Key drivers of cloud response to surface-active organics.

Aerosol-cloud interactions constitute the largest source of uncertainty in global radiative forcing estimates, hampering our understanding of climate evolution. Recent empirical evidence suggests surface tension depression by organic aerosol to significantly influence the formation of cloud droplets, and hence cloud optical properties. In climate models, however, surface tension of water is generally assumed when predicting cloud droplet concentrations. Here we show that the sensitivity of cloud microphysics, optical properties and shortwave radiative effects to the surface phase are dictated by an interplay between the aerosol particle size distribution, composition, water availability and atmospheric dynamics. We demonstrate that accounting for the surface phase becomes essential in clean environments in which ultrafine particle sources are present. Through detailed sensitivity analysis, quantitative constraints on the key drivers - aerosol particle number concentrations, organic fraction and fixed updraft velocity - are derived for instances of significant cloud microphysical susceptibilities to the surface phase.

Microphysical explanation of the RH-dependent water affinity of biogenic organic aerosol and its importance for climate.

A large fraction of atmospheric organic aerosol (OA) originates from natural emissions that are oxidized in the atmosphere to form secondary organic aerosol (SOA). Isoprene (IP) and monoterpenes (MT) are the most important precursors of SOA originating from forests. The climate impacts from OA are currently estimated through parameterizations of water uptake that drastically simplify the complexity of OA. We combine laboratory experiments, thermodynamic modeling, field observations, and climate modeling to (1) explain the molecular mechanisms behind RH-dependent SOA water-uptake with solubility and phase separation; (2) show that laboratory data on IP- and MT-SOA hygroscopicity are representative of ambient data with corresponding OA source profiles; and (3) demonstrate the sensitivity of the modeled aerosol climate effect to assumed OA water affinity. We conclude that the commonly used single-parameter hygroscopicity framework can introduce significant error when quantifying the climate effects of organic aerosol. The results highlight the need for better constraints on the overall global OA mass loadings and its molecular composition, including currently underexplored anthropogenic and marine OA sources.

Resistant starch, large bowel fermentation and a broader perspective of prebiotics and probiotics.

The metabolic end products of the large bowel microbiota contribute significantly to human health. After weaning to solid foods, some of the most important of these are the short chain fatty acids (SCFA) produced by the fermentation of undigested dietary components and endogenous secretions. The main SCFA are acetate, propionate and butyrate which have numerous documented effects promoting large bowel function. Of the major acids, butyrate seems especially important. It is a major metabolic fuel for colonocytes and promotes a normal phenotype in these cells, potentially lowering the risk of diseases such as colo-rectal cancer. Imbalances in the microbiota are thought to predispose to large bowel dysfunction and probiotics are being developed to correct this. However, most commercial products contain bacteria (lactobacilli and bifidobacteria) which are dominant species in milk-fed infants but have limited roles in adults. Prebiosis is defined usually by the specific stimulation of these bacteria. However, the end products of most probiotics do not include butyrate or propionate which raises questions about their effectiveness in promoting bowel health in adults. Resistant starch (RS) is a dietary fibre component and its fermentation generally favours butyrate production. Dietary RS intakes and faecal butyrate levels are high in populations at low risk of diet-related large bowel diseases. Conversely, RS intakes and faecal butyrate levels are very low in high risk groups. This raises the possibility that greater RS consumption could be of health benefit. RS is not regarded widely as a prebiotic but (according to the accepted definition) most forms show the requisite features in stimulating specific bacteria, giving raised total SCFA and butyrate levels and a consequent benefit to the host. Current efforts to improve public health through increasing RS consumption could be facilitated by greater recognition of its prebiotic role.

Population and virulence factor dynamics in fecal Escherichia coli from healthy adults consuming weight control diets.

