New Findings: What is the central question of this study? The study was designed to investigate the age- and sex-related differences in muscle sympathetic nerve activity (MSNA) in relation to age- and sex-related differences in haemodynamics, blood volume and left ventricular size and function. What is the main finding and its importance? Age-related increases in MSNA were associated with decreases in blood volume in men only, and only in men was there a significant relation between MSNA and forearm vascular resistance. These results show that age and sex independently alter the neural control of blood pressure and may provide insights into the pathogenesis of hypertension. We compared the effect of age- and sex-related differences in haemodynamics, blood volume (BV) and left ventricular (LV) size and mass on resting muscle sympathetic nerve activity (MSNA) in healthy, normotensive adults. Twenty young men (19-47 years old) and 20 young women (21-46 years old) as well as 15 older men (62-80 years old) and 15 older women (60-82 years old) were studied. Cardiac output (acetylene rebreathing), total peripheral resistance, forearm vascular resistance (FVR; venous occlusion plethysmography) and MSNA were measured during supine rest. Blood volume was calculated (CO rebreathing), and LV mass, end-diastolic (LVEDV) and end-systolic volumes (LVESV) were measured using magnetic resonance imaging. Cardiac index (P < 0.001 and P = 0.016), BV (both P < 0.001), LV mass (P < 0.001 and P = 0.002), LVEDV (P < 0.001 and P = 0.002) and LVESV (both P < 0.001) were lower in the older and female groups, respectively. Total peripheral resistance was significantly higher in the older (P < 0.001) and female groups (P = 0.014), but FVR was increased in the female groups (P = 0.048) only (age, P = 0.089). The MSNA was greater in the older groups (P < 0.001) only (sex, P = 0.228). Increased MSNA was shown to correlate with a decrease in BV (P = 0.004) in men only when adjusted for age (women, P = 0.133). There was a positive relation between MSNA and FVR (P = 0.020) in men but not women (P = 0.422). There were no significant relations between MSNA and LV mass, LVEDV or LVESV. The findings suggest that the increase in resting MSNA with age may be related to the decline in BV in men only, but it is unknown whether sex differences in sympathetic adrenergic vasoconstriction occur independently of these changes.
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