Free amino acid concentrations in normal and abnormal third trimester placental villi.

Homogenates of placental villi were obtained from normotensive pregnancies and from pregnancies complicated by pre-eclampsia and growth retardation. The homogenates were analysed for free amino acid concentrations using high performance liquid chromatography (HPLC). The study shows that there are significantly higher concentrations of two essential amino acids, L-arginine and phenylalanine (P < 0.05), in the growth-retarded group compared to the term normotensive and pre-eclamptic groups. There are significantly higher concentrations of the non-essential amino acids: glutamic acid in the growth retarded group compared to the normotensive group, and of tyrosine in the growth-retarded group compared to the pre-eclamptic group. However, there is no increase in the glycine/valine ratio, a characteristic marker for kwashiorkor or a reduction in total protein concentration in the pre-eclamptic or growth-retarded groups. These findings suggest that there may be abnormalities in placental metabolism, amino acid transfer across the basal membrane into the fetal circulation, and also fetoplacental perfusion in pregnancies complicated by pre-eclampsia and growth retardation.

Free amino acid concentrations in human first and third trimester placental villi.

Homogenates of first (10.1 +/- 1.0 weeks) and third trimester placental villi were analysed for free amino acid concentrations. As has been previously reported, several amino acids showed increased concentrations during early pregnancy when compared to term. In addition, marked differences were seen in the levels of ethanolamine (which was increased fivefold in term placentae) and phosphoethanolamine (which was decreased by almost 97 per cent of the value measured at 10 weeks gestation). The implications of these results are discussed.

Studies of water movement across the gut using oral rehydration solutions in a rat perfusion model.

AIM: To measure water influx and efflux, as well as net water, sodium and potassium absorption from a range of oral rehydration solutions (ORS) in which the glucose content had been partially replaced with the amino acid leucine or with food supplements. METHODS: A series of in vivo steady-state perfusion studies in normal rat intestine. The oral rehydration solutions contained 60 or 90 mmol/L of sodium. The reference solution used was the World Health Organization (WHO) formula. RESULTS: There was a significant negative correlation between the oral rehydration solution osmolality and net water absorption (r = -0.722, P < 0.05). The highest net water absorption occurred using comminuted chicken supplemented oral rehydration solution containing 60 mmol/L sodium (P < 0.001). This oral rehydration solution also showed a significant increase in the rate of influx of water (P < 0.05) in comparison with the WHO formula containing 60 mmol/L sodium. CONCLUSION: This work provides further evidence that food-based oral rehydration solutions, including non-vegetable sources, may have a useful role to play in the management of patients with acute diarrhoea.

Evaluation of the effects of varying solute content on the efficacy of oral rehydration solutions in a rat model of secretory diarrhoea.

A series of in vivo steady-state perfusion studies in cholera toxin-induced secreting rat intestine were carried out to investigate net water, sodium, and potassium absorption and water influx and efflux from a range of oral rehydration solutions (ORSs) in which the glucose content had been partially replaced by amino acids or food supplements and the sodium content had been reduced to 60 mM. The reference solution used was the World Health Organization formula. There was a significant correlation between the osmolality of the ORS and the net water absorption (r = -0.911; p < 0.02). The greatest net water absorption occurred using comminuted chicken- and tapioca-supplemented ORS.

Plasma amino acid profiles in preterm infants receiving Vamin 9 glucose or Vamin infant.

Amino acid profiles were measured in 29 low-birth-weight infants receiving either Vamin 9 glucose (n = 18, group A) or Vamin Infant (n = 11, group B) as the amino acid source in parenteral nutrition; intake was otherwise identical. Infants were sampled when receiving 430 mgN/kg per day (3.2 g/kg per day amino acids) and 90 non-protein kcal/kg per day. There was no difference between groups in birth weight, gestational or postnatal age. The percentage N retention was similar in both (68 and 60%, groups A and B respectively). Phenylalanine and tyrosine levels were higher in those who received Vamin 9 glucose but 55% of infants given Vamin Infant had tyrosine levels below the lower limit of the target range. Cysteine levels were low in both groups. Further modification of the amino acid composition of parenteral solutions for the newborn is necessary. If sufficient non-protein energy can be provided the risk of abnormally high amino acid levels is reduced.

Kinetics of influx of peptides and amino acids into hamster jejunum in vitro: physiological and theoretical implications.

