1. Protein loss in muscle can be caused by decreased protein synthesis, increased breakdown or both. In small animals the tracer incorporation technique is mostly used to measure protein synthesis, but for degradation measurements in vitro or ex vivo settings are required. In human and large animal studies the arteriovenous dilution technique is used because it enables the measurement of synthesis and breakdown rates simultaneously. The applicability in small animals has not yet been proven. We used a starvation model to compare both techniques. 2. A primed constant infusion of l -[2,6- 3 H]phenylalanine was given to male Lewis rats after 16, 40, 64 and 112 h starvation. Protein synthesis rates of the gastrocnemius muscle were measured by the incorporation technique and compared with hindquarter protein turnover calculated in a two- and three-compartment arteriovenous dilution model. 3. Whole-body phenylalanine rate of appearance decreased from 456 ± 32 after 16 h to 334 ± 34 (nmol min −1 100 g −1 body weight) after 112 h starvation. Protein synthesis rates of the gastrocnemius muscle measured by the tracer incorporation technique decreased from 3.6 ± 0.4 after 16 h starvation to 2.2 ± 0.3 after 64 h starvation and 1.8 ± 0.4 (%/day) after 112h starvation. Hindquarter protein breakdown, calculated with the tracer dilution model, increased after 112 h starvation from 28 ± 12 to 77 ± 15 nmol min −1 100 g −1 body weight. Using the tracer dilution model, however, the calculated protein synthesis rate across the hindquarter also increased after prolonged starvation (29 ± 7 and 68 ± 16 nmol min −1 100 g −1 body weight after 16 and 112h respectively). In conjunction with this, calculated bidirectional membrane transport rates were also enhanced. Using valine and glutamine as tracers, the enhanced amino acid turnover rates were confirmed. 4. In conclusion, our results show that during short periods of starvation both methods give similar results. After prolonged starvation, however, an opposite change in disappearance rate and protein synthesis rate was observed. Assumptions made to calculate protein turnover using the arteriovenous dilution model may account for the discrepancy and care must be taken with the interpretation when using only one model in anaesthetized small animals.

1. Sodium pump function has been assessed by measurement of ouabain-sensitive 86 Rb uptake in human erythrocytes after incorporation of palmitic, stearic, oleic and linoleic acids into the erythrocyte membrane. 14 C-labelled fatty acids were used to measure membrane uptake of these substances. 2. For palmitic, oleic and linoleic acids, up to 1000 nmol of the fatty acid/ml of packed cells can be incorporated without causing significant haemolysis. For stearic acid, 270 nmol/ml of packed cells was incorporated in similar conditions. More than 88% of the fatty acid incorporated could be extracted with a 50 μmol/l fatty-acid-free albumin solution and was, therefore, in a non-esterified form in the erythrocyte membrane. The concentrations of palmitic, stearic, oleic and linoleic acids incorporated in these experiments represent a five- to ten-fold increase above the normal concentrations of these fatty acids in the membrane. 3. Up to 1000 nmol of palmitic, oleic and linoleic acids/ ml of packed cells and up to 270 nmol of stearic acid/ml of packed cells could be incorporated without a significant change in mean ouabain-sensitive 86 Rb uptake with respect to control cells. Mean percentage changes in ouabain-sensitive 86 Rb uptake for all these experiments were: palmitic acid, 3.7% ( sd 11.4, n = 15); stearic acid, 4.0% ( sd 5.7, n = 7); oleic acid, −4.8% ( sd 19, n = 17); linoleic acid, 2.2% ( sd 15.6, n = 19). 4. The demonstration of near-normal sodium pump activity in the presence of greatly elevated membrane levels of these fatty acids makes it extremely unlikely that they act as modulators of sodium pump function in vivo.