Individual susceptibility to gastrointestinal infection is seen commonly in food poisoning outbreaks, but factors (such as diet) which may modulate this variability are understood poorly. Similarly, factors altering the population dynamics of enteric non-pathogenic Escherichia coli or of pathogenic E. coli containing toxin-signature DNA sequences in the colonic flora of healthy individuals are largely unknown. Feces were collected 4 times over a 12 week period from 41 healthy volunteer adults on a weight control diet (high or low in fiber). E. coli strains were examined by conventional culture followed by PCR for virulence genes stx1, stx2, eae and hlyA, and polymorphic beta-glucuronidase. Total E. coli counts ranged from undetectable to 8.75 log10 CFU/g feces and were unaffected by dietary fiber consumption or gender. Total E. coli counts were correlated positively with age (r = 0.401, P < 0.05). Fifty-eight percent (n = 24) of study individuals harboured more than 1 morph of beta-glucuronidase, indicating the presence of more than 1 strain of E. coli. Virulence genes were detected in 12 of 41 adults, comprising 10 stx1, 3 stx2, 3 eae, and 0 hlyA, but occurrence was not associated with diet, gender, or age. Factors influencing strain mobility over time did not appear to include diet or gender, while the positive relationship between total E. coli numbers and increasing age suggests that some older individuals are "more permissive" to mobile E. coli, including those with toxin genes.

Inhalation two-generation reproductive toxicity study of methyl isobutyl ketone in rats.

To evaluate whether methyl isobutyl ketone (MIBK) affects reproductive performance, a two-generation reproduction study was conducted. MIBK was administered to 30 Sprague-Dawley rats/sex/group via whole-body inhalation at concentrations of 0, 500, 1000, or 2000 ppm, 6 h daily, for 70 days prior to mating. F(0) and F(1) females were exposed from mating through gestation day 20 and from postnatal day 5; F(2) litters were maintained through postnatal day 21. No treatment-related mortality of adult animals occurred. There was a dose-related increase in adult animals with no or a decreased response to a sound stimulus at 1000 and 2000 ppm; however, no adverse clinical signs occurred 1 h after exposure, suggesting this was a transient sedative effect. Clinical signs of central nervous system (CNS) depression in the pups were observed and one F(1) pup died after initial exposure to 2000 ppm on postnatal day 22; subsequently exposure was delayed until postnatal day 28. Decreased body weight gain and slight decreased food consumption were observed during the first 2 weeks of exposure in both generations at 2000 ppm. There were no adverse effects on male and female reproductive function or landmarks of sexual maturation. Increased F(0) and F(1) liver weights with associated centrilobular hypertrophy occurred in rats at 2000 ppm, indicative of an adaptive response. Increased male kidney weights at all exposure concentrations, associated with hyaline droplets, were indicative of male rat-specific nephropathy. Other than acute sedative effects, the no-observed-adverse-effect level (NOAEL) for parental systemic effects (excluding male rat kidney) was 1000 ppm, based on transient decreased body weight and food consumption; for reproductive effects, 2000 ppm, the highest concentration tested; and for neonatal toxicity, 1000 ppm (based on acute CNS depressive effects).

The impact of involvement of biomedical scientists in specimen dissection and selection of blocks for histopathology: a study of time benefits and specimen handling quality in Ayrshire and Arran area laboratory.

AIMS: To assess possible time benefits of specimen dissection by biomedical scientists (BMSs) and the quality of specimen handling by BMSs, in a department where BMSs trim those specimens requiring simple descriptions, from which standard blocks are taken. METHODS: Specimen handling by BMSs and consultant pathologists was compared. Time taken for each specimen trimmed was recorded prospectively. To determine specimen handling quality, adherence to dissection standard operating procedures (SOPs) was assessed by recording retrospectively whether or not each action in the SOP had been performed. Information on subsequently required extra levels or blocks was recorded. RESULTS: Analysis of data from 672 specimens trimmed by consultants showed that any given action in the SOPs was performed on average on 60.2% of applicable/assessable specimens; for 660 similar specimens trimmed by BMSs, each action was performed on average on 80.1% of specimens. Of the specimens where data on extra blocks were recorded, extra blocks were required in 3% of those trimmed by pathologists and in 4% of those trimmed by BMSs. Extra levels were required in 12% of those trimmed by pathologists and in 16% of those trimmed by BMSs. BMS trimming saves 16 hours of consultant time each month. The difference between pathologists and BMSs in time for each specimen trimmed is negligible. CONCLUSIONS: The advantages of increased adherence to trimming SOPs and saving consultant time outweigh the relatively small number of extra blocks and levels required when BMSs trim. There is no reduction in quality of dissection.

