The impact of menstrual phase on brachial artery flow‐mediated dilatation during handgrip exercise in healthy premenopausal women

New Findings What is the central question of this study? The aim of this study was to determine the influence of menstrual phase on flow‐mediated dilatation in response to sustained, exercise‐induced increases in shear stress. What is the main finding and its importance? We showed, for the first time, that in healthy, premenopausal women the flow‐mediated dilatation stimulated by exercise‐induced increases in shear stress did not fluctuate across two phases of the menstrual cycle, despite significant fluctuations in oestrogen. This suggests that endothelial function is not consistently augmented in the high‐oestrogen phase. Flow‐mediated dilatation (FMD) in response to a sustained shear‐stress stimulus (e.g. via handgrip exercise; HGEX) is emerging as a useful tool for assessing endothelial function; however, the impact of menstrual phase on HGEX‐FMD is unknown. The purpose of this study was to determine whether HGEX‐FMD fluctuates with cyclical changes in oestrogen concentrations over two discrete phases (low and high oestrogen) of the menstrual cycle. Brachial artery (BA) diameter and blood velocity were assessed with two‐dimesional and Doppler ultrasound, respectively. Shear stress was estimated using shear rate (SR = BA blood velocity/BA diameter). Participants (12 healthy, regularly cycling women, 21 ± 2 years of age) completed two experimental visits: (i) low oestrogen (early follicular, EF); and (ii) high oestrogen (late follicular, LF). Reactive hyperaemia‐stimulated FMD (RH‐FMD) and HGEX‐FMD (6 min of handgrip exercise) were assessed during each visit. Results are mean values ± SD. Oestrogen increased from the EF to LF phase (EF, 33 ± 9 pg ml−1; LF, 161 ± 113 pg ml−1, P = 0.003). However, neither the SR stimuli (HGEX, P = 0.501; RH, P = 0.173) nor the FMD responses differed between phases (EF versus LF: HGEX‐FMD, 4.8 ± 2.8 versus 4.6 ± 2.2%, P = 0.601; RH‐FMD, 7.9 ± 4.3 versus 6.4 ± 3.1%, P = 0.071). These results extend existing RH‐FMD findings indicating that not all women experience fluctuations in FMD with the menstrual cycle. Further research is needed to investigate the mechanisms that underlie variability in the impact of menstrual phase on FMD.