1. Erythrocyte choline transport has been studied in nine patients on maintenance haemodialysis for chronic renal failure, six patients on continuous ambulatory peritoneal dialysis, 31 patients with renal transplants and in nine normal control subjects. 2. The mean maximum rate of choline influx ( V max. , measured at an extracellular choline concentration of 250 μmol/l) was 66.7 ( sd 14.1) μmol h −1 l −1 cells in patients on haemodialysis, 87.8 ( sd 18.5) μmol h −1 l −1 cells in patients on continuous ambulatory peritoneal dialysis and 30.5 ( sd 4.9) μmol h −1 l −1 cells in control subjects. The increase in choline flux in patients on haemodialysis and patients on continuous ambulatory peritoneal dialysis compared with control subjects was highly significant ( P < 0.001). 3. Renal transplant patients showed variable values for the V max. of choline influx (range 17.7-71.7 μmol h −1 l −1 cells). The values showed a signifcant negative correlation with creatinine clearance and this correlation correctly extrapolated to the maximum choline flux in normal subjects and in patients on dialysis. 4. The kinetics of choline transport have been studied in erythrocytes of patients on haemodialysis and control subjects in ‘zero-trans’ conditions after depletion of intracellular choline. The mean V max. in these conditions was 38.4 ( sd 4.6) μmol h −1 l −1 cells in patients on haemodialysis compared with 14.2 ( sd 3.7) μmol h −1 l −1 cells in control subjects. The mean K m under ‘zero-trans’ conditions was 19.4 ( sd 2.4) μmol/l in patients on haemodialysis and 7.4 ( sd 1.4) μmol/l in control subjects. These differences were significant ( P < 0.001).

1. The initial rate of l -lysine influx into erythrocytes from 13 patients with chronic renal failure has been measured using 14 C-labelled lysine. Ten patients were on maintenance haemodialysis and three had never been dialysed. The results are compared with data obtained from 12 normal individuals. 2. The rate of lysine influx into washed cells from buffered saline containing 0.02–0.5 mmol of l -lysine/l has been calculated. The results can be fitted with a model in which influx has a single saturable component obeying Michaelis–Menten kinetics, and a linear non-saturable component. 3. In uraemic erythrocytes the saturable component had a mean V max. of 0.762 mmol h −1 litre −1 of cells ( n = 13, sem 0.072) and a mean K m of 68.2 μmol/l ( sem 5.7). These values in normal erythrocytes were 0.566 mmol h −1 litre −1 of cells ( n = 12, sem 0.033) and 70.5 μmol/l ( sem 4.1), respectively. The mean apparent diffusion constant ( K D ) for the linear component of influx was 0.224 h −1 ( sem 0.039) in uraemic cells and 0.178 h −1 ( sem 0.028) in normals. 4. The 35% increase in mean V max. seen in uraemic erythrocytes was statistically significant ( P = 0.02). A similar increase in V max. in uraemic cells compared with controls was seen in erythrocytes which were studied in zero-trans conditions after depletion of intracellular amino acids. The mean values of K m and K D were not significantly different in uraemia. The origins of this increased membrane transport capacity for lysine in uraemia are discussed.

1. The aim of this study was to compare certain renal and erythrocyte functions in the Milan hypertensive strain (MHS) of rats and in normotensive control animals (MNS). 2. Under experimental conditions close to those in conscious unmanipulated animals, the glomerular filtration rate (GFR) per g kidney weight was higher and the kidney weight/body weight ratio lower in MHS than in MNS rats ( P < 0.001). This suggests that there is increased transtubular ion transport in MHS animals. 3. The MHS rats had a higher rate constant for outward Na + -K + cotransport than the MNS animals ( P < 0.02), their intra-erythrocyte sodium concentrations were lower ( P < 0.05) and their erythrocyte volume was smaller ( P < 0.001). Therefore, accelerated cell membrane sodium outward cotransport seems to reset the volume and the sodium content of the erythrocytes of MHS rats at a lower level. 4. We speculate that increased transport of ions across the cell membrane may be an abnormality common to erythrocytes and renal tubular cells in MHS rats.