Ingestion of the anorexigenic drug dexfenfluramine (Dfen) is a risk factor for development of pulmonary arterial hypertension (PAH). We have previously shown that Dfen induces PAH in wildtype (WT) mice whilst protecting against the exaggerated hypoxia-induced PAH observed in mice over-expressing the 5HT transporter (5HTT+ mice)^1,2. In the present study we investigated the effects of Dfen on proliferation of pulmonary arterial fibroblasts (PAFs) from WT and 5HTT+ mice. Mice (C57BL/6, 5-6 months, 25-35g, n=4) were euthanised using sodium pentobarbitone (2g/kg), pulmonary arteries dissected out, and PAFs cultured as described previously³. PAFs were grown to 60% confluency, quiesced for 24 hours, and exposed to normoxia or hypoxia (5%O₂) in the presence of Dfen (0.3-10μM) for a further 24 hours. Proliferation was measured by [3H]thymidine incorporation. The effects of Dfen (3μM) were also assessed in the presence or absence of 5HT_2A receptor antagonist ketanserin (30nM), 5HT_2B receptor antagonist SB204741 (300nM) or 5HTT inhibitor citalopram (100nM). Values shown are the mean±s.e.m. for 4 replicate plates from the same experiment. Experiments were repeated at least in triplicate and the results shown typical of those obtained. Statistical comparisons were made by one-way ANOVA with a Dunnetts multiple comparison test. Dfen had no effect on proliferation of WT PAFs under normoxia (disintegrations per minute (DPMs): 0.3μM 31.9±6.5, 1μM 42.6±10.0, 3μM 35.7±9.8, 10μM 24.6±2.7, P>0.05 cf. control: 26.9±3.4) or hypoxia (DPMs: 0.3μM 32.3±6.1, 1μM: 37.8±7.9, 3μM: 46.8±7.7, 10μM 38.3±2.7 DPM, P>0.05 cf. control: 40.4±7.6). Neither did Dfen have any effect on proliferation of 5HTT+ PAFs under normoxic conditions (DPMs: 0.3μM 28.0±4.0, 1μM: 22.6±6.0, 3μM: 22.7±5.2, 10μM 22.5±3.8 DPM, P>0.05 cf. control: 27.1±1.2). However, hypoxic exposure led to proliferation of PAFs from 5HTT+ mice (713.5±23.6 DPM cf 81.6±16.8 DPM in normoxia, P<0.01), an effect which was inhibited by Dfen (3μM; 71.4±22.6DPM, P<0.01). The hypoxic response was restored when PAFs were pre-incubated with citalopram before addition of Dfen (806.5±93.2 DPM, P0.05). In conclusion, Dfen has no effect on proliferation of normoxic cells from WT or 5HTT+ mice, or on hypoxic WT cells. However, Dfen inhibits hypoxic-induced proliferation of PAFs from 5HTT+ mice through a 5HTT mediated mechanism. This may explain why Dfen can protect against the exaggerated hypoxia-induced PAH observed in 5HTT+ mice.