Increased human erythrocyte sodium-lithium countertransport in hyperlipidaemic patients may indicate increased membrane lipid fluidity.

Erythrocyte sodium-lithium countertransport (SLC) activity, membrane fluidity, plasma triglyceride and cholesterol were measured in hyperlipidaemic patients and normal subjects. Fluidity was assessed by the fluorescence anisotropy (inversely related to fluidity) of the probes 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1,4-trimethylammonium-3,5-hexatriene (TMA-DPH). In a second group of patients the maximum velocity (Vmax) and external sodium affinity constant (km) of SLC was also measured. In the first group of patients, SLC activity was increased compared with the controls (0.279 +/- 0.019 vs. 0.213 +/- 0.013, P = 0.006) as was membrane fluidity in the deep hydrophobic regions (DPH anisotropy 0.211 +/- 0.0007 vs. 0.215 +/- 0.0011, P = 0.007). There was a strong correlation between SLC and DPH anisotropy (Rs = -0.72, P = < 0.001) which was due to the correlation between Vmax and DPH anisotropy (Rs = -0.90, P = < 0.001). Increases in Vmax of SLC in hyperlipidaemic patients may be due to differences in lipid organisation in the deep hydrophobic regions of the membrane which may affect the turnover rate of the transporter.