The role of the lymphoid system in the regulation of the blood glucose level.

Recent findings have led to a new hypothesis in which it is proposed that the immune system plays a role in regulating the increase in blood glucose levels after a meal. The relevant findings are: (1) the primary lymphoid tissue, the lymph nodes are mostly present within adipose tissue depots throughout the body (there are at least 12 such depots and about 10 (12) lymphocytes, 99% of which are present in lymph nodes); (2) lymphocytes and other immune cells utilize glucose at a high rate but almost all of it is converted to lactate which accumulates in the cells prior to release; (3) glutamine, some of which is synthesized in muscle from glucose, is utilized at a high rate by immune cells, the end-product of which is mainly aspartate, which also accumulates in the cells prior to release; and (4) finally, there is a common blood supply to the lymph node and the adipose tissue depot and the blood flow through the depot and hence the node is increased after a meal. It is proposed that, after a meal, some of the absorbed glucose is taken up from the blood by the lymphocytes and converted to lactate and glutamine is converted to aspartate. These are released slowly into the blood from where they are removed and converted to glycogen by the liver. Hence the immune cells provide a temporary buffer for glucose in the form of lactate and aspartate and, in this way, restrict the rise in blood glucose during and after a meal.

Integration of biochemical and physiologic effects of insulin on glucose metabolism.

The major effects of insulin on tissues are: (1) Carbohydrate metabolism: (a) It increases the rate of transport of glucose across the cell membrane in adipose tissue and muscle, (b) it increases the rate of glycolysis in muscle and adipose tissue, (c) it stimulates the rate of glycogen synthesis in a number of tissues, including adipose tissue, muscle, and liver. It also decreases the rate of glycogen breakdown in muscle and liver, (d) it inhibits the rate of glycogenolysis and gluconeogenesis in the liver. (2) Lipid metabolism: (a) It decreases the rate of lipolysis in adipose tissue and hence lowers the plasma fatty acid level, (b) it stimulates fatty acid and triacylglycerol synthesis in tissues, although only to a minor extent in humans, (c) it increases the rate of very-low-density lipoprotein formation in the liver, (d) it increases the uptake of triglyceride from the blood into adipose tissue and muscle, (e) it decreases the rate of fatty acid oxidation in muscle and liver, (f) it increases the rate of cholesterol synthesis in liver. (3) Protein metabolism: (a) It increases the rate of transport of some amino acids into tissues, (b) it increases the rate of protein synthesis in muscle, adipose tissue, liver, and other tissues, (c) it decreases the rate of protein degradation in muscle (and perhaps other tissues), (d) it decreases the rate of urea formation.--These insulin effects serve to encourage the synthesis of carbohydrate, fat and protein.

Dietary supplementation with gamma-linolenic acid or fish oil decreases T lymphocyte proliferation in healthy older humans.

Animal and human studies have shown that greatly increasing the amounts of flax seed oil [rich in the (n-3) polyunsaturated fatty acid (PUFA) alpha-linolenic acid (ALNA)] or fish oil [FO; rich in the long chain (n-3) PUFA eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)] in the diet can decrease mitogen-stimulated lymphocyte proliferation. The objective of this study was to determine the effect of dietary supplementation with moderate levels of ALNA, gamma-linolenic acid (GLA), arachidonic acid (ARA), DHA or FO on the proliferation of mitogen-stimulated human peripheral blood mononuclear cells (PBMC) and on the production of cytokines by those cells. The study was randomized, placebo-controlled, double-blinded and parallel. Healthy subjects ages 55-75 y consumed nine capsules/d for 12 wk; the capsules contained placebo oil (an 80:20 mix of palm and sunflower seed oils) or blends of placebo oil with oils rich in ALNA, GLA, ARA or DHA or FO. Subjects in these groups consumed 2 g of ALNA or 770 mg of GLA or 680 mg of ARA or 720 mg of DHA or 1 g of EPA plus DHA (720 mg of EPA + 280 mg of DHA) daily from the capsules. Total fat intake from the capsules was 4 g/d. The fatty acid composition of PBMC phospholipids was significantly changed in the GLA, ARA, DHA and FO groups. Lymphocyte proliferation was not significantly affected by the placebo, ALNA, ARA or DHA treatments. GLA and FO caused a significant decrease (up to 65%) in lymphocyte proliferation. This decrease was partly reversed by 4 wk after stopping the supplementation. None of the treatments affected the production of interleukin-2 or interferon-gamma by PBMC and none of the treatments affected the number or proportion of T or B lymphocytes, helper or cytotoxic T lymphocytes or memory helper T lymphocytes in the circulation. We conclude that a moderate level GLA or EPA but not of other (n-6) or (n-3) PUFA can decrease lymphocyte proliferation but not production of interleukin-2 or interferon-gamma.

