Do racial differences exist in mechanoreflex sensitivity in young healthy males?

Cardiovascular morbidity and mortality rates are highest among Black Americans (BA), the mechanisms of which are complex and not completely understood. Altered autonomic reflexes or sympathetic transduction could contribute to elevations in baseline blood pressure or an exaggerated exercise pressor response, thereby increasing CVD risk; though no studies have explored whether mechanoreflex sensitivity, a component of the exercise pressor reflex, is altered in BA. Purpose: Using passive leg movement (PLM), as model of activating the mechanoreflex and an assessment of lower limb vascular function, the aim of this study was to compare the central and peripheral hemodynamic responses in young healthy Black (BA) and White American (WA) males. Methods: Young (21±4 yr) healthy BA (n = 9) and WA (n = 10) males, matched for physical activity, were instrumented to continuously monitor central and peripheral hemodynamics using Finger Photoplethysmography (Finometer) and frequency domain multi-distance near-infrared spectroscopy (NIRS) of the vastus lateralis (Oxiplex TS), respectively. After 1 minute of baseline, subjects underwent continuous PLM at 1 hz for two minutes, while tissue oxygen saturation (StO2 %), cardiac output (CO), heart rate (HR), stroke volume (SV), and mean arterial pressure (MAP) were continuously recorded. Results: Resting HR (61±3 vs. 62±3 beats/min), SV (89±5 vs. 92±5 mL/beat), CO (5.4±0.4 vs. 4.4±0.5 L/min), and MAP (92±5 vs. 86±3 mmHg) were not different between BA and WA, respectively (all, p>0.05). The individual peak PLM-induced changes in HR (6±2 vs. 11±3 Δbeats/min, p = 0.06), SV (7.3±1.6 vs. 11.4±1.9 ΔmL/beat, p = 0.04), and CO (0.7±0.2 vs. 1.0±0.2 ΔL/min, p = 0.09) were lower in BA, however the MAP response (5.3±1.4 vs. 6.0±0.8 ΔmmHg, p > 0.05) was not different. The peak PLM-induced change in StO2 was significantly attenuated in BA (1.6±0.5 vs. 3.8±0.4 Δ%, p=0.01). As a surrogate of nitric oxide, urinary nitrate+nitrate were not different between groups (2.89±1.67 vs. 2.00±1.35 μM, p>0.05). Conclusion: A novel insight from the current study reveals that Black American men had an attenuated mechanoreflex response to PLM as compared to White American men. However, Black Americans had a lower peripheral hemodynamic response, perhaps the result of the altered mechanoreflex or suggestive of lower limb vascular dysfunction, which might explain a propensity towards elevated rates of peripheral vascular disease in Black Americans.