Abstract 139

Excessive proliferation of vascular smooth muscle cells (VSMC) can lead to serious consequences, in patients with hypertensive diseases or those following vascular surgery and heart transplantation. VSMC constitute a useful in vitro model for testing drugs that could control VSMC hyperplasia/hypertrophy. In this study, we evaluated the effects of trapidil, pentoxifylline and magnesium sulfate on VSMC growth in vitro following stimulation by phorbol myristate acetate (PMA) or platelet-derived growth factor-A (PDGF-AA), which are both potent promoters of VSMC growth. VSMC from human aortic tissue were grown in F12K medium with 2% fetal bovine serum to near confluence in 96-well plates. Cells were pre-incubated for 20 hours with or without the inhibitor trapidil, pentoxifylline or magnesium sulfate and were then stimulated by the addition of PDGF-AA (8 ng/ml) or by PMA (10-8 M). Twenty-four to 48 hours later, 3H-thymidine was incorporated into the cells during a pulse of 15 hours. The cell cultures were assayed in quadruplicate. RT-PCR was used to determine the effect of trapidil on the expression of PDGF-A mRNA. The response of VSMC to PDGF-AA was inhibited by trapidil at 250 to 1000 µg/ml and by pentoxifylline at 250 to 1000 µM in a dose-dependent manner. Magnesium sulfate from 1.6 to 4.8 mM inhibited the VSMC response to PMA, also in a dose-dependent manner. The transcription of PDGF-A mRNA was not affected by trapidil. Protein kinase C activation, common to both PDGF-AA and PMA induction of cell growth, normally results in extracellular calcium influx. Magnesium sulfate probably inhibits cell growth/proliferation by its known inhibition of extracellular calcium uptake. Although trapidil did not affect gene transcription for PDGF-A, it inhibited the activity of PDGF-AA added to the cell culture medium, either by inhibiting the binding of PDGF to its receptor (Gesualdo et al., 1994, Kidney Int 46:1002-9) or by activation of cellular protein kinase A (Bonisch et al., 1998, Mol Pharmacol 54:241-8). Similarly, trapidil would also prevent the autocrine/paracrine stimulation of VSMC by PDGF-AA. Pentoxifylline, which inhibits phosphodiesterase activity and results in increased intracellular cAMP, also effectively reduced VSMC proliferation. All of these drugs inhibit growth/proliferation of VSMC in vitro. Although the mechanisms of their activity are very different, their ultimate effects are similar. These drugs might be very useful in the treatment of many types of cardiovascular problems, by controlling VSMC proliferation.