I. Rafecas

Formaldehyde derived from dietary aspartame binds to tissue components in vivo.

Adult male rats were given an oral dose of 10 mg/kg aspartame 14C-labelled in the methanol carbon. At timed intervals of up to 6 hours, the radioactivity in plasma and several organs was investigated. Most of the radioactivity found (>98% in plasma, >75% in liver) was bound to protein. Label present in liver, plasma and kidney was in the range of 1-2% of total radioactivity administered per g or mL, changing little with time. Other organs (brown and white adipose tissues, muscle, brain, cornea and retina) contained levels of label in the range of 1/12 to 1/10th of that of liver. In all, the rat retained, 6 hours after administration about 5% of the label, half of it in the liver. The specific radioactivity of tissue protein, RNA and DNA was quite uniform. The protein label was concentrated in amino acids, different from methionine, and largely coincident with the result of protein exposure to labelled formaldehyde. DNA radioactivity was essentially in a single different adduct base, different from the normal bases present in DNA. The nature of the tissue label accumulated was, thus, a direct consequence of formaldehyde binding to tissue structures. The administration of labelled aspartame to a group of cirrhotic rats resulted in comparable label retention by tissue components, which suggests that liver function (or its defect) has little effect on formaldehyde formation from aspartame and binding to biological components. The chronic treatment of a series of rats with 200 mg/kg of non-labelled aspartame during 10 days resulted in the accumulation of even more label when given the radioactive bolus, suggesting that the amount of formaldehyde adducts coming from aspartame in tissue proteins and nucleic acids may be cumulative. It is concluded that aspartame consumption may constitute a hazard because of its contribution to the formation of formaldehyde adducts.

Is leptin an insulin counter‐regulatory hormone?

Leptin, the product of the ob gene, controls appetite through the hypothalamus and may affect many other tissues because of the widespread distribution of its receptors. Leptin is synthesized by white adipose tissue (WAT) under conditions of high energy availability and insulin stimulus. Glucocorticoids enhance this synthesis and catecholamines hamper leptin production. Leptin diminishes insulin secretion by the pancreatic beta cells and induces insulin resistance. In fact leptin hampers insulin action on WAT itself in a negative feedback loop. The evidence acquired in studies on diabetics, starvation, refeeding and insulin and glucose clamps supports this interpretation, which may also explain part of the difficulties encountered by the current postulate that links leptin to WAT mass size signalling to the brain. Leptin may be, essentially, a counter‐regulatory hormone limiting the insulin drive to store energy in the form of fat, its effects reaching from a decrease in food intake to lower insulin secretion and increased resistance to insulin and lower glucose uptake and fat synthesis by WAT.

EFFECT OF DIETARY PROTEIN CONTENT ON TISSUE PROTEIN SYNTHESIS RATES IN ZUCKER LEAN RATS

Abstract Protein synthesis rates were determined in representative tissues of lean Zucker rats fed diets containing varying amounts of protein for 30 days. The control group (RD) received a standard commercial diet (20% of protein-derived energy), the high protein group (HP) received a high protein diet (36% of protein-derived energy) and the hypoproteic group (LP) received a low protein diet (9.5% of protein-derived energy). The contribution of skin and skeletal muscle to the whole body weight is higher in Zucker lean rats than in other rat strains. In all dietary groups the tissues with the highest fractional protein synthesis rates were the small intestine, kidney and liver, followed by lungs, skin and finally muscle. The HP group showed significantly lower protein synthesis in the muscle and skin than the RD group; the LP group showed lower rates in skin, small intestine, kidneys and lungs. The data presented indicate that the adaptations to a diet rich in protein did not affect the rate of protein synthesis, and also that the limited availability of protein reduced the rate of protein synthesis in some visceral organs but not in the skeletal muscle nor the skin. This implies that nitrogen partitioning is adapted to protein availability in the diet.

Lipid synthesis: A thermogenic mechanism in cold-exposed zucker fa/fa rats

1. The oxygen consumption and carbon dioxide production of Wistar and Zucker lean (Fa/?) and obese (fa/fa) rats was measured at 4, 10, 20 and 30 degrees C. 2. There was a net synthesis of lipid at the expense of carbohydrate in Wistar rats at 20 degrees C, with active lipid oxidation at 4 degrees C, and increasing heat production at lower temperature. Zucker lean rats also showed this trend. 3. Zucker fa/fa rats synthesized lipid at 4, 10 and 20 degrees C, showing a less marked increase in heat production with lowering temperature. 4. It is postulated that Zucker obese rats synthesize lipids as a way to obtain residual metabolic heat to maintain their body temperature. This is part of a process--fully functional in Wistar and Zucker lean rats, and truncated in Zucker obese rats--in which liver lipogenesis can combine with brown adipose tissue lipolysis to generate enough heat to maintain body functions under a cold environment.

