Bioavailability of lysine for kittens in overheated casein is underestimated by the rat growth assay method.

Growth assays were performed to determine lysine bioavailability for kittens and rats in untreated and heated casein; these values were compared with estimates obtained with an in vitro method. Body weight, food intake, nitrogen and dry matter digestibility, and plasma lysine were determined during an 80-day growth trial using kittens (n = 16). Body weight and food intake were determined during a 21-day growth trial using weanling rats (n = 80). The growth data showed bioavailable lysine to be 102.4% and 100.2% (for untreated casein) and 66.1% and 51.7% (for heated casein) for kittens and rats, respectively. There was no relationship between plasma lysine and dietary lysine concentrations for kittens. There were no significant differences in nitrogen or dry matter digestibility among diets for kittens. The chemically reactive lysine content of untreated casein was 99.6%, and of heated casein was 67.1%. Heat treatment of casein resulted in significantly decreased lysine bioavailability as estimated by all methods. For untreated casein, both growth assays showed good agreement with the in vitro method for available lysine. For heated casein, the rat growth assay significantly underestimated bioavailable lysine as determined in kittens while the in vitro method closely approximated this value for the cat.

Ammonia intoxication in the near-adult cat as a result of a dietary deficiency of arginine.

Near-adult cats, fasted overnight, and given a single meal of a complete amino acid diet without arginine, developed hyperammonemia and showed clinical symptoms of ammonia toxicity within 2 hours. One cat (2.7 kilograms) died 4.5 hours after ingesting only 8 grams of the diet. Since ornithine also prevented hyperammonemia, it appears that the domestic cat cannot synthesize ornithine.

Myocardial failure in cats associated with low plasma taurine: a reversible cardiomyopathy.

Thousands of pet cats die each year with dilated cardiomyopathy, the cause of which is unknown. Although taurine is present in millimolar concentrations in the myocardium of all mammals, taurine depletion has not previously been associated with a decrease in myocardial function in any species. In this study, low plasma taurine concentrations associated with echocardiographic evidence of myocardial failure were observed in 21 cats fed commercial cat foods and in 2 of 11 cats fed a purified diet containing marginally low concentrations of taurine for 4 years. Oral supplementation of taurine resulted in increased plasma taurine concentrations and was associated with normalization of left ventricular function in both groups of cats. Since myocardial concentrations of taurine are directly related to plasma concentrations and low plasma concentrations were found to be associated with myocardial failure in cats, a direct link between decreased taurine concentration in the myocardium and decreased myocardial mechanical function is proposed.

Testosterone increases urinary free felinine, N-acetylfelinine and methylbutanolglutathione excretion in cats (Felis catus).

Two days after castration, urinary free felinine plus N-acetylfelinine decreased 24% in male cats, but, by day 5, the concentration had not decreased to that routinely found in males that have been castrated for several months. In a second experiment, three groups of castrated adult male cats received different subcutaneous injections: control (carrier), testosterone, testosterone plus estradiol. A fourth group of intact adult female cats received a testosterone injection. Urine was collected and analysed for free felinine, N-acetylfelinine and 3-methylbutanolglutathione. Baseline blood testosterone and estradiol concentrations were low during the pre-period, but increased sharply after hormone injections. The concentration of all three urinary metabolites increased as a result of testosterone injections with estradiol not modulating the effect. The effect of testosterone was not gender dependent. The concentration of free felinine, N-acetylfelinine and 3-methylbutanolglutathione in the urine remained low in the placebo control group throughout the study. The relative molar contribution of free felinine to the total amount of felinine containing compounds increased due to testosterone treatment, while the contribution of 3-methylbutanolglutathione and N-acetylfelinine decreased. Testosterone increases free felinine, N-acetylfelinine and 3-methylbutanolglutathione excretion in castrated adult male and intact female cats, whereas estradiol does not modulate this effect.

Dietary crude protein has minimal effect on the activity of selected enzymes of methionine catabolism in kittens fed diets near-limiting in methionine.