The life and work of Dermot Hedley ("Derek") Williamson (1929-1998).

Derek Williamson's scientific career spanned the "Golden Age" of research into metabolic regulation, to which he made an important and sustained contribution. Derek joined Hans Krebs' laboratory at Sheffield University in 1946 and moved to Krebs' MRC Unit in Oxford in 1960. He elaborated an enzymic method for the determination of acetoacetate and 3-hydroxybutyrate [Williamson, Mellanby and Krebs, Biochem. J. (1962) 82, 90-96], which opened up the field of ketone body metabolism and its regulation and became a Citation Classic. Another Citation Classic followed [Williamson, Lund and Krebs, Biochem. J. (1967) 103, 514-527]. He moved with Krebs to the Metabolic Research Laboratory at the Radcliffe Infirmary in 1967, where he blossomed, formulating his ideas about the integrated regulation of metabolic pathways, particularly with regard to fatty acid oxidation, lipid synthesis and ketone body metabolism. His success was illustrated by more than 200 publications. Derek implanted and nurtured a sense of the excitement of scientific discovery in his colleagues and students, and he worked hard to provide a friendly, supportive and encouraging environment. Many lives have been enriched by the privilege of working with him.

The relation between glutamine and the immunodepression observed in exercise.

Glutamine is the most abundant amino acid in the body. It is an important fuel for some key cells of the immune system. Both the plasma concentration of glutamine and the functional ability of immune cells in the blood are decreased after prolonged, exhaustive exercise. Glutamine feeding has had beneficial effects in clinical situations, and the provision of glutamine after intensive exercise has decreased the incidence of infections, particularly of upper respiratory tract infections. However, the precise effect of glutamine on immunodepression in this situation is not yet established.

Dietary supplementation with eicosapentaenoic acid, but not with other long-chain n-3 or n-6 polyunsaturated fatty acids, decreases natural killer cell activity in healthy subjects aged >55 y.

BACKGROUND: Animal studies showed that dietary flaxseed oil [rich in the n-3 polyunsaturated fatty acid alpha-linolenic acid (ALA)], evening primrose oil [rich in the n-6 polyunsaturated fatty acid gamma-linolenic acid (GLA)], and fish oil [rich in the long-chain n-3 polyunsaturated fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)] can decrease natural killer (NK) cell activity. There have been no studies of the effect on NK cell activity of adding these oils to the diet of humans. OBJECTIVE: Our objective was to determine the effect of dietary supplementation with oil blends rich in ALA, GLA, arachidonic acid (AA), DHA, or EPA plus DHA (fish oil) on the NK cell activity of human peripheral blood mononuclear cells. DESIGN: A randomized, placebo-controlled, double-blind, parallel study was conducted. Healthy subjects aged 55-75 y consumed 9 capsules/d for 12 wk; the capsules contained placebo oil (an 80:20 mix of palm and sunflower seed oils) or blends of placebo oil and oils rich in ALA, GLA, AA, DHA, or EPA plus DHA. Subjects in these groups consumed 2 g ALA, 770 mg GLA, 680 mg AA, 720 mg DHA, or 1 g EPA plus DHA (720 mg EPA + 280 mg DHA) daily, respectively. Total fat intake from the capsules was 4 g/d. RESULTS: The fatty acid composition of plasma phospholipids changed significantly in the GLA, AA, DHA, and fish oil groups. NK cell activity was not significantly affected by the placebo, ALA, GLA, AA, or DHA treatment. Fish oil caused a significant reduction (mean decline: 48%) in NK cell activity that was fully reversed by 4 wk after supplementation had ceased. CONCLUSION: A moderate amount of EPA but not of other n-6 or n-3 polyunsaturated fatty acids can decrease NK cell activity in healthy subjects.

