Molecular mechanisms of the effect of ultrasound on the fibrinolysis of clots

Our objective was to identify mechanisms for the ultrasound-enhanced acceleration of tissue-type plasminogen activator (t-PA)-induced fibrinolysis of clots. Measurements of turbidity as a function of time, used to characterize quantitatively the effects of ultrasound, showed that the ultrasound pulse-repetition frequency affected clot lysis times, but there were no thermal effects. Ultrasound in the absence of t-PA produced a slight but consistent decrease in turbidity, suggesting a decrease in fibrin diameter due solely to the action of ultrasound, likely caused by an increase in protofibril tension because of vibration from ultrasound. Changes in fibrin network structure during lysis with ultrasound were visualized in real time by deconvolution microscopy, revealing that the network becomes unstable when 30–40% of the network was digested, whereas without ultrasound, the fibrin network was digested gradually and retained structural integrity. Fluorescence recovery after photobleaching during lysis was used to characterize the kinetics of binding/unbinding and transport. Ultrasound causes a decrease in the diameter of fibers due to tension as a result of vibration, leading to increased binding sites for plasmin(ogen)/t-PA. The positive feedback of this structural change together with increased mixing/transport of t-PA/plasmin(ogen) is likely to account for the observed enhancement of fibrinolysis by ultrasound.