Previous experiments have shown that increasing the dietary crude protein (CP) of cats does not increase urea cycle enzymes or alanine amino transferase as occurs in rats. Also when an essential amino acid (EAA) is limiting in a diet for growing kittens, the kittens do not exhibit an amino acid imbalance when other EAAs are added to the diet. To study the metabolic basis for these observations which are different from that found in omnivores and herbivores, the hypothesis that increased dietary CP decreases methionine catabolism, so more is spared for growth, was tested. Fifteen male kittens were randomly assigned to one of three dietary treatments. Each diet contained 2.5 g l-methionine/kg diet and 200, 300 or 500 g CP/kg diet. The livers and kidneys were removed and assayed for methionine transaminase (MTA), cystathionase (CASE) and cystathionine synthase (CS). Free amino acid concentrations were determined in liver, kidney and plasma. The 300 and 500 g CP/kg groups had significantly greater kidney weights and body weight gains than the 200 g CP/kg group. Hepatic MTA activity was lower in the 300 than the 200 or 500 g CP/kg groups (p < 0.05). Renal MTA and CASE activities were 35% and 50% greater, respectively, for the 500 g CP/kg group than for the 200 g CP/kg diet group (p < 0.05). Renal CS activities for the 300 and 500 g CP/kg groups were 29% (p > 0.05) and 38% (p < 0.05) greater, respectively, than the 200 g CP/kg group. Cyst(e)ine concentrations were lower in the livers of the 500 g CP/kg group than the 200 g CP/kg group (p < 0.05). Cystathionine was lower in plasma and kidney from the 500 g CP/kg diet group than from the 200 g CP/kg diet group (p < 0.05). It was concluded that the metabolic basis for the increased growth of kittens fed diets marginally limiting in methionine, with increasing concentrations of dietary CP, was not mediated through decreased enzyme activity associated with the catabolism of methionine, but was the result of an increase in food (methionine) intake.

Phenotypic correction of feline lipoprotein lipase deficiency by adenoviral gene transfer.

Previous studies have revealed that adenovirus-mediated ectopic liver expression of human LPL (huLPL) can efficiently mediate plasma triacylglycerol (TG) catabolism in mice despite its native expression in adipose and muscle tissue. We aimed to explore the feasibility of liver-directed gene transfer and enzyme replacement for human LPL deficiency in a larger, naturally occurring feline animal model of complete LPL deficiency that is remarkably similar in phenotype to the human disorder. A cohort of LPL-deficient (LPL -/-) cats was given an intravenous injection of 8 x 10(9) PFU/kg of a CMV promoter/enhancer-driven, E1/E3-deleted adenoviral (Ad) vector containing a 1.36-kb huLPL cDNA (Ad-LPL) or reporter alkaline phosphatase gene (Ad-AP). After Ad-LPL administration, active, heparin-releasable huLPL was readily detected along with a 10-fold reduction in plasma TGs, disappearance of plasma TG-rich lipoproteins up to day 14, and enhanced clearance of an excess intravenous fat load on day 9. However, antibody against the huLPL protein was detected on day 14 in cats receiving Ad-LPL and adenovirus-specific neutralizing antibody was present 7 days after gene transfer in both cat cohorts. Tissue-specific expression of the huLPL transgene relative to controls was confirmed by RT-PCR. While huLPL expression was evident in the liver, other tissues including spleen and lung expressed huLPL message, in direct correlation with histological evidence of increased Oil red O (ORO)-positive neutral lipid influx. In contrast, intravenous LPL enzyme replacement therapy (ERT) led to rapid disappearance of 9000 mU/kg of active bovine LPL enzyme from the circulation, with t1/2 occurring at <10 min in two LPL-/- cats. Heparin injection 1 hr later released <10% of the original bovine LPL, further indicating its rapid systemic clearance, inactivation, or degradation as well as its ineffectiveness as a viable therapeutic alternative for complete LPL deficiency. Although LPL gene transfer and expression via this first-generation Ad vector was limited by the immune response against both the human LPL protein and adenovirus our results clearly provide a key advance supporting further development of LPL gene therapy as a viable therapeutic option for clinical LPL deficiency.

Fluctuations in plasma pyrroline-5-carboxylate concentrations during feeding and fasting.