1. This paper reports a comparison of the kinetics of influx into hamster jejunum of a series of dipeptides of neutral, basic and acidic amino acids, and a tripeptide of neutral amino acids, with those of corresponding free amino acids. 2. Kt, the substrate concentration at which the transport rate is half the maximal transport rate, and Vmax, the maximal transport rate, were more similar from one peptide to another than among amino acids, with the result that, over a wide range of concentrations, rates of influx of individual peptides varied much less than those of amino acids. 3. It is suggested that this may account for the rates of absorption of amino acids being closely related to the amino acid composition of the protein fed, instead of being widely dissimilar as with corresponding mixtures of free amino acids. 4. With neutral amino acids, both Kt and Vmax. fell with increasing length of the side-chain, as observed on many previous occasions. This did not occur with the corresponding homologous dipeptides, which shows that the hypothesis that the apparent affinity for transport is related to the lipophilic properties of the side-chain cannot be applied to peptides.

A comparison of the structural integrity of several commonly used preparations of rat small intestine in vitro.

The structural integrities of various preparations of rat small intestine for the study of absorption in vitro have been compared after incubation or perfusion. Perfused intestines removed from anaesthetized rats, and thus never deprived of a supply of oxygen, maintain their structural integrity even after perfusion for 1 h provided that a Krebs-Henseleit bicarbonate perfusate is used. However, intestines removed from freshly killed rats show severe villus disruption and oedema after perfusion for only 20 min. Extensive damage to both crypts and villi is observed in everted sacs of small intestine incubated for 20 min, regardless of the buffer system used. Intestinal rings show damage at the tips of the villi after incubation for 2 min, but otherwise remain morphologically intact; this damage is progressive with time. It is concluded that the exact mode of preparation of intestinal tissue is critical for preservation of structural and functional integrity and that this is especially important in quantitative studies on transport processes. Further, it is recommended that routine monitoring of the integrity of intestinal preparations in vitro is desirable and that histological assessment is an appropriate technique.

Effects of sodium replacement on uptake of the dipeptide glycylsarcosine by hamster jejunum in vitro.

We report observations on transport of the hydrolysis-resistant dipeptide glycylsarcosine by rings of everted hamster jejunum in vitro in the presence and absence of Na+, using several substituents for Na+: Li+, K+, Cs+, Tris, choline and mannitol. At most concentrations, mediated influx of glycylsarcosine was depressed by Li+, K+, Cs+ and Tris, though not abolished. At high concentrations, it was moderately increased by choline and mannitol. Under conditions in which the tissue could concentrate the peptide in the presence of Na+, uptake was greatly depressed by all the substituents and the ability to concentrate was abolished. The Kt of mediated influx was affected in different ways according to the substituent used. Kt was reduced by Li+ replacement of Na+ and increased by choline replacement. Vmax was greatly reduced by all metallic substituents but not by non-metallic substituents. Though the results cannot yet be satisfactorily interpreted, they suggest possible reasons for previous conflicting results and show that it is impossible to make the unqualified statement that transport of glycylsarcosine is 'Na+-dependent'. It is now doubtful whether transport of this peptide into the intestinal cells is by co-transport with Na+, and the whole matter of Na+-dependence of intestinal peptide transport is in question.

Uptake of a series of neutral dipeptides including L-alanyl-L-alanine, glycylglycine and glycylsarcosine by hamster jejunum in vitro.

This paper is the last of a set reporting an investigation of the kinetics of jejunal uptake and inhibitory ability of a series of neutral dipeptides, glycylglycine, L-ananyl-L-alanine, L-valyl-L-valine and L-leucyl-L-leucine, with progressively longer and more lipophilic side chains. The results suggested that at pH 5, uptake of L-alanyl-L-alanine, like that of L-valyl-L-valine and L-leucyl-L-leucine, was the result of two processes, uptake of intact peptide and uptake of free amino acid released extracellularly. On the other hand, uptake of glycylglycine was entirely in the form of intact peptide. In contrast to uptake of L-valyl-L-valine and L-leucyl-L-leucine, the proportion of intact L-alanyl-L-alanine taken up by mediated transport was greatest at the lowest concentration studied and smallest at the highest concentration. Taking the series of results as a whole, whereas the corresponding series of amino acids, glycine, L-alanine, L-valine and L-leucine, showed a progressive increase in apparent affinity for uptake and a decrease in Vmax, we could find no such regular progression with the peptides. The results of work on inhibition of uptake of one dipeptide by another were unexpectedly complex. Examples were the very powerful inhibitory effect of L-valyl-L-valine on uptake of glycylsarcosine, not suggested by the Kt of the former peptide, and the failure of glycylsarcosine to cause complete inhibition of uptake of L-alanyl-L-alanine and L-leucyl-L-leucine, though it could completely inhibit uptake of L-valyl-L-valine.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetics of uptake of L-leucine and glycylsarcosine into normal and protein malnourished young rat jejunum.

