Effect of metformin and phenformin on vascular smooth muscle cell proliferation assessed by thymidine incorporation and cell counting. A. Human vSMCs were treated with metformin (10–100 μM) and phenformin (10–100 μM) in the presence of PDGF (50 ng/ml) and assessed for [3H]-thymidine incorporation. Data represent the mean ± SEM, *P < 0.05 vs PDGF. B. Human vSMCs were treated with metformin (10–300 μM) and phenformin (10–300 μM) in the presence of 5% serum for 3 days and then counted on a Coulter counter. Data represent the mean ± SEM from 2 experiments in triplicate **P < 0.01, ***P < 0.001 vs the 5% FBS.