Serotonin uptake by bovine pulmonary artery endothelial cells in culture. I. Characterization

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Abstract
Serotonin (5-HT) uptake by bovine pulmonary artery endothelial cells in culture was measured at 5-HT concentrations of 1.5 .times. 10-8 to 10-3 M. The uptake increased linearly at concentrations of 10-5 to 10-3 M 5-HT, and the slope for uptake was 1.276 .times. 106 pmol 5-HT .cntdot. 106 cells-1 .cntdot. 30 min-1 .cntdot. M concentration-1. The slope for uptake at 5-HT concentrations < 10-7 M was 7.10 .times. 106 pmol .cntdot. 106 cells-1 .cntdot. 30 min-1 .cntdot. M concentration-1. With subtraction of the slope for linear uptake obtained at high concentrations of 5-HT from total uptake of 5-HT, a curve showing saturable uptake at concentrations > 10-6 to 10-5 M was obtained. Lineweaver-Burk analysis of the data yielded a saturated value of high-affinity transport of 6.25 pmol .cntdot. 106 cells-1 .cntdot. 30 min-1 and a Km of 10-6 M. At 5-HT concentrations of 1.5 .times. 10-8 to 5 .times. 10-7 M, uptake was inhibited 75-80% by exposure to 4.degree. C or by absence of sodium in the medium; absence of sodium did not affect uptake at 5-HT concentrations > 10-5 M, and 4.degree.C was less effective in inhibiting uptake at high 5-HT concentrations. Imipramine markedly inhibited uptake at low, but not at high, 5-HT concentrations. Ouabain only partially inhibited 5-HT uptake, but inhibition ws more pronounced at low 5-HT concentrations. The data are compatible with predominance of active transport of 5-HT by bovine pulmonary artery endothelial cells at low 5-HT concentrations and predominance of accumulation of 5-HT by diffusion as 5-HT concentration is elevated above 10-5 M.