BACKGROUND: Older females (OF) have greater cardiovascular risk and blood pressure (BP) reactivity compared to younger females (YF). In cohorts of males and females, aging augments BP reactivity and reduces sympathetic reactivity to chemoreflex stimulation. However, there is limited data on how aging affects BP reactivity to chemoreflex stimulation in females. Therefore, we tested the hypothesis that OF vs. YF would exhibit greater BP reactivity and a lower sympathetic reactivity during an end-expiratory breath-hold (EEBH; chemoreflex stimulus). METHODS: We measured beat-to-beat BP and Modelflow-derived hemodynamics (photoplethysmography; 13 YF, 8 OF) during a two-minute rest and maximal voluntary duration EEBH. In a subset, we also measured muscle sympathetic nerve activity (MSNA via microneurography; 9 YF, 5 OF). We tested YF during the early follicular phase and OF were post-menopausal. We compared variables at rest and in response to the EEBH between groups using unpaired, two-tailed t-tests for normally distributed data and Mann-Whitney U tests for non-normally distributed data. All data are presented as OF vs. YF with mean±SD or median[IQR]. RESULTS: Age (OF 66±6 vs. YF 23±2 years, p<0.001), but not BMI (OF 21.8[3.7] vs. YF 22.6[3.6] kg/m2, p=0.491), was different between groups. OF had a longer EEBH duration (OF 46[28] vs. YF 29[9] s, p<0.001). At rest, mean BP (OF 99±13 vs. YF 81±4 mmHg, p<0.001), MSNA burst frequency (OF 32±9 vs. YF 9±7 bursts/min, p<0.001), and MSNA total activity (OF 802±296 vs. YF 90±121 AU, p<0.001) were higher in OF. The change in MAP (OF 27[10] vs. YF 15[13] mmHg, p=0.045) was larger in OF whereas the relative change in MSNA total activity (OF 369±182 vs. YF 1014±620 %, p=0.014) was smaller in OF during the EEBH. However, the increases in MSNA burst frequency (OF 15±7 vs. YF 16±9 bursts/min, p=0.833) during the EEBH were not different between groups. Additionally, the change in cardiac output was lower in OF (OF -0.3±0.5 vs. YF 0.2±0.3 L/min, p=0.005) but the change in systemic vascular resistance was greater in OF (OF 9.4±2.8 vs. YF 3.2±1.9 mmHg/L/min, p<0.001). CONCLUSION: The smaller relative increases in MSNA total activity during EEBH in OF suggest reduced chemoreflex sensitivity in female aging. Interestingly, these preliminary data suggest that augmented BP reactivity in OF was driven by vascular resistance despite smaller increases in MSNA during EEBH.

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