Insulin stimulation of hepatic triacylglycerol secretion and the etiology of insulin resistance.

The recent observations that insulin can either stimulate or inhibit triacylglycerol secretion by the liver, depending on prior metabolic (possibly insulinemic) state, have rationalized the many apparently contradictory observations, obtained over the past three decades, on the effects of the hormone on this aspect of hepatic metabolism. Extrapolation to the situation in vivo suggests that frequent stimulation of insulin secretion may result in a chronic stimulation of VLDL secretion, and increased delivery of acyl moieties to muscle, where they induce insulin resistance if provided in excess of the oxidative needs (mostly due to exercise) of the tissue. High fructose/sucrose diets, which also stimulate hepatic VLDL secretion, will have the same effect, especially if consumed frequently during the diurnal cycle. Due to the quantitative importance of muscle as a site for insulin-sensitive glucose metabolism, these effects may initiate the metabolic vicious cycle that results in the development of the metabolic syndrome, well in advance of overt obesity or the diagnosis of type-2 diabetes.

Institutional validation of breast cancer treatment guidelines.

Several groups have developed clinical guidelines for the management of breast cancer, yet little data exist regarding their validation. Therefore, we examined the effect of published National Comprehensive Cancer Network (NCCN) guidelines for invasive breast cancer on survival, quality of life (QOL), and hospital cost. From 260 consecutive breast cancer patients, 129 patients were identified for analysis: 93 patients (72%) were treated according to the guidelines (NCCN+), while the treatment of 36 patients (28%), with a similar stage distribution, deviated from the guidelines (NCCN-). Patients were excluded from analysis with a diagnosis of carcinoma in situ, inflammatory cancer, stage IV disease, and comorbid conditions that affected treatment. The 5-year survival was 87.6% for the NCCN+ patients versus 83.3% for NCCN- patients (P = 0.319 by Kaplan-Meier). Twelve QOL parameters were evaluated using a Likert-type scale (1 = severe and 5 = none). NCCN+ patients had a cumulative QOL score of 4.18 +/- 0.08 versus 4.24 +/- 0.14 for NCCN- patients (P = 0.745). Treatment-related costs were $20,300 +/- 1800 for NCCN+ patients versus $59,700 +/- 25,200 for NCCN- patients (P = 0.016 by t test). Although deviation from NCCN breast cancer guidelines had no effect on perceived quality of life or survival, there was a significant decrease in cost in the NCCN+ group. These findings suggest that adherence to NCCN guidelines can significantly reduce the cost of breast cancer care without adversely affecting either survival or quality of life.

Short-chain fatty acids and human colonic function: roles of resistant starch and nonstarch polysaccharides.

Resistant starch (RS) is starch and products of its small intestinal digestion that enter the large bowel. It occurs for various reasons including chemical structure, cooking of food, chemical modification, and food mastication. Human colonic bacteria ferment RS and nonstarch polysaccharides (NSP; major components of dietary fiber) to short-chain fatty acids (SCFA), mainly acetate, propionate, and butyrate. SCFA stimulate colonic blood flow and fluid and electrolyte uptake. Butyrate is a preferred substrate for colonocytes and appears to promote a normal phenotype in these cells. Fermentation of some RS types favors butyrate production. Measurement of colonic fermentation in humans is difficult, and indirect measures (e.g., fecal samples) or animal models have been used. Of the latter, rodents appear to be of limited value, and pigs or dogs are preferable. RS is less effective than NSP in stool bulking, but epidemiological data suggest that it is more protective against colorectal cancer, possibly via butyrate. RS is a prebiotic, but knowledge of its other interactions with the microflora is limited. The contribution of RS to fermentation and colonic physiology seems to be greater than that of NSP. However, the lack of a generally accepted analytical procedure that accommodates the major influences on RS means this is yet to be established.

