Human erythrocyte choline uptake in uraemia: the role of intracellular substrate and an investigation into the effects of haemodialysis.

1. Erythrocyte choline transport was studied in 10 haemodialysis patients immediately before and after a haemodialysis session and in 10 control subjects. Choline uptake was measured in erythrocytes from normal and uraemic patients after washing in vitro and subsequent incubation in autologous plasma. Amines present in uraemic plasma were examined for their effect on choline transport in normal erythrocytes. 2. NMR spectroscopy was used to measure choline, trimethylamine and dimethylamine in erythrocyte extracts from nine control subjects, 32 subjects with renal impairment and nine samples from haemodialysis patients. 3. The increased choline influx in uraemic erythrocytes is significantly decreased by prior haemodialysis (mean Vmax pre-dialysis 146 +/- 20 mumol h-1 l-1, post-dialysis 113 +/- 13 mumol h-1 l-1 (P < 0.005). After in vitro washing there is a fall in Vmax, and no longer any significant difference between pre- and post-dialysis samples. There remains a significant difference in the erythrocyte choline Vmax between samples from patients with chronic renal failure and from normal subjects (P < 0.005). 4. Human plasma was found to contain factors capable of increasing choline uptake. Trimethylamine and dimethylamine were found to inhibit choline uptake. Trimethylamine and trimethylamine-N-oxide trans-stimulated choline efflux, but the major transport substrate present in erythrocyte extracts from all groups was choline, which was higher in those with renal impairment (71 +/- 10 mumol/l) than in haemodialysis patients (47 +/- 10 mumol/l) and control subjects with normal renal function (40 +/- 9 mumol/l). 5. Our data suggest that erythrocyte choline transport is increased in uraemia as a consequence of increased transporter number or activity, rather than the presence of intracellular substrate.