Vasomotor sympathetic neural control is maintained during sustained upright posture in humans

Abstract

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.

Figure 1

Original tracings of blood pressure, heart rate, respiratory waveforms, and muscle sympathetic nerve activity from one subject in the supine position, and during the initial 1 min and the last 2 min of 60 deg upright tilt.

Figure 2

Muscle sympathetic nerve activity (A), systolic blood pressure (B), diastolic blood pressure (C), and heart rate (D) responses during 45 min of 60 deg upright tilt. Values are mean ±s.d.*P < 0.05 compared to the supine position. †P < 0.05 compared to the 5th minute of upright tilt.

Figure 3

Correlation between total peripheral resistance and muscle sympathetic nerve activity in the supine position and during 45 min upright tilt in each subject.

Figure 4

Sympathetic baroreflex sensitivity in the supine position, and during the early, middle and late stages of 60 deg upright tilt. Values are mean ±s.d. Tilt5, between the 2nd and 5th minute; Tilt20, between the 17th and 20th minute; Tilt45, between the 42nd and 45th minute of upright tilt. *P < 0.05 compared to the supine position.

Figure 5

Correlation between muscle sympathetic nerve activity and stroke volume in the supine position and during 45 min upright tilt in each subject.