Individual amino acid balances in young lean and obese Zucker rats fed a cafeteria diet

The amino acid composition of the diet ingested by reference and cafeteria diet-fed lean and obese Zucker rats has been analyzed from day 30 to 60 after birth. Their body protein amino acid composition was measured, as well as the urinary and faecal losses incurred during the period studied. The protein actually selected by the rats fed the cafeteria diet had essentially the same amino acid composition as the reference diet. The mean protein amino acid composition of the rat showed only small changes with breed, age or diet.Cafeteria-fed rats had a higher dietary protein digestion/absorption efficiency than reference diet-fed rats. Obese rats wasted a high proportion of dietary amino acids when given the reference diet, but not on the cafeteria diet. In all cases, the amino acids lost as such in the urine were a minimal portion of available amino acids.In addition to breed, the rates of protein accretion are deeply influenced by diet, but even more by the age — or size — of the animals: cafeteria-fed rats grew faster, to higher body protein settings, but later protein accrual decreased considerably; this is probably due to a limitation in the ‘blueprint for growth’ which restricts net protein deposition when a certain body size is attained. Obese rats, however, kept accuring protein with high rates throughout.Diet composition — and not protein availability or quality-induced deep changes in amino acid metabolism. Since the differences in the absolute levels of dietary protein or carbohydrate energy ingested by rats fed the reference or cafeteria diets were small, it can be assumed that high (lipid) energy elicits the changes observed in amino acid metabolism by the cafeteria diet. The effects induced in the fate of the nitrogen ingested were more related to the fractional protein energy proportion than to its absolute values. Cafeteria-fed rats tended to absorb more amino acids and preserve them more efficiently; these effects were shown even under conditions of genetic obesity.There were deep differences in handling of dietary amino acids by dietary or genetically obese rats. The former manage to extract and accrue larger proportions of their dietary amino acids than the latter. The effects of both ‘models’ of amino acid management were largely additive, suggesting that the mechanisms underlying the development of obesity did not run in parallel to those affecting the control of amino acid utilization. Obesity may be developed in both cases despite a completely different strategy of amino acid assimilation, accrual and utilization. (Mol Cell Biochem121: 45–58, 1993)

Water balance in zucker obese rats

The water balance in Wistar, Zucker obese and Zucker lean rats, aged 60 days, was measured by determining the amount of water they drank, that contained in the solid food eaten, the water lost through urine and droppings, the net water accrued (estimated from the composition of the body and the daily increase in body weight), the measurement of the water vapour lost and the calculation of metabolic water production by means of the measurement of oxygen consumption, carbon dioxide production and protein oxidation in a 24 hr period. 1. Despite widely different body weights, all three groups of animals accrued a similar proportion of their daily water budget (4.3-4.9%, i.e. 1.2-1.4% of the total rat water mass). 2. Wistar and Zucker obese rats had a similar daily water budget despite very different body weights, lean Zucker rats had lower water budgets. 3. Obese and lean Zucker rats produced a more concentrated, and excreted much less urine (the highest urea concentration was found in obese rats) than Wistar rats. 4. The water lost in the droppings was in the same range as that in urine for obese rats, slightly less for lean Zucker rats and much less in Wistar rats. Obese rats produced a higher amount of stool with respect to the amount of food eaten than the lean animals studied. 5. The contribution of metabolic water to the daily water budget was a 23.6% for Zucker obese, 22.5% for Zucker lean and 15.9% for Wistar rats.

Rat splanchnic net oxygen consumption, energy implications.

1. The blood flow, PO2, pH and PCO2 have been estimated in portal and suprahepatic veins as well as in hepatic artery of fed and overnight starved rats given an oral glucose load. From these data the net intestinal, hepatic and splanchnic balances for oxygen and bicarbonate were calculated. The oxygen consumption of the intact animal has also been measured under comparable conditions. 2. The direct utilization of oxygen balances as energy equivalents when establishing the contribution of energy metabolism of liver and intestine to the overall energy expenses of the rat, has been found to be incorrect, since it incorporates the intrinsic error of interorgan proton transfer through bicarbonate. Liver and intestine produced high net bicarbonate balances in all situations tested, implying the elimination (by means of oxidative pathways, i.e. consuming additional oxygen) of high amounts of H+ generated with bicarbonate. The equivalence in energy output of the oxygen balances was then corrected for bicarbonate production to 11‐54% lower values. 3. Intestine and liver consume a high proportion of available oxygen, about one‐half in basal (fed or starved) conditions and about one‐third after gavage, the intestine consumption being about 15% in all situations tested and the liver decreasing its oxygen consumption with gavage.