Physical exercise and normobaric hypoxia: independent modulators of peripheral cholecystokinin metabolism in man.

The purpose of the present investigation was to determine the independent effects of hypoxia and physical exercise on peripheral cholecystokinin (CCK) metabolism in humans. Thirty-two physically active men were randomly assigned in a double-blind manner to either a normoxic (N; n = 14) or hypoxic (H; n = 18) group. During the acute study, subjects in the H group only participated in two tests, separated by 48 h, which involved a cycling test to exhaustion in normobaric normoxia and normobaric hypoxia (inspired O(2) fraction = 0.21 and 0.16, respectively). In the intermittent study, N and H groups cycle-trained for 4 wk at the same relative exercise intensity in both normoxia and hypoxia. Acute normoxic exercise consistently raised plasma CCK during both studies by 290-723%, which correlated with increases in the plasma ratio of free tryptophan to branched chain amino acids (r = 0.58-0.71, P < 0.05). In contrast, acute hypoxic exercise decreased CCK by 7.0 +/- 5.5 pmol/l, which correlated with the decrease in arterial oxygen saturation (r = 0.56, P < 0.05). In the intermittent study, plasma CCK response at rest and after normoxic exercise was not altered after physical training, despite a slight decrease in adiposity. We conclude that peripheral CCK metabolism 1) is more sensitive to acute changes than chronic changes in energy expenditure and 2) is potentially associated with acute changes in tissue PO(2) and metabolic precursors of cerebral serotoninergic activity.

Influence of age and dietary fish oil on plasma soluble adhesion molecule concentrations.

Soluble forms of intercellular adhesion molecule-1, vascular cell adhesion molecule-1 and E-selectin (termed sICAM-1, sVCAM-1 and sE-selectin respectively) are found in the plasma, and are elevated during inflammatory conditions in which there is increased expression of the cellular forms of the molecules on endothelial and other cells. sICAM-1, sVCAM-1 and sE-selectin concentrations were measured in the plasma of 140 healthy Caucasian subjects aged between 18 and 75 years (100 males/40 females). sICAM-1 concentrations varied between 59.9 and 299.7 ng/ml (median 150 ng/ml), sVCAM-1 concentrations varied between 222.8 and 1672.9 ng/ml (median 662 ng/ml) and sE-selectin concentrations varied between 12.4 and 90.3 ng/ml (median 45.5 ng/ml). There were significant positive linear correlations between age and the plasma concentrations of sICAM-1 (r=0.580; P<0.001) and sVCAM-1 (r=0.392; P<0.001), which were retained when the effects of gender, body mass index and fasting plasma triacylglycerol and total cholesterol concentrations were controlled for. The significant positive linear correlation between age and the plasma concentration of sE-selectin (r=0.234; P=0.027) was lost when other variables were controlled for. Male subjects <40 years of age had significantly lower plasma concentrations of both sICAM-1 and sVCAM-1 than males >55 years of age (both P<0.001), but the difference in plasma sE-selectin concentrations between the age groups did not reach significance (P=0.073). Subgroups of 16 males aged <40 years and 12 elderly subjects (>55 years of age) participated in a doubled-blind, placebo-controlled study of fish oil supplementation over 12 weeks. The level of eicosapentaenoic acid in plasma phospholipids did not change with placebo supplementation, but was significantly increased with fish oil supplementation in both young male and elderly subjects (median increase 200%). sICAM-1, sVCAM-1 and sE-selectin concentrations were unaffected by supplementation with placebo in either young male or elderly subjects. sICAM-1 concentrations were unaffected by fish oil supplementation. sE-selectin concentrations were significantly increased by fish oil supplementation in young males (P=0.043; median increase 38%), but fish oil tended to decrease plasma sE-selectin concentrations in the elderly subjects (P=0.075), with a median decrease of 11%. sVCAM-1 concentrations were unaffected by fish oil supplementation in young males. Fish oil supplementation significantly decreased plasma sVCAM-1 concentrations in the elderly subjects (P=0.043), with a median decrease of 20% (range 16-60%). These observations suggest that fish oil decreases endothelial activation in elderly subjects.

