Arterial baroreflex regulation of muscle sympathetic nerve activity at rest and during stress.

Spontaneous sympathetic baroreflex sensitivity (sBRS) is commonly quantified as the slope of the relationship between variations in absolute diastolic blood pressure (DBP) and muscle sympathetic nerve activity (MSNA) burst incidence or strength. This relationship is well maintained at rest but not during stress. We assessed whether sBRS could be calculated at rest and during stress (static handgrip, rhythmic handgrip, mental stress) using blood pressure variables that quantify relative change: beat-to-beat DBP change (ΔDBP), ΔDBP rate-of-change (ΔDBP rate), pulse pressure (PP) and PP rate-of-change (PP rate). Sixty-six healthy participants underwent continuous measures of blood pressure (finger photoplethysmography) and multi-unit MSNA (microneurography). At rest, absolute DBP (91%), ΔDBP (97%) and ΔDBP rate (97%) each yielded higher proportions of participants with strong linear relationships (r ≥ 0.6) with MSNA burst incidence compared to PP (57%) and PP rate (56%) and produced similar sBRS slopes (DBP: -4.5 ± 2.0 bursts 100 heartbeats-1 /mmHg; ΔDBP: -5.0 ± 2.1 bursts 100 heartbeats-1 /ΔmmHg; ΔDBP rate: -4.9 ± 2.2 bursts 100 heartbeats-1 /ΔmmHg s-1 ; P > 0.05). During stress, ΔDBP (74%) and ΔDBP rate (74%) yielded higher proportions of strong linear relationships with MSNA burst incidence than absolute DBP (43%), PP (46%) and PP rate (49%) (all P < 0.05). The absolute DBP associated with a 50% chance of a MSNA burst (T50 ) was shifted rightward during static handgrip (Δ+15 ± 11 mmHg, P < 0.001) and mental stress (Δ+11 ± 7 mmHg, P < 0.001); however, the ΔDBP T50 was shifted rightward during static handgrip (Δ+2.5 ± 3.7 mmHg, P = 0.009) but not mental stress (Δ0.0 ± 4.4 mmHg, P = 0.99). These findings suggest that calculating sBRS using absolute DBP alone may not adequately characterize arterial baroreflex regulation of MSNA, particularly during stress.