Coarse brown rice increases fecal and large bowel short-chain fatty acids and starch but lowers calcium in the large bowel of pigs.

Young male pigs were fed a diet formulated from human foods including either boiled white rice plus rice bran or heat-stabilized brown rice at equivalent levels of fiber for 3 wk. Stool and starch excretion were low in pigs fed white rice during the first 2 wk of the experiment. In pigs fed brown rice, their excretion was high during wk 1 but declined in wk 2 while short-chain fatty acid (SCFA) excretion was higher at both times. Large bowel digesta mass, measured during wk 3, was higher in pigs fed brown rice but only in the proximal colon. Large bowel and fecal starch concentrations were higher in pigs fed brown rice but the difference was insufficient to explain the increase in large bowel digesta mass. In pigs with a cecal cannula, digesta starch concentrations were equally higher when white or brown rice was fed compared with the corresponding rice which had been finely milled, indicating that particle size was a determinant of ileal digestibility. Concentrations and pools of total and individual SCFA were higher in all regions of the colon but not the cecum of pigs fed brown rice. Large bowel Ca(2+) concentrations were lower in pigs fed brown rice, suggesting greater absorption. The data confirm earlier findings that brown rice raises large bowel digesta mass and SCFA through greater fermentation of starch but show that starch itself makes a relatively small contribution to digesta and stool mass. Apparently, the rate of passage of digesta is a determinant of the concentrations and pools of SCFA in the distal colon and in feces.

Modulation of the acute phase response by altered expression of the IL-1 type 1 receptor or IL-1ra.

A complete understanding of the role for endogenously produced interleukin-1 (IL-1), tumor necrosis factor-alpha (TNF-alpha), and IL-1 receptor antagonist (IL-1ra) in the acute phase response to inflammation remains unknown. In the present studies, knockout mice lacking either a functional IL-1 type I receptor (IL-1RI(-/-)), a TNF type I receptor (TNFR-I(-/-)), or both IL-1 type I and TNF type I receptors (IL-1RI(-/-)/TNFR-I(-/-)) received a turpentine abscess. Additional mice deficient in IL-1ra protein (IL-1ra(-/-)) or overexpressing IL-1ra protein (IL-1ra(tg)) were similarly treated. After a turpentine abscess, IL-1 receptor knockout mice exhibited an attenuated inflammatory response compared with wild-type or animals lacking a functional TNFR-I. Mice overexpressing IL-1ra also had an attenuated hepatic acute phase protein response, whereas IL-1ra knockout mice had a significantly greater hepatic acute phase response. We conclude that the inflammatory response to a turpentine abscess is the result of a balance between IL-1ra expression and IL-1 binding to its type I receptor. Endogenously produced IL-1ra plays a central role in mitigating the magnitude of the IL-1-mediated inflammatory response and, ultimately, the outcome to a turpentine abscess.

Starches, resistant starches, the gut microflora and human health.