Influence of dietary supplementation with long-chain n-3 or n-6 polyunsaturated fatty acids on blood inflammatory cell populations and functions and on plasma soluble adhesion molecules in healthy adults.

Greatly increasing the amounts of flaxseed oil [rich in alpha-linolenic acid (ALNA)] or fish oil (FO); [rich in eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)] in the diet can decrease inflammatory cell functions and so might impair host defense. The objective of this study was to determine the effect of dietary supplementation with moderate levels of ALNA, gamma-linolenic acid (GLA), arachidonic acid (ARA), DHA, or FO on inflammatory cell numbers and functions and on circulating levels of soluble adhesion molecules. Healthy subjects aged 55 to 75 yr consumed nine capsules per day for 12 wk. The capsules contained placebo oil (an 80:20 mix of palm and sunflowerseed oils) or blends of placebo oil with oils rich in ALNA, GLA, ARA, or DHA or FO. Subjects in these groups consumed 2 g ALNA; approximately 700 mg GLA, ARA, or DHA; or 1 g EPA plus DHA (720 mg EPA + 280 mg DHA) daily from the capsules. Total fat intake from the capsules was 4 g per day. None of the treatments affected inflammatory cell numbers in the bloodstream; neutrophil and monocyte phagocytosis or respiratory burst in response to E. coli; production of tumor necrosis factor-alpha, interleukin-1beta, and interleukin-6 in response to bacterial lipopolysaccharide; or plasma concentrations of soluble intercellular adhesion molecule-1. In contrast, the ALNA and FO treatments decreased the plasma concentrations of soluble vascular cell adhesion molecule-1 (16 and 28% decrease, respectively) and soluble E-selectin (23 and 17% decrease, respectively). It is concluded that, in contrast to previous reports using higher amounts of these fatty acids, a moderate increase in consumption of long-chain n-6 or n-3 polyunsaturated fatty acids does not significantly affect inflammatory cell numbers or neutrophil and monocyte responses in humans and so would not be expected to cause immune impairment. Furthermore, we conclude that moderate levels of ALNA and FO, which could be incorporated into the diet, can decrease some markers of endothelial activation and that this mechanism of action may contribute to the reported health benefits of n-3 fatty acids.

Diminished central fatigue by inhibition of the L-system transporter for the uptake of tryptophan.

Nagase genetically analbuminemic rats (NAR) were run to fatigue. Administration of branched chain amino acids (BCAA) before exhaustive exercise, resulted in a post-fatigue decreased tryptophan uptake (-22%, p < 0.05) and 5-hydroxytryptophan (5-HTP) uptake (-29%, p < 0.01) into the synaptosomes isolated from the striatum when compared with saline administration. At the same time, NAR who received either BCAA or 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid (BCH, a specific inhibitor for the L-system transporter) had a considerably prolonged run time to exhaustion (by twofold), compared to those who received either saline or albumin treatments. When classified by run time, it was of interest that, when the data for BCAA and BCH treatments for the longer run time NAR (Group B) was combined, it gave rise to a significant decrease in synaptosomal tryptophan and 5-HTP of a similar magnitude to that observed with BCAA alone. These levels were lower than those observed in NAR in the shorter run time group (Group A) for all treatments. These results support the view that an activated serotonergic function may be involved in central fatigue, which can be diminished by inhibition of the L-system transporter.

Encapsulated fish oil enriched in alpha-tocopherol alters plasma phospholipid and mononuclear cell fatty acid compositions but not mononuclear cell functions.

