The impact of menstrual phase on lower limb microvascular function, ERα, eNOS and p-eNOS protein in premenopausal females

There is variability in the impact of menstrual phase on microvascular function with some studies reporting an increase from the early follicular (EF) to late follicular (LF) phase. Estradiol (E2) may increase nitric oxide bioavailability and thereby microvascular function through increasing estrogen receptor alpha (ERα), endothelial nitric oxide synthase (eNOS), and phosphorylated eNOS (p-eNOS) protein. It is unknown whether variability in ERα, eNOS, and p-eNOS protein levels drives menstrual cycle-related changes in microvascular endothelial function. We hypothesized that microvascular function would be positively related to ERα, eNOS, and p-eNOS protein across the menstrual cycle. Premenopausal females (21±3 years) completed two visits (EF and LF phase) to assess leg microvascular function (n=23) and protein levels (n=17). Microvascular function was quantified by passive leg movement hyperemia (leg blood flow area under the curve (LBF AUC) and change to peak (LBF Δpeak)). eNOS, p-eNOS, and ERα content was quantified from quadricep muscle biopsies. E2 increased from the EF to LF phase (P=0.002). There were no phase differences in LBF AUC (P=0.252) and LBF Δpeak (P=0.477), or eNOS (P=0.722), p-eNOS (P=0.079), and ERα (P=0.182) protein assessed via immunoblotting, or eNOS (P=0.610) and p-eNOS (P=0.510) assessed via immunofluorescence. E2, eNOS and p-eNOS protein were positively related to microvascular function (P<0.05). This study does not support a group-level role of the EF to LF menstrual phase transition in influencing leg microvascular function or ERα, eNOS, and p-eNOS protein. Rather, it highlights that individual quantification of E2 and eNOS protein may be more indicative of microvascular function.