Vasomotor sympathetic activity plays an important role in arterial pressure maintenance via the baroreflex during acute orthostasis in humans. If orthostasis is prolonged, blood pressure may be supported additionally by humoral factors with a possible reduction in sympathetic baroreflex sensitivity. We tested the hypothesis that baroreflex control of muscle sympathetic nerve activity (MSNA) decreases during prolonged upright posture. MSNA and haemodynamics were measured supine and during 45 min 60 deg upright tilt in 13 healthy individuals. Sympathetic baroreflex sensitivity was quantified using the slope of the linear correlation between MSNA and diastolic pressure during spontaneous breathing. It was further assessed as the relationship between MSNA and stroke volume, with stroke volume derived from cardiac output (C2H2 rebreathing) and heart rate. Total peripheral resistance was calculated from mean arterial pressure and cardiac output. We found that MSNA increased from supine to upright (17 ± 8 (S.D.) versus 38 ± 12 bursts min-1; P < 0.01), and continued to increase to a smaller degree during sustained tilt (39 ± 11, 41 ± 12, 43 ± 13 and 46 ± 15 bursts min-1 after 10, 20, 30 and 45 min of tilt; between treatments P < 0.01). Sympathetic baroreflex sensitivity increased from supine to upright (-292 ± 180 versus -718 ± 362 units beat-1 mmHg-1; P < 0.01), but remained unchanged as tilting continued (-611 ± 342 and -521 ± 221 units beat-1 mmHg-1 after 20 and 45 min of tilt; P = 0.49). For each subject, changes in MSNA were associated with changes in stroke volume (r = 0.88 ± 0.13, P < 0.05), while total peripheral resistance was related to MSNA during 45 min upright tilt (r = 0.82 ± 0.15, P < 0.05). These results suggest that the vasoconstriction initiated by sympathetic adrenergic nerves is maintained by ongoing sympathetic activation during sustained (i.e. 45 min) orthostasis without obvious changes in vasomotor sympathetic neural control.
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