BACKGROUND: Several studies have reported that dietary fish oil (FO) supplementation alters cytokine production and other functional activities of peripheral blood mononuclear cells (PBMC). However, few of these studies have been placebo controlled and few have related the functional changes to alterations in PBMC fatty acid composition PATIENTS AND METHODS: Healthy subjects supplemented their diets with 9 g day-1 of encapsulated placebo oil (3 : 1 mix of coconut and soybean oils), olive oil (OO), safflower oil (SO), evening primrose oil (EPO) or FO [providing 2.1 g eicosapentaenoic acid (EPA) plus 1.1 g docosahexaenoic acid (DHA) per day] for 12 weeks; the capsules also provided 205 mg alpha-tocopherol per day. Blood was sampled at 4-weekly intervals and plasma and PBMC prepared. Plasma phospholipid and PBMC fatty acid composition, plasma alpha-tocopherol and thiobarbituric acid-reactive substance concentrations, plasma total antioxidant capacity, the proportions of different PBMC subsets, the proportions of PBMC expressing the adhesion molecules CD2, CD11b and CD54, and PBMC functions (lymphocyte proliferation, natural killer cell activity, cytokine production) were measured. All measurements were repeated after a 'washout' period of 8 weeks. RESULTS: The placebo, OO and SO capsules had no effect on plasma phospholipid or PBMC fatty acid composition. The proportion of dihomo-gamma-linolenic acid in plasma phospholipids was elevated in subjects taking EPO and was decreased in subjects taking FO. There was no appearance of gamma-linolenic acid in the plasma phospholipids or PBMC in subjects taking EPO. There was a marked increase in the proportion of EPA in the plasma phospholipids (10-fold) and PBMC (four-fold) of subjects taking FO supplements; this increase was maximal after 4 weeks of supplementation. There was an increase in the proportion of DHA in plasma phospholipids and PBMC, and an approximately 20% decrease in the proportion of arachidonic acid in plasma phospholipids and PBMC, during FO supplementation. Plasma concentrations of alpha-tocopherol were significantly elevated during supplementation in all subjects and returned to baseline values after the washout period. There were no effects of supplementation with any of the capsules on total plasma antioxidant activity or plasma thiobarbituric acid-reactive substances or on the proportion of different PBMC subsets, on the proportion of PBMC expressing adhesion molecules, on natural killer cell activity, on the proliferation of mitogen-stimulated whole blood cultures or PBMC, or on the ex vivo production of a range of cytokines by whole blood cultures or PBMC cultures stimulated by either concanavalin A or lipopolysaccharide. CONCLUSION: Supplementation of the diet with 3.2 g EPA plus DHA per day markedly alters plasma phospholipid and PBMC fatty acid compositions. The lack of effect of FO upon PBMC functions may relate to the level of alpha-tocopherol included in the supplements.

Comparison of cytokine production in cultures of whole human blood and purified mononuclear cells.

In order to examine inter- and intra-individual variations in cytokine production, blood was collected from 48 healthy subjects on each of 4 occasions separated by 4 weeks. Whole blood (diluted 1:10) and mononuclear cell (MNC) cultures were stimulated for 24 h with either concanavalin A (Con A) or bacterial lipopolysaccharide (LPS) and the concentrations of IL-1alpha, IL-1beta, IL-2, TNF-alpha, IL-10 and IFN-gamma in the culture medium measured. There were highly significant inter-individual variations in the production of each of the cytokines measured. However, the level of the production of each cytokine appeared to be characteristic of an individual. There were significant correlations between production of each cytokine in whole blood and MNC cultures. It is concluded that there is significant inter-individual variation in cytokine production which is unaffected by time or by the stimulus used to elicit cytokine production, and that whole blood cultures can be used instead of MNC cultures to measure cytokine production.

Manipulation of the type of fat consumed by growing pigs affects plasma and mononuclear cell fatty acid compositions and lymphocyte and phagocyte functions.