Starches are important as energy sources for humans and also for their interactions with the gut microflora throughout the digestive tact. Largely, those interactions promote human health. In the mouth, less gelatinised starches may lower risk of cariogensis. In the large bowel, starches which have escaped small intestinal digestion (resistant starch), together with proteins, other undigested carbohydrates and endogenous secretions are fermented by the resident microflora. The resulting short chain fatty acids contribute substantially to the normal physiological functions of the viscera. Specific types of resistant starch (e.g. the chemically modified starches used in the food industry) may be used to manipulate the gut bacteria and their products (including short chain fatty acids) so as to optimise health. In the upper gut, these starches may assist in the transport of probiotic organisms thus promoting the immune response and suppressing potential pathogens. However, it appears unlikely that current probiotic organisms can be used to modulate large bowel short chain fatty acids in adults although resistant starch and other prebiotics can do so. Suggestions that starch may exacerbate certain conditions (such as ulcerative colitis) through stimulating the growth of certain pathogenic organisms appear to be unfounded. Short chain fatty acids may modulate tissue levels and effects of growth factors in the gut and so modify gut development and risk of serious disease, including colo-rectal cancer. However, information on the relationship between starches and the microflora is relatively sparse and substantial opportunities exist both for basic research and food product development.

The effect of repeated methyl iso-butyl ketone vapor exposure on schedule-controlled operant behavior in rats.

Methyl iso-butyl ketone (MiBK), a commercial solvent, was selected by the US Environmental Protection Agency (US EPA) for testing under the Multi-Substance Rule for the Testing of Neurotoxicity (US EPA, 1993) using schedule-controlled operant behavior (SCOB) to determine if subchronic exposure to MiBK vapor had the potential to alter behavior as an indicator of neurotoxicity. Food-restricted and ad libitum-fed Sprague-Dawley male rats were exposed to 0, 250, 750, or 1500 ppm MiBK for 6 h/day, 5 d/wk for 13 weeks. SCOB testing of food-restricted animals, using a multiple fixed ratio (FR)/fixed interval (FI) schedule (FR20:FI120), was conducted prior to each exposure to maintain the operant behavior; the data from Weeks -1, 4, 8, and 13 were evaluated for evidence of neurotoxicity. SCOB testing was also evaluated for two weeks following the cessation of exposures. Ad libitum-fed animals were included to assess systemic effects using routine indicators such as changes in body weight, food consumption, and organ weight. No significant differences were seen in fixed-ratio run rate, FR pause duration, fixed-interval response rate, and index of curvature values at any concentration. Animals exposed to 750 and 1500 ppm MiBK exhibited clinical signs associated with transient reduced activity levels, but only during exposure. No signs of reduced activity were observed immediately after exposure for either group. No other treatment-related abnormalities were observed during exposure. Food-restricted animals did not demonstrate any increased or decreased sensitivity to the CNS depressive effects of MiBK relative to the ad libitum-fed animals. No treatment-related body weight differences were observed within either the food-restricted groups or the ad libitum-fed groups, although body weights of the former were clearly depressed compared with those of the latter. Relative and absolute liver, and relative kidney weights were significantly greater for the 750 and 1500 ppm ad libitum-fed animals. No differences in kidney weight were observed for food-restricted animals, but absolute and/or relative liver weights were significantly higher for all the treated food-restricted groups. The results of this study indicate that repetitive exposures to high concentrations of MiBK vapors do not result in adverse effects on operant behavior in the rat.

Aleurone flour is a rich source of bioavailable folate in humans.

With the use of novel milling technology, it has become commercially viable to isolate the aleurone layer of cells from wheat grain and to prepare a novel flour from this fraction that has a natural folate concentration of approximately 500 microgram/100 g. The aim of this study was to determine the relative bioavailability of natural folate from aleurone flour when ingested as a cereal. Using a series of randomized, short-term intervention trials with a cross-over involving eight men and eight women aged between 29 and 50 y, we compared the increment of plasma folate following ingestion of 1) 100 g wheat bran cereal (low folate control), 2) 100 g aleurone cereal, and 3) a tablet containing 500 microgram folic acid taken together with 100 g wheat bran cereal (high folate control). Folate absorption was measured by estimating the area under the plasma folate concentration versus time curve. The extent of increase in plasma folate over the 7-hour period following ingestion of aleurone cereal was more than fourfold greater than that observed following the wheat bran cereal (P < 0.0001) and not different from that observed following the 500 microgram folic acid tablet taken with wheat bran cereal. Differences were significant when data for males and females were analyzed separately (P < 0.001). This study has shown that cereal made from wheat aleurone flour is a good source of bioavailable, natural folate.