Pyrroline-5-carboxylate (P5C) is the oxidized metabolite of proline. In cultured cells redox-sensitive metabolic pathways are influenced by these two amino acids. To determine whether plasma P5C levels fluctuate enough physiologically to regulate such pathways in vivo, we measured venous plasma P5C concentrations hourly for 48 h in five normal subjects while eating and in four subjects while fasting. Two fed subjects had one or more plasma P5C excursions per day exceeding 10 times the baseline level; the other fed subjects had lesser variations. These patterns suggest a relationship of plasma P5C peaks to meals. This relationship is supported by the finding of no significant P5C elevations above baseline levels in four subjects during fasting for 24 h. Despite the changes in plasma P5C concentrations, those of proline and other metabolically related amino acids were constant. The variation among subjects in the degree of plasma P5C fluctuation could not be explained by differences in total protein and caloric intake or body weight. Among plasma constituents, the large peak to basal plasma P5C excursions in some of the normal subjects we studied are matched only by those of peptide hormones.

The potency of dietary amino acids in elevating plasma cholecystokinin immunoreactivity in cats is related to amino acid hydrophobicity.

Incomplete agreement exists on the relative potency of amino acids in stimulating endocrine secretion of cholecystokinin (CCK). Species and methodological variations have been suggested to account for the apparent inconsistencies. In the present research, the CCK-releasing potency of dietary amino acids was evaluated in cats using plasma CCK-like immunoreactivity (CCK-LI) as an indicator of CCK secretion rather than pancreatic protein and enzyme secretion, as has been used in past research. Oral-gastric administrations of a casein-simulating amino acid mixture increased (P < 0.05) plasma CCK-LI but not to the extent of that observed for casein or sodium oleate. The response in plasma CCK-LI to administrations of 50 mM solutions of amino acids was significant (P < 0.05) for tryptophan, phenylalanine, leucine, and isoleucine and the response increased linearly (P < 0.01) with increasing amino side-chain hydrophobicity. Control administrations of water and saline also evoked elevation in plasma CCK-LI, but the responses were so transient that amino acid effects were not obscured. This was substantiated by the finding of a significant linear (P < 0.001) dose response to phenylalanine administration. Cholecystokinin-8, 33 and 58 were among the CCK molecular forms identified by HPLC in plasma after administrations of phenylalanine and water. The present findings indicate that lipophilic amino acids released during digestion account for at least part of the endocrine CCK response in cats to ingested protein. The greater CCK-releasing potency observed for intact protein relative to free amino acids may have been the result of a slow digestive release of amino acids, elaboration of peptide secretogogues or protection of protease-sensitive releasing factors.

Elevation of plasma cholecystokinin (CCK) immunoreactivity by fat, protein, and amino acids in the cat, a carnivore.

The cat requires a diet high in protein and certain nutrients that are found only in animal tissue. It is possible that secretogogues of intestinal CCK in the cat may be different from those observed in non-carnivorous species. Plasma CCK concentrations were determined in cats (n = 6) given by oral-gastric tube either casein, whey protein, corn oil, or corn starch suspended in water. CCK was measured by RIA with a tyrosine sulfate-specific, C-terminal antibody, DINO. HPLC of plasma revealed that most CCK-immunoreactivity (CCK-LI) was associated with CCK-33 and a late eluting peak, presumably CCK-58. Casein, whey protein, and corn oil increased (P < 0.05) post-administration plasma CCK-LI, and at least for casein, the effect was dose related. An amino acid mixture approximating the residue composition of casein increased plasma CCK-LI (P < 0.05), however, the increase tended to be less than that caused by casein. Evaluation of post-administration levels of plasma amino acids indicated that intact protein and amino acids in the intestinal lumen affect CCK release by different mechanisms. Collectively, the results indicated that although cats are carnivores cats and humans secrete CCK in response to the same nutrients.

Changes in cerebrospinal fluid and plasma amino acid concentrations with elevated dietary protein concentration in dogs with portacaval shunts.

Effects of dietary protein concentration on plasma and cerebrospinal fluid (CSF) amino acids (AA) in dogs with portacaval shunts (PCS) were examined. An 18% protein purified diet (18P) was fed to 4 PCS dogs and 2 controls; at week 10, 2 of the PCS dogs were switched to 36% protein (36P) until week 28. Effects of the diet switch on plasma and CSF AA in 8 normal dogs were determined in another experiment. Neither surgery nor protein level significantly affected average food intake (weeks 10-28). Plasma amino acid patterns typical of PCS animals were observed: phenylalanine and tyrosine increased and branched chain AA decreased with shunting (p less than 0.05). Plasma phenylalanine increased further with 36P in PCS dogs (p less than 0.05), but was not affected by dietary protein concentration in controls. With 36P: CSF arginine, serine, histidine, tryptophan, phenylalanine, glutamate and glutamine increased in PCS dogs; but only arginine decreased in CSF of controls (p less than 0.05). In PCS dogs, significant CSF AA changes with elevated dietary protein were unrelated to plasma AA changes.

