NITRIC OXIDE DOES NOT BUFFER BLOOD PRESSURE VARIABILITY DURING HEAD-UP TILT

Abstract

Recent studies in animals suggest that nitric oxide (NO) plays an important role in buffering short-term blood pressure variability. However, data from humans addressing this hypothesis are scarce. In this study, we evaluated the effects of NO synthase inhibition on blood pressure (BP) and R-R interval variability in eight healthy subjects in the supine position and during 60° head-up tilt. Systemic NO systhase was blocked by intravenous infusion of L-NMMA (loading dose, 5mg/kg for 15 min; maintenance dose, 5ug/kg/min, usually 30 to 45 min). ECG and BP (Finapres) were recorded continuously for 6 minutes before and after L-NMMA in each body position. Respiratory excursion was monitored by pneumograph. BP and R-R variability were quantified by power spectral analysis. The transfer function between these variables was estimated to evaluate spontaneous baroreflex function. Spectral power, mean transfer function gain and phase were calculated in the low frequency (LF; 0.05–0.15 Hz) and high frequency (HF; 0.15–0.30 Hz) range, respectively. Result: L-NMMA infusion augmented supine BP significantly by 7% (systolic, 130 ± 5 vs. 139 ± 5; diastolic, 73 ± 4 vs. 78 ± 4 mmHg). R-R variability at all frequencies remained unchanged though BP variability in the HF range decreased significantly by 24% (1.04 ± 0.19 vs. 0.79 ± 0.21 mmHg2). Before L-NMMA, HF R-R variability decreased significantly by 85% and LF BP variability increased by 122% during tilt. Transfer function gain decreased significantly in both the low and high frequency ranges (LF, 12 ± 2 vs. 5 ± 1; HF, 18 ± 3 vs. 3 ± 1 ms/mmHg). After L-NMMA, similar changes in R-R and BP variability were observed during tilt. We conclude that inhibition of NO synthase does not affect short-term BP variability and/or baroreflex function in humans. We speculate that tonic release of NO is important for control of systemic vascular tone and thus steady-state blood pressure. However, its role in dynamic BP regulation appears less essential.