2-Ethylhexanoic acid: subchronic oral toxicity studies in the rat and mouse.

Groups of 10 male and 10 female Fischer 344 rats and B6C3F1 mice were fed diets containing either 0.0, 0.1, 0.5 or 1.5% 2-ethylhexanoic acid (EHA) for 13 wk. Additional groups of 10 male and 10 female rats or mice. were fed either 0.0 or 1.5% EHA for 13 wk followed by a 4-wk recovery (non-treatment) period. Based on food consumption and body weight, the EHA diets provided doses of 61, 303 or 917 mg/kg/day for male rats and 71, 360 or 1068 mg/kg/day for female rats. The EHA diets provided doses of 180, 885 or 2728 mg/kg/day for male mice and 205, 1038 or 3139 mg/kg/day for female mice. No mortality or significant clinical signs of toxicity were observed during the study. Body weights and food consumption of both rats and mice fed 1.5% EHA were lower beginning after the first week of treatment, consistent with a reduction in food consumption. Other groups were unaffected by treatment. After 13 wk, lower triglyceride levels occurred in male mice fed 1.5% EHA and female mice fed 0.5 or 1.5% EHA, but not in other groups. Cholesterol levels were higher in all male rat test groups and in female rats and male and female mice fed either 0.5 or 1.5% EHA, although this effect was reversible following a 28-day recovery period. The principal effects of EHA involved the liver or metabolic processes associated with the liver. The 0.5 and 1.5% diets in both rats and mice were associated with increased relative liver weight and histological changes in hepatocytes, specifically hepatocyte hypertrophy and reduced cytoplasmic vacuolization. Observed histopathological and clinical pathological changes were reversible following recovery. These results indicate that EHA does not produce persistent. overt toxicity in rats or mice following subchronic dietary exposure at concentrations up to 1.5% in feed. The no-observed-adverse-effect level (NOAEL) for male rats was 61 mg/kg/day and the no-observed-effect level (NOEL) for female rats was 71 mg/kg/day, while 180 and 205 mg/kg/day represent NOELs for male and female mice, respectively.

White and wholemeal flours from wheats of low and higher apparent metabolizable energy differ in their nutritional effects in rats.

Wheats used for feeding poultry differ considerably in the ratio of soluble to insoluble non-starch polysaccharides (NSP) and apparent metabolizable energy (AME). We have examined effects of whole and white flour from a wheat of low (12.02 MJ/kg of dry matter) and high (14.52 MJ/kg of dry matter) AME in rats fed a cholesterol-free diet. NSP concentrations were higher in whole flour from the low AME wheat but similar in both white flours. In contrast to chickens, food intake and body weight gain of rats were unaffected by diet. Plasma cholesterol concentrations were lower in rats fed whole wheat and low AME wheat. Cecal bile acids and neutral sterol pools were larger in rats fed whole wheat but were unaffected by wheat type. Negative correlations were found between digesta steroid pools and plasma cholesterol, consistent with greater fecal steroid excretion. Cecal digesta was greater in rats fed whole wheat and low AME wheat. Digesta pH was lower in rats fed whole wheat, but there was a significant interaction between wheat and flour type with lower pH in rats fed low AME white flour. Total volatile fatty acids (VFA) and acetate and butyrate pools were larger in rats fed whole wheat than in those fed white flour. Total and individual VFA pools were larger in rats fed low AME flours than in those fed high AME flours, apparently due to greater cecal starch fermentation in the former. Factors affecting wheat AME in chickens affect important metabolic variables in rats and may have similar actions in other species including humans.

Fecal numbers of bifidobacteria are higher in pigs fed Bifidobacterium longum with a high amylose cornstarch than with a low amylose cornstarch.

