(A) Popliteal LVs, trimmed to contain a single valve, were cannulated with pipettes to allow control of upstream (P_in) and downstream (P_out) pressure. Assays were conducted in calcium-free medium to prevent vessel contraction. P_in was held constant, and P_out was increased until valve closure, assessed by a drop in upstream pressure as measured with the use of a servo-null micropipette (Psn) and by a decrease in vessel diameter. The position of the diameter-tracking window is shown. The amount of adverse pressure (P_out− P_in) required for valve closure over a range of P_in and initial vessel diameters was determined. (B) Examples of traces from valve-closure tests performed with LVs from littermate Rasa1^fl/fl and Rasa1^fl/fl Ub^ert2cre mice administered tamoxifen 9 weeks previously. For control Rasa1^fl/fl LVs, the point of valve closure is indicated with an asterisk. Adverse pressure required for valve closure is represented by the difference between the dotted and P_in lines. Note that increased amounts of adverse pressure are required for valve closure at increased P_in and initial vessel diameter. Note also apparent failed valve closure in the Rasa1^fl/fl Ub^ert2cre LVs up to maximum tested adverse pressures of 30 cm H₂O at either tested P_in. (C) Plots of adverse pressure required for valve closure at different vessel diameters (D) represented as D/D_max, where D_max is defined as vessel diameter at a maximum tested P_in of 10 cm H₂O. Individual valves from littermate Rasa1^fl/fl (n = 4) and Rasa1^fl/fl Ub^ert2cre (n = 5) mice treated with tamoxifen 9 weeks previously are represented by the same colored symbols. (D) Plots of adverse pressure required for valve closure over a range of D/D_max of LVs from littermate Rasa1^fl/fl (n = 6) and Rasa1^fl/fl Ub^ert2cre (n = 6) mice treated with tamoxifen 1 week previously.