Effect of amino acid imbalance and deficiency on dietary choice patterns of rats.

Detailed dietary choice patterns were determined with a computerized feeding monitoring system in groups of Sprague Dawley rats kept on a 12:12 hr light-dark cycle and offered in sequence a series of dietary choice regimens involving amino acid-imbalanced or deficient diets with threonine as the most limiting amino acid. Animals established their preference for a threonine-basal diet over a threonine-imbalanced or a threonine-devoid (devoid of threonine) diet shortly (within 2-3 hr) after the consumption of small quantities of either diet in the beginning of the first dark-cycle. An intensive sampling process characterized by frequent small bouts was evident throughout the light period. Both the meal size and the meal frequency of the imbalanced or devoid diet were curtailed after prolonged choices. Animals preferred the threonine-corrected (imbalance corrected by threonine supplementation) over the threonine-basal diet initially with an increase in meal frequency. But no clear choice for either diet was observed thereafter. Animals did not establish their preference for the threonine-corrected diet when paired with the threonine-devoid diet until after 5 days with a steady decrease in the meal size of the devoid diet but not the meal frequency. When the protein-free diet was introduced as an alternative for the threonine-imbalanced diet, animals selected the protein-free diet during the first dark-cycle after consuming a small amount of the imbalanced diet. Initially there was a drastic reduction in meal size of the imbalanced diet and subsequently a decrease in meal frequency as well. Nevertheless, animals immediately rejected the protein-free diet and chose the threonine-basal diet when it replaced the imbalanced diet as an alternative. The almost exclusive preference for the basal diet occurred in the beginning of the first dark-cycle with an increase in meal size but no change in meal frequency. The sampling bouts of small quantities, which followed the first introduction of the diets in the choice regimens, may be an inherent investigative behavior whereby the physical or oropharyngeal properties of the diets are recognized. The establishment of the choices for the alternative diets in the present experiments provides additional information about the rapid time course of the food intake control mechanisms in rats fed amino acid-imbalanced or deficient diets.

Effects of hippocampal lesions on adaptive intake of diets with disproportionate amounts of amino acids.

Bilateral double electrolytic overlapping lesions were placed in dorsal-lateral hippocampus of male 230 g rats, and their food intake responses to the ingestion of diets containing disproportionate amounts of amino acids were examined. Rats with such lesions and intact control rats maintained their normal intakes of the 6% casein basal diet or a threonine basal amino acid diet postoperatively. However, they exhibited marked initial food intake depression, similar to that of intact rats, when fed the threonine imbalanced amino acid diet. Also, animals with lesions in certain areas of the dorsal-lateral hippocampus showed facilitated adaptation to the amino acid imbalanced diet. Similar severe reduction in food intake with relative lack of adaptation were observed in both the intact controls and rats with hippocampal lesions when fed amino acid diets completely devoid of threonine. Initial food intake of rats with hippocampal lesions was inhibited drastically as was the case with the intact controls when fed a 75% casein high protein diet. All rats, either intact or lesioned, showed similar slow adaptation patterns with the prolonged ingestion of the high protein diet. The initial food intake responses and facilitated adaptation of the animals bearing lesions in certain areas of the hippocampus suggest that such areas are not crucially involved in the inhibition of food intake of rats fed disproportionate amounts of dietary amino acids. Rather, such areas of lesions in the hippocampus may play a role in a system governing the behavioral adaptation of the intake of amino acid imbalanced diets but not of diets containing amino acids in general excess. This would also indicate that different mechanisms control the intake of amino acid imbalanced diets and diets containing amino acids in excess.

Aversion of the cat to dietary medium-chain triglycerides and caprylic acid.

