Flow mediated dilation response to oscillatory vs. steady shear: evidence for the transduction of the mean shear stimulus

Abstract

OBJECTIVE: 1) To compare the flow mediated dilation (FMD) response to an increase in brachial artery (BA) shear stress (SS) via forearm heating (FH) vs. forearm exercise (FE). 2) To isolate the effects of the oscillatory pattern of exercise SS. BA diameter (BAD) and mean blood velocity (MBV) were measured with ultrasound in 16 healthy subjects. BA MBV was elevated to the same mean level for 10min via 3 protocols. 1) FHstdy ‐ A steady MBV was created by controlled release of BA compression upstream of heat‐induced forearm vasodilation. 2) FHosc: rhythmic cuff inflation simulated exercise induced oscillations in MBV. 3) FE ‐ Rhythmic isometric handgrip exercise.RESULTS: The mean increase in shear rate (MBV/BAD) (±SD) was the same in all conditions (FHstdy: 52.2±14.1s−1; FHosc: 51.7±15.7s−1; FE: 50.1±13.0s−1 P=0.13). Neither the percent change in BAD (End‐trial: FHstdy: 7.0±3.0%; FHosc: 7.8±3.4%; FE: 6.7±3.0% P=0.20) nor the initial response speed (tau‐time to 63% of max response) (FHstdy:27.8±14.4s; FHosc: 22.6±16.7s; FE:25.9±23.3s P=0.69) were different between conditions.CONCLUSION: The FMD was determined by the mean shear independent of stimulus pattern (oscillatory vs. steady) and mode of increase (heating vs. exercise). These data suggest that the endothelium transduces the mean shear stimulus and indicate that exercise may provide a viable technique to investigate human FMD.Funded by NSERC, ACSM and HSFC