1. Sodium pump function has been assessed by measurement of ouabain-sensitive 86Rb uptake in human erythrocytes after incorporation of palmitic, stearic, oleic and linoleic acids into the erythrocyte membrane. 14C-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 86Rb uptake with respect to control cells. Mean percentage changes in ouabain-sensitive 86Rb 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.