Serotonin uptake and configurational change of bovine pulmonary artery smooth muscle cells in culture

Although it is well known that endothelial cells transport serotonin (5‐HT) from extracellular to intracellular locations, it has been generally assumed that smooth muscle cells do not accumulate 5‐HT but, rather, respond to 5‐HT through a receptor activity unrelated to uptake of this amine or via stimulation of endothelial‐derived relaxing factor. In the present study smooth muscle cells (PASMC), isolated and cultured from bovine pulmonary artery, were evaluated for 5‐HT uptake under a variety of conditions. 5‐HT uptake was linear up to 15 min and the rate was seven‐ to eightfold higher than that by bovine pulmonary artery endothelial cells. There was intracellular metabolism of 5‐HT to 5‐hydroxy‐indoleacetic acid (5‐HIAA). The uptake was inhibited by exposure to 4°C, absence of Na + from the medium, and agents such as imipramine, verapamil, ketanserin, and methiothepin. Like that of endothelial cells, 5‐HT uptake by PASMC was stimulated by exposure of cells to anoxia for 24 hr. Unlike endothelial cells that showed no morphological changes, PASMC at early passage showed dendritic formation after 30–60 min exposure to 5‐HT at a concentration as low as 10 −8 M. Although this configurational change in response to 5‐HT was lost with passage of cells, transport of 5‐HT by these cells was retained. The configurational change was blocked by agents that inhibited 5‐HT uptake, such as imipramine, verapamil, ketanserin, and methiothepin; it was unaffected by inhibitors of protein kinase C, phospholipase C, and calmodulin or absence of Ca 2+ from the medium. We conclude that PASMC, as well as endothelial cells, accumulate 5‐HT; there appears to be a close relationship between 5‐HT uptake and configurational change of early passaged PASMC in culture. The factor(s) required for the configurational change are absent in endothelial cells and lost during passage of PASMC.