Serotonin increases DNA synthesis in rat proximal and distal pulmonary vascular smooth muscle cells in culture

Cultured smooth muscle cells obtained from rat lung periphery (RPC) and proximal pulmonary artery (RSMC) expressed mRNA for serotonin (5-HT) type 2 receptor (5-HT₂) and 5-HT transporter (by Northern blot analysis). Functional expression of these genes was evident since both cell types 1) bound ¹²⁵I-labeled lysergic acid diethylamide (LSD; 5-HT₂ receptor antagonist) that was equally effectively displaced by either ketanserin or mianserin; and 2) transported 5-[³H]HT in an imipramine-sensitive manner. Serotonin (10^-9-10^-5 M) stimulated DNA synthesis (as measured by [³H]thymidine uptake) in RPC and RSMC. The 5-HT-induced increase in DNA synthesis was significantly inhibited in both cell types by the 5-HT₂ receptor antagonist, ketanserin (10^-7-10^-6 M), and by fluoxetine (10^-6 M), a putative 5-HT transport inhibitor. Acute exposure to 5-HT (1-100 μM) caused an abrupt rise in intracellular calcium ([Ca²⁺](i)) in single pulmonary vascular smooth muscle cells as microspectrofluorometrically determined using the calcium-sensitive dye, fura 2. The 5-HT-induced change in [Ca²⁺](i) was completely abolished in the presence of 10^-6 M ketanserin as well as imipramine or fluoxetine (10^-6 M). The calcium transients due to 5-HT persisted in a Na⁺-free condition (in which the transporter activity was completely abolished) and imipramine and fluoxetine (and ketanserin) were effective inhibitors of 5-HT under these conditions. Therefore, the 5-HT₂ receptor, but not the transporter, is responsible for initiating the acute effects (e.g., calcium transients) of 5-HT in cultured rat pulmonary vascular smooth muscle cells and fluoxetine (1 μM) may have 5- HT₂-receptor antagonist properties. These molecular and pharmacologic data are consistent with the presence of both a 5-HT₂ receptor and transporter in cultured rat pulmonary vascular smooth muscle cells and suggest that 5-HT is a potential intrapulmonary growth factor.