Fatty acid utilization by young Wistar rats fed a cafeteria diet

The content and accretion of fatty acids in 30, 45 and 60-day old Wistar rats fed either reference chow or a cafeteria diet has been studied, together with their actual fatty acid intake during that period. Diet had a small overall effect on the pattern of deposition of fatty acids, but the deposition of fat was much higher in cafeteria rats. The fat-rich cafeteria diet allowed the direct incorporation of most fatty acids into lipid storage, whilst chow-feeding activated lipogenesis and the deposition of a shorter chain and more saturated type of fatty acids. During the second month of the rats life, the elongation pathway as well as Δ9-desaturase became functional, thus helping to shape the pattern of fatty acids actually accrued. The 60-day rats showed a relative impairment in the operation of Δ5-desaturase, since their lipids had a higher C20:4/C20:3 ratio than those of the diet ingested. Cafeteria-diet feeding minimized this effect since the large supply of dietary polyunsaturated fatty acids made the operation of the elongation-desaturase pathways practically unnecessary.

The thermogenic effect of a sucrose gavage on the fa/fa rat

Abstract The heat production of homozygous fa/fa and +/+ Zucker rats as well as that of Wistar control adult female rats was estimated with a direct chamber calorimeter, after a gavage of 5.2 mmoles of sucrose given with a stomach cannula. The effects of mild food deprivation (12 hours) affected the thermic response to the gavage. Obese fa/fa rats had very low basal metabolic rates, increasing very little with dietary state or gavage. Lean +/+ rats had basal rates comparable to those of Wistar rats and showed a higher effect of gavage in the food deprived state. Wistar rats had a comparable, albeit magnified, pattern. Zucker rats increased their thermogenesis with gavage with a deep decrease following suit. It is postulated that the deficient thermogenic apparatus of fa/fa rats prevented them showing a diet induced thermogenesis after forced sucrose feeding. Both lean Zucker (+/+) and Wistar rats showed a marked thermogenic response to the sucrose gavage, which was somewhat less pronounced in the Zucker lean rats.

Changes in UCP expression in tissues of Zucker rats fed diets with different protein content.

The effect of dietary protein content on the uncoupling proteins (UCP) 1, 2 and 3 expression in a number of tissues of Zucker lean and obese rats was studied. Thirty-day-old male Zucker lean (Fa/?) and obese (fa/fa) rats were fed on hyperproteic (HP, 30% protein), standard (RD, 17% protein) or hypoproteic (LP, 9% protein) dietsad libitum for 30 days. Although dietary protein intake affected the weights of individual muscles in lean and obese animals, these weights were similar. In contrast, huge differences were observed in brown adipose tissue (BAT) and liver weights. Lean rats fed on the LP diet generally increased UCP expression, whereas the HP group had lower values. Obese animals, HP and LP groups showed higher UCP expression in muscles, with slight differences in BAT and lower values for UCP3 in subcutaneous adipose tissue. The mean values of UCP expression in BAT of obese rats were lower than in their lean counterpart, whereas the expression in skeletal muscle was increased. Thus, expression of UCPs can be modified by dietary protein content, in lean and obese rats. A possible thermogenic function of UCP3 in muscle and WAT in obese rats must be taken into account.ResumenSe determina el efecto de cambios moderados en el contenido proteico de la dieta sobre la expresión de las proteínas desacoplantes 1, 2 y 3 en distintos tejidos de la rata Zucker. A ratas Zucker macho de 30 días de edad, tanto delgadas (Fa/?) como obesas (fa/fa), se les administraad libitum una dieta hiperproteica (HP, 30% de contenido proteico), estándar (RD, 17% de contenido proteico) o hipoproteica (LP, 9% de contenido proteico) durante 30 días. Los pesos de los músculos individuales son similares en los animales delgados y en los obesos, aunque ligeramente influenciados por el tipo de dieta; este hecho contrasta con las grandes diferencias detectadas en el hígado y en el tejido adiposo marrón. Los animales delgados alimentados con dieta LP incrementan la expresión de las UCP, mientras que los alimentados como dieta HP muestran valores más bajos. Los animales obesos, tanto los alimentados con dieta LP como con dieta HP, muestran una mayor expresión de UCP en músculo, con pequeñas diferencias en el tejido adiposo marrón y valores muy bajos para la UCP3 en el tejido adiposo subcutáneo. Los valores promedio de la expresión de UCP en el tejido adiposo marrón de las ratas obesas son menores de los de las ratas delgadas, mientras que la expresión en los músculos es superior. Así, la expresión de las UCP puede ser modificada por la ingesta crónica de dietas con dstinto contenido proteico, tanto en ratas delgadas como obesas. Se debe considerar el posible papel termogénico de la UCP3 en el músculo y en el tejido adiposo blanco de las ratas obesas.