Young, specific-pathogen-free cats were fed purified diets containing different sources of fat. Food intake was depressed and cats lost weight when the diet contained either hydrogenated coconut oil (HCO) or medium-chain triglycerides (MCT). With an MCT preparation enriched in 8:0 (MCT8), cats would not eat after first tasting the diet. When cats were offered a choice of two high-fat diets, they chose the basal diet over a diet containing 30% HCO, by a ratio of 4.5:1. Low levels of MCT8 (5% or 10% by weight) were also rejected, whereas cats did not reject 5% or 15% MCT12. Caprylic acid, at 0.1-1.0% of the diet, was rejected. In other studies, food intake and body weight decreased when HCO was added to a fat-free diet. Cats fed 25% or 35% HCO lost weight. When 5% safflower seed oil was added to the HCO diets, body weights and food intake improved, but were still less than optimal. These studies indicate that the food intake depression in cats fed dietary HCO and MCT is primarily a result of impalatability, and that the fatty acid moiety may be responsible for the aversion.

Norepinephrine and amino acids in prepyriform cortex of rats fed imbalanced amino acid diets.

Monoamines and amino acids were measured in anterior prepyriform cortex (PPC) and anterior cingulate cortex (CC) of male Sprague-Dawley rats after they were offered basal, imbalanced (IMB) or corrected amino acid diets, limited in threonine (THR) or isoleucine (ILE). In the THR study, brains were taken after 2.5 hr of feeding, when intake of THR-IMB was just depressed. In the ILE study the brains were taken after 3.5 hr on ILE-IMB, a less severely imbalanced ration, before the onset of food intake depression. The PPC has been shown to be involved in the acute response of animals to imbalanced amino acid diets. In the PPC from the IMB diet groups, NE was reduced by 30%, but the other monoamines were unchanged. In CC, an area involved in the adaptive, but not the acute feeding response to imbalanced diets, the monoamines were unchanged in the IMB diet groups. In both studies, in both tissues, the limiting amino acids were decreased in the IMB groups, although the decrease of ILE in the CC failed to reach significance. The remaining indispensable amino acids, added to create the imbalance, were slightly reduced in the THR-IMB group, but not in the ILE-IMB group in both tissues. Thus, the amino acid patterns were altered in the PPC and CC, as they are in whole brains from animals fed imbalanced amino acid diets. These results also suggest that the concentration of NE in the PPC may be associated with the initial food intake response of animals to imbalanced amino acid diets.

Self-selection of dietary casein and soy-protein by the cat.

Growing specific-pathogen-free kittens were fed for two weeks a choice between two complete diets differing only in protein content. When casein diets containing 18, 36 and 54% protein were offered in the three possible combinations, the kittens consistently avoided the higher casein diets and kittens offered the two highest levels of casein significantly reduced their total food intake. In one soy-protein choice study, 16, 31 and 63% protein diets were each offered with a protein-free (PF) diet. When diets were similar in physical consistency, kittens selected similar amounts of both diets with the result that the PF:16% group consumed below their requirement of protein. In another soy-protein experiment the 16, 31 and 63% protein diets were offered in their three possible combinations. Kittens in all three groups selected similar amounts of both diets. Except for their avoidance of casein, the kittens did not regulate in a consistent manner their intake of protein and therefore, behaved very differently from the rat in the self-selection of dietary protein.

Dietary amino acid imbalance and neurochemical changes in three hypothalamic areas.

The impact of feeding imbalanced amino acid diets on monoamine, metabolite and amino acid concentrations was measured in the ventromedial hypothalamus (VMH), lateral hypothalamus (LH) and paraventricular nucleus (PVN). After rats were fed either an isoleucine imbalanced diet, a threonine imbalanced diet, or the appropriate basal or corrected control diets, regional differences were found in neurochemical concentrations. Contrary to our expectations, the limiting amino acid was unchanged in the imbalanced groups, tending to be decreased only in the isoleucine imbalanced-diet group in the PVN. This is the first report that the limiting amino acid was not reduced uniformly in the brain after imbalanced amino acid feeding. In the VMH, norepinephrine (NE) was increased by 22% and 63% in the threonine and isoleucine imbalanced-diet groups, respectively. Since the concentration of NE was affected even before the decrease in feeding, both in the VMH, and, as previously reported, in the prepyriform cortex, the NE system may be involved in very early responses to imbalanced amino acid diets.

Influence of taste on dietary choice of rats fed amino acid imbalanced or deficient diets.

