Aging augments renal vasoconstrictor response to 1 orthostatic stress in humans 2 3 4 5 Christine

29 The ability of the human body to maintain arterial blood pressure (BP) during orthostatic 30 stress is determined by several reflex neural mechanisms. Renal vasoconstriction 31 progressively increases during graded elevations in lower body negative pressure (LBNP). 32 This sympathetically mediated response redistributes blood flow to the systemic circulation 33 to maintain BP. However, how healthy aging affects the renal vasoconstrictor response to 34 LBNP is unknown. Therefore, ten young (25 ± 1 years; mean ± SE) and ten older (66 ± 2 35 years) subjects underwent graded LBNP (-15 and -30 mmHg) while beat-to-beat renal 36 blood flow velocity (RBFV; Doppler ultrasound), arterial blood pressures (BP; Finometer), 37 and heart rate (HR; electrocardiogram) were recorded. Renal vascular resistance (RVR), an 38 index of renal vasoconstriction, was calculated as mean BP/RBFV. All baseline 39 cardiovascular variables were similar between groups, except diastolic BP was higher in 40 older subjects (P < 0.05). Increases in RVR during LBNP were greater in the older group 41 compared to the young group (older: -15 mmHg ∆10 ± 3%, -30 mmHg ∆20 ± 5%; young: 42 15 mmHg ∆2 ± 2%, -30 mmHg ∆6 ± 2%; P < 0.05). RBFV tended to decrease more (P = 43 0.10) and mean BP tended to decrease less (P = 0.09) during LBNP in the older group 44 compared to the young group. Systolic and diastolic BP, pulse pressure, and HR responses 45 to LBNP were similar between groups. These findings suggest that aging augments the 46 renal vasoconstrictor response to orthostatic stress in humans. 47