To investigate the immunological effect of feeding pigs different dietary lipids, 3-wk-old, weaned pigs were fed for 40 d on one of five diets, which differed only in the type of oil present (the oil contributed 5% by weight of the diet and the total fat content of the diets was 8% by weight). The oils used were soybean (control diet), high-oleic sunflower oil (HOSO), sunflower oil (SO), canola oil (CO), and fish oil (FO; rich in long-chain [n-3] polyunsaturared fatty acids). There were no significant differences in initial or final animal weights, weight gains, or health scores among the groups. There were no significant differences in the concentration of anti-Escherichia coli vaccine antibodies in the gut lumens of pigs fed the different diets. The fatty acid composition of the diet markedly affected the fatty acid composition of the plasma and of mononuclear cells (a mixture of lymphocytes, monocytes, and macrophages) prepared from the blood, lymph nodes, or thymus. The FO feeding resulted in a significant increase in the number of circulating granulocytes. The FO feeding significantly decreased the proportion of phagocytes engaged in uptake of E. coli and decreased the activity of those phagocytes that were active. The proliferation of lymphocytes in cultures of whole blood from pigs fed the HOSO, SO, or FO diets was less than in those from pigs fed the CO diet. Proliferation of lymph node lymphocytes from SO- or FO-fed pigs was less than that from control, CO-, or HOSO-fed pigs. The natural killer cell activity of blood lymphocytes from pigs fed the FO diet was significantly reduced compared with those from pigs fed the CO diet. The concentration of PGE2 in the medium of cultured blood, lymph node, or thymic mononuclear cells was lower if the cells came from pigs fed the FO diet. Thus, the type of oil included in the diet of growing pigs affects the numbers and functional activities of immune cells in different body compartments.

The role of tryptophan in fatigue in different conditions of stress.

Tryptophan is the precursor for the neurotransmitter 5-hydroxytryptamine (5-HT), which is involved in fatigue and sleep. It is present in bound and free from in the blood, where the concentration is controlled by albumin binding to tryptophan. An increase in plasma free tryptophan leads to an increased rate of entry of tryptophan into the brain. This should lead to a higher level of 5-HT which may cause central fatigue. Central fatigue is implicated in clinical conditions such as chronic fatigue syndrome and post-operative fatigue. Increased plasma free tryptophan leads to an increase in the plasma concentration ratio of free tryptophan to the branched chain amino acids (BCAA) which compete with tryptophan for entry into the brain across the blood-brain barrier. The plasma concentrations of these amino acids were measured in chronic fatigue syndrome patients (CFS) before and after exercise (Castell et al., 1998), and in patients undergoing major surgery (Yamamoto et al., 1997). In the CFS patients, the pre-exercise concentration of plasma free tryptophan was higher than in controls (p < 0.05) but did not change during or after exercise. This might indicate an abnormally high level of brain 5-HT in CFS patients leading to persistent fatigue. In the control group, plasma free tryptophan was increased after maximal exercise (p < 0.001), returning towards baseline levels 60 min later. The apparent failure of the CFS patients to change the plasma free tryptophan concentration or the free tryptophan/BCAA ratio during exercise may indicate increased sensitivity of brain 5-HT receptors, as has been demonstrated in other studies (Cleare et al., 1995). In post-operative recovery after major surgery plasma free tryptophan concentrations were markedly increased compared with baseline levels; the plasma free tryptophan/BCAA concentration ratio was also increased after surgery. Plasma albumin concentrations were decreased after surgery: this may account for the increase in plasma free tryptophan levels. Provision of BCAA has improved mental performance in athletes after endurance exercise (Blomstrand et al., 1995, 1997). It is suggested that BCAA supplementation may help to counteract the effects of an increase in plasma free tryptophan, and may thus improve the status of patients during or after some clinically stressful conditions.

Glutamine and the effects of exhaustive exercise upon the immune response.