Diets with added quinine as the negative taste cue or saccharin as the positive taste cue were employed to determine the influence of taste on dietary choice of rats offered diets containing different proportions of amino acids (amino acid imbalance) and which differed in acceptability. The quinine was added to the protein-free or the corrected (corrected for amino acid imbalance) diet that animals normally preferred and the saccharine was added to the amino acid imbalanced or deficient diets that animals normally avoided. There appeared to be a balance in acceptance of a diet between the undesirability of the quinine and the degree of metabolic benefit from the diets with favorable metabolic or nutritional characteristics. Although the presence of higher levels of quinine could interfere with the normal dietary preference based on metabolic consequences, the animals invariably selected the metabolically favorable diet if they were forced to experience the metabolic characteristics of the diets by having to consume them exclusively. The presence of the taste cues appeared to enhance the acceptance or avoidance of diets in the choice regimens, possibly by aiding in their identification, especially to animals previously not exposed to the taste cues.

Effect of pre-feeding ammonium acetate on food intake of rats fed high protein diets.

The effect on intake of a 75% casein diet after prefeeding for one week a 6% casein basal diet with additional 0%, 2%, 5%, 8% or 15% ammonium acetate was examined in rats trained to eat in three hours per day. Food intake was measured from 0-15, 15-30, 30-90, and 90-180 minutes for the first two days that the ammonium acetate diets were presented. Rats eating 5% and 8% or 15% ammonium acetate diet depressed their intake significantly for one day and for four days respectively. Rats eating 2%, 5%, 8%, or 15% ammonium acetate diets depressed their intake significantly from 0-30 minutes. When presented with the 75% casein diet, rats prefed 0% to 5% and 8% and 15% ammonium acetate diets ate 55% to 58% and 72% and 94% of their respective baseline intakes. It is suggested that prefeeding 15% ammonium acetate apparently induces sufficient metabolic adaptation to ammonia intake so that the rat is able to offset the metabolic consequences of intake of the 75% casein diet, thus preventing the usual food intake depressing effect of the high protein diet.

Cingulate lesions and behavioral adaptation to amino acid imbalanced diets.

The effect of anterior cingulate cortex lesions on dietary intake and adaptation of disproportionate amounts of amino acids was examined. Rats with bilateral electrolytic lesions in the anterior cingulate cortex and sham-operated rats were fed, in turn, amino acid basal, imbalanced or devoid diets involving threonine and isoleucine as the growth limiting amino acids, and then a low protein (6% casein) followed by a high protein (75% casein) diet. Lesions of the anterior cingulate cortex did not prevent the initial depression in food intake of the amino acid imbalanced diets, but shortened the duration of anorexia associated with dietary amino acid imbalances. Cingulate lesions did not influence the food intake of rats fed amino acid devoid diets. When switched from a low protein to a high protein diet, animals bearing lesions and sham-operated controls reduced markedly their initial food intake and adapted to the high protein diet in similar manner. It was concluded that the initial food intake depression associated with a dietary amino acid imbalance is a direct response to postingestive cues which influence food intake. Moreover, that the difference in adaptive intakes of the cingulate cortex lesioned animals who ingested a diet of imbalanced amino acids or of high protein, indicates that separate mechanisms act to control food intake of animals fed diets containing imbalanced amino acid mixtures or diets with excessive amounts of protein.

Increase in plasma ammonia and amino acids when rats are fed a 44% casein diet.

Rats were trained to eat a 6% casein basal diet during a 3-hour period per day. They were then fed either the same 6% casein diet or a 44% casein diet for 3 hours. No food intake depression was observed in the rats eating 44% casein diet during the 3-hour period. Plasma ammonia and amino acids and brain amino acids were measured at 0, 4, 12 and 24 hours after presentation of the 6% or 44% casein diets. Plasma ammonia rose to 134 (p less than 0.01) and 110 micromolar (p less than 0.05) in the 44% casein fed rats at 4 and 12 hours, respectively, as compared to 67 and 53 micromolar, respectively, for the 6% casein fed rats. All plasma amino acid concentrations except methionine and glutamate were elevated (p less than 0.05) at 4 hours. In the brain, threonine, glutamine and tyrosine concentrations were elevated (p less than 0.05) at 4 hours after diet presentation. At 24 hours, valine, isoleucine, leucine, phenylalanine, and methionine concentrations were also elevated (p less than 0.05). Because intake of the 44% casein diet decreases the second day of its presentation, as noted in an earlier experiment, the increases in plasma ammonia and its possible entry into the brain as reflected by increased brain glutamine together with changes in amino acid concentrations should be considered collectively among possible metabolic signals affecting intake of high protein diets.