Effect of plasma membrane fluidity on serotonin transport by endothelial cells.

To evaluate the effect of plasma membrane fluidity of lung endothelial cells on serotonin transport, porcine pulmonary artery endothelial cells were incubated for 3 h with either 0.1 mM cholesterol hemisuccinate, 0.1 mM cis-vaccenic acid, or vehicle (control), after which plasma membrane fluidity and serotonin transport were measured. Fluorescence spectroscopy was used to measure fluidity in the plasma membrane. Serotonin uptake was calculated from the disappearance of [14C]-serotonin from the culture medium. Cholesterol decreased fluidity in the subpolar head group and central and midacyl side-chain regions of the plasma membrane and decreased serotonin transport, whereas cis-vaccenic acid increased fluidity in the central and midacyl side-chain regions of the plasma membrane and also increased serotonin transport. Cis-vaccenic acid had no effect on fluidity in the subpolar head group region of the plasma membrane. These results provide evidence that the physical state of the central and midacyl chains within the pulmonary artery endothelial cell plasma membrane lipid bilayer modulates transmembrane transport of serotonin by these cells.