There is a high incidence of infections in athletes undergoing intense, prolonged training or participating in endurance races (e.g., the marathon), in particular, upper respiratory tract infections. Prolonged, exhaustive exercise can lower the plasma level of the amino acid, glutamine, which is an important fuel for some cells of the immune system and may have specific immunostimulatory effects. This could therefore be an important factor in the event of an impaired response of immune cells to opportunistic infections. The effects of feeding glutamine to sedentary individuals and to marathon and ultramarathon runners before and after prolonged, exhaustive exercise has been investigated in a series of studies that monitored the incidence of infections and some acute-phase response markers. Oral glutamine, compared with a placebo, appeared to have a beneficial effect on the incidence of infections reported by runners after a marathon.

Furosemide decreases the sensitivity of glucose transport to insulin in skeletal muscle in vitro.

The effects of the diuretic furosemide on the sensitivity of glucose disposal to insulin were investigated in rat soleus muscle in vitro. At basal levels of insulin, the rates of 3-O-methylglucose transport, 2-deoxyglucose phosphorylation and lactate formation were not affected significantly by furosemide (0.5 mmol/l). However, furosemide significantly decreased these rates at physiological and maximal levels of insulin. The contents of 2-deoxyglucose and glucose 6-phosphate in the presence of furosemide were not significantly different from those in control muscles at all levels of insulin studied. It is concluded that furosemide decreases the sensitivity of glucose utilization to insulin in skeletal muscle by directly inhibiting the glucose transport process.

Inactivation of aconitase and oxoglutarate dehydrogenase in skeletal muscle in vitro by superoxide anions and/or nitric oxide.

Strips of rat soleus muscle were incubated in media containing a superoxide generating system and/or the nitric oxide donor sodium nitroprusside (SNP) before the maximal catalytic activities of aconitase, citrate synthase, and oxoglutarate dehydrogenase were measured. The maximal activities of aconitase and oxoglutarate dehydrogenase were both decreased by 25-30% by superoxide anions; however, only the maximal activity of aconitase was decreased, by approximately 50%, by incubation of muscles with SNP. Furthermore, when both superoxide and NO were present in the medium, aconitase activity was decreased by 70%. The maximal activity of citrate synthase was not affected by any of the treatments. This is the first time that superoxide anions or NO has been shown to inactivate aconitase and oxoglutarate dehydrogenase in skeletal muscle. It is suggested that these effects may be responsible for some alterations in skeletal muscle metabolism, and these possibilities are discussed.

Dietary fish oil suppresses human colon tumour growth in athymic mice.

1. Human colon tumour growth, initiated by subcutaneous inoculation of HT29 cells, was measured in athymic mice fed ad libitum on high-fat (210 g/kg) diets rich in coconut oil (CO), olive oil (OO), safflower oil (SO) or fish oil (FO); a low fat (LF; 25 g/kg) diet was used as the control. In one experiment the mice were fed the experimental diets for 3 weeks before HT29 cell inoculation and were killed 2 weeks post-inoculation. In a second experiment the mice were maintained on the LF diet until 4 days post-HT29 cell inoculation; they were then fed the experimental diets for 17 days. 2. Compared with mice fed the LF diet, tumour size was increased in mice fed the CO, OO or SO diets for 3 weeks before HT29 cell inoculation; FO feeding did not significantly increase tumour size. 3. Feeding mice the CO or OO diets from 4 days post-inoculation increased tumour growth rate and tumour size compared with feeding the LF, SO or FO diets; tumour growth rate and size did not differ among mice fed the latter diets. 4. The fatty acid composition of the tumours was markedly influenced by the fatty acid composition of the diet. 5. We conclude that human colon tumour growth is influenced by the type of fat consumed in the diet. Human colon tumour growth in this model is promoted by feeding high fat diets rich in medium chain saturated fatty acids (CO) or monounsaturated fatty acids (OO). A high fat diet, rich in long chain n - 3 polyunsaturated fatty acids (FO), does not promote colon tumour growth. The effect of a high fat diet rich in n - 6 polyunsaturated fatty acids (SO) depends upon the time at which it is fed: if fed before tumour cell inoculation such a diet promotes tumour growth, whereas if fed once tumour growth is initiated it does not. This suggests that n - 6 polyunsaturated fatty acids promote the initiation of colon tumour growth, but do not exert growth-promoting effects on colon tumours once they are established.