1. Human erythrocyte membrane cholesterol, fluidity and basal and calmodulin-stimulated calcium pump (Ca2+-Mg2+-ATPase) activities were compared in 24 patients with primary combined hyperlipidaemia and 20 age-matched normolipidaemic control subjects.
2. There was no correlation between serum and membrane cholesterol. Despite the differences in serum cholesterol levels between the two groups, membrane cholesterol levels were similar.
3. 1,6-Diphenyl-1,3,5-hexatriene anisotropy was lower in the hyperlipidaemic group, suggesting increased fluidity in the hydrocarbon core of the phospholipid membrane bilayer.
4. Basal calcium pump activity was lower in the hyperlipidaemic group with increased membrane fluidity.
5. These results suggest that membrane adaptive mechanisms can maintain membrane cholesterol within a narrow range, that serum triacylglycerol is more important than serum cholesterol in determining membrane fluidity and that increased membrane fluidity reduces basal calcium pump activity.