The impact of malnutrition on peptide and amino acid absorption has been studied in the immediate postweaning period. At this time peptide uptake is quantitatively more important than amino acid uptake and the vulnerability of the infant to malnutrition is great. Everted rings of rat jejunum were used to investigate the uptake of the peptide glycylsarcosine (Gly-Sar) and the amino acid L-leucine. The animals had been weaned on to isocaloric diets containing 18% or 4% protein. The rats deprived of protein at this age showed a marked growth disturbance with considerable reduction in gut length in addition to poor weight gain. Mediated influx of Gly-Sar and leucine per centimeter of jejunum was reduced in the malnourished animals: Vmax, 77 +/- 7.1 (SEM) and 65 +/- 3.6 compared with 85 +/- 10.6 and 77 +/- 4.4 nmol . min-1 . cm-1., respectively. But, when expressed in relation to body weight, the maximal transport capacity showed a marked increase with malnutrition, values being 126 and 111 nmol-1 . cm-1 . 100 g-1 body weight compared with 39 and 35 nmol-1 . cm-1 . 100 g-1 body weight for Gly-Sar and leucine respectively.

Trans-mucosal passage of intact peptides in the guinea-pig small intestine in vivo: a re-appraisal.

1. Re-examination of data for the chemical analysis of peptide-bound amino acids in the mesenteric venous blood of anaesthetized guinea-pigs suggests that there are small, but significant, amounts of small peptides in the blood of fasted animals. 2. Furthermore, there is a significant increase in the peptide content of the mesenteric venous blood during intraduodenal infusion of a partial digest of casein. 3. The data are consistent with the view that some 10% of the amino nitrogen entering the mesenteric blood during absorption of a casein digest in vivo may be in the form of small peptides, although it is not possible to define confidence limits for this estimate.

Use of a peptide rather than free amino acid nitrogen source in chemically defined "elemental" diets.

Previous studies have shown that amino acid (AA) residues are absorbed more rapidly from di- tripeptides than from free AA. In the present study, an intestinal perfusion technique has been used in normal human subjects to compare absorption of AA residues and total alpha-amino nitrogen (N) from 4 partial enzymic hydrolysates of protein (50--80% of the N contents present as small peptides) and their respective equimolar free AA mixtures. alpha-Amino N absorption was greater from 2 casein hydrolytes and a lactalbumin hydrolysate than from the respective free AA mixtures but similar to that from a fish protein hydrolysate and its AA mixture. The considerable variation in absorption of individual AA residues from the AA mixtures was much reduced when the protein hydrolysates were perfused, as a number of AA which were poorly absorbed from the AA mixtures were absorbed to a greater extent from the protein hydrolysates. The casein and lactalbumin hydrolysates had a stimulatory effect on jejunal absorption of water and electrolytes. In contrast, the fish protein hydrolysate appeared to cause a mean net secretion of fluid and electrolytes. The findings indicate that when absorption is limited by diminished luminal hydrolysis or absorptive capacity, serious consideration might be given to using partial enzymic hydrolysates of whole protein rather than free AA mixtures as the N source in "elemental" diets. Care should be taken, however, in ensuring that the preparation of choice does not promote a net secretion of fluid and electrolytes for such a property could have a deleterious effect in the clinical setting.

Kinetics of uptake of lysine and lysyl-lysine by hamster jejunum in vitro.

1. The kinetics of 2-min uptake of L-lysine and L-lysyl-L-lysine have been studied by using rings of everted hamster intestine in vitro, and values for Kt and Vmax. established. 2. On a molar basis, mediated uptake was more rapid for the amino acid than for the peptide. Non-mediated uptake was more rapid for the peptide than for the amino acid. 3. Comparison of relative rates of uptake of lysine from equivalent solutions of lysine and lysyl-lysine showed that at low concentrations, uptake of lysine was less rapid from the peptide than from the amino acid, whereas at high concentrations, uptake of lysine was more rapid from the peptide than from the amino acid. This type of effect of concentration on relative rates of uptake from equivalent solutions of amino acid and peptide has not previously been described.