacyclin production in the EPA-supplemented rats was unexpected and without obvious explanation.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

Leukotrienes as mediators of nasal inflammation.

The three sets of studies that we have described support the notion that leukotrienes play a role in the nasal inflammatory response to allergen challenge. Strong inhibition of both leukotrienes and symptoms by A-64077, a trend toward inhibition of both leukotrienes and symptoms by SC-45662, and a lack of effect on either leukotrienes or symptoms by dietary eicosapentaenoic acid provide a consistent association between alterations of leukotrienes and nasal symptoms. A great deal of evidence has now accumulated that leukotrienes are important mediators of inflammation in vivo, but we also know that the chronic stimulus of hay fever produces a much more complex picture than seen in an acute challenge model and that many other mediators such as kinins, other peptides, histamine, and eicosanoids produced outside the 5-lipoxygenase pathway are involved (34,35). Further clinical research will be needed to develop optimal dosing regimens and delivery systems to achieve high concentrations of potent and safe 5-lipoxygenase inhibitors at sites that are critical for producing clinically useful results by modulation of the 5-lipoxygenase pathway.

Arachidonate 5-Lipoxygenase↗
PubMed1994

Asymmetric incorporation of dietary n-3 fatty acids into membrane aminophospholipids of human erythrocytes.

Dietary supplementation with different classes of polyunsaturate fatty acids is known to result in their incorporation into cell membranes, but the effects of this on eicosanoid formation and other cell functions frequently does not correspond to the degree of alteration in total membrane fatty acids. This phenomenon may be related to the compartmentalization of polyunsaturate fatty acids both within the organelles and within membranes. Aminophospholipids are asymmetrically distributed across the membrane bilayers of most human cells. These phospholipids are highly enriched in polyunsaturated fatty acids, and are known to have specific interactions with a number of membrane proteins. To determine whether dietary n-3 fatty acids are preferentially incorporated into membrane lipids in a particular spatial pattern, we have utilized the nonpermeant aminophospholipid probe, trinitrobenzenesulfonic acid, to study the transmembrane molecular species distribution of human erythrocyte ethanolamine phospholipids and phosphatidylserines before and at the end of 4 weeks of dietary supplementation with n-3 fatty acids. Selective incorporation of n-3 fatty acids occurred in the inner membrane leaflet ethanolamine phospholipids, particularly into the alkenyl-acyl species. The n-3 species in phosphatidylserines, particularly 18:0 and 22:6 n-3 (sn-1 and sn-2, respectively), replaced n-6 and n-9 species. These data may provide a basis for different cell responses to n-3 fatty acid enrichment, and for different degrees of diet-induced alteration in responses involving inner and outer membrane leaflet functions.

Dietary Fats↗