NO-stimulated VSMC contraction is blocked in the presence of exogenous and endogenous TSP1 and S1P. Type I collagen gels (3 mg/mL) were prepared and seeded with either HAVSMCs (A-B) (50 000 cells in 75 μL gel/well) or VSMCs harvested from aortic segments from WT or TSP1-null mice (C-D) (75 000 cells in 75 μL gel/well) and divided into aliquots in 96-well plates (Nunc, Denmark) and incubated overnight. Wells treated with TSP1 were preincubated overnight with 2.2 nM TSP1. Following release of the gels, contraction was initiated with either 10% FCS or 100 nM S1P ± 10 μM DETA/NO, and contraction was determined. *P < .05 versus FCS + NO, #P < .05 versus S1P + NO, &P < .05 versus TSP ± S1P, Student t test. Results are presented as the mean ± SD.