Twelve young male pigs consumed a purified diet containing wheat bran as fiber source. Starch provided 50% of total daily energy either as a low amylose cornstarch or as a high amylose (amylomaize) starch. The pigs were given a supplement of a freeze-dried probiotic organism (Bifidobacterium longum CSCC 1941). A block crossover design was used so that at any one time two groups of three pigs consumed either the high or low amylose cornstarch without probiotic and a further two groups of three pigs consumed either high or low amylose cornstarch with probiotic. Neither food intake nor body weight gain was affected by diet. Fecal output was higher when pigs were fed the high amylose cornstarch, but moisture content was unaffected. Fecal concentrations and excretion of total volatile fatty acids were higher when pigs were fed the high amylose cornstarch. Concentrations of acetate were unaffected by dietary starch, but those of propionate and butyrate were higher when the high amylose cornstarch was consumed. Fecal excretion of all three acids was higher during high amylose cornstarch feeding. Bifidobacteria were detected in the feces only when pigs were fed Bifidobacterium longum. Fecal bifidobacteria counts (expressed per gram of wet feces) and their daily fecal excretion were higher when pigs were fed high amylose cornstarch. Feeding the probiotic did not alter fecal starch or volatile fatty acids. None of the variables studied was affected by the order of feeding of starch or probiotic. The data show that a high amylose starch acts as a prebiotic in promoting the fecal excretion of probiotic organisms.

Arginine-enhanced enteral nutrition augments the growth of a nitric oxide-producing tumor.

BACKGROUND: Arginine-enhanced diets have been shown to be beneficial in tumor-bearing hosts, but no data exist regarding their effects in hosts bearing nitric oxide (NO)-producting tumors. OBJECTIVE: To examine the effect of arginine supplementation on the growth of a NO-producing murine breast cancer cell line. METHODS: EMT-6 cells were grown in various concentrations of arginine in the presence or absence of the inducible nitric oxide synthase (iNOS) inhibitor, aminoguanidine (1 mmol/L). Forty-eight hours later, nitrite accumulation and viable cell number were assessed. BALB/c mice were then pair-fed basal purified diets (n = 10), 4% casein diets (isonitrogenous control, n = 5), or 4% arginine-enhanced diets (n = 10). One week later, 10(5) EMT-6 cells were implanted subcutaneously into the dorsal flank. After tumor implantation, five mice fed basal purified diets and five mice fed arginine-enhanced diets also received aminoguanidine (100 mg/kg subcutaneously twice daily). Two weeks after tumor cell implantation, tumor size (mean diameter), animal weight, serum and tumor nitrite and nitrate levels were measured. RESULTS: There was minimal nitrite accumulation in arginine-free media, while increasing the arginine concentration increased nitrite levels. Viable cell number did not increase in arginine-free media, but increased nearly twofold in 100 and 1000 mumol/L arginine. In 5000 and 10,000 mumol/L arginine, the difference in viable cell number was not statistically different than that seen in arginine-free media, whereas the addition of aminoguanidine blocked nitrite accumulation and increased viable cell number at these arginine concentrations. Arginine-enhanced diets stimulated tumor growth in vivo more than twofold over tumor growth in mice fed isonitrogenous control or basal purified enteral diets. Mice fed arginine-enhanced diets also had increased serum nitrite and nitrate levels over mice fed basal purified enteral diets, whereas tumors from mice fed arginine-enhanced diets had nitrite and nitrate levels similar to mice fed basal purified enteral diets. Aminoguanidine blocked the increase in serum nitrite and nitrate, but failed to block the increased tumor growth in mice receiving the arginine-supplemented diets. CONCLUSIONS: Arginine concentration influences the growth of EMT-6 tumor cells in vitro and dietary arginine supplementation augments tumor growth in vivo. The mechanism of the growth modulation in vitro is NO-dependent whereas the enhanced tumor growth in vivo is NO-independent.