HypvrtvwtUm Clinical Studies Failure of Plasma Norepinephrine to Consistently Reflect Sympathetic Activity in Humans

SUMMARY To determine whether venous plasma norepinephrine concentrations consistently re-flect changes in sympathetic nervous activity, the influence of mental arithmetic, static handgrip, and submaximal bicycle exercise on infra-arterial blood pressure, heart rate, and plasma norepinephrine was studied in 51 subjects with untreated essential hypertension (mean age, 46 years; range, 16-69 years). At rest, plasma norepinephrine was unrelated to age or blood pressure. Mental arithmetic increased mean arterial pressure from 108 ± 18 to 127 ± 18 mm Hg (mean ± S.D.; p<0.001) and heart rate from 69 ± 7 to 93 ± 13 beats/min (p<0.001) but not plasma norepinephrine (547 ± 297 to 518 ± 250 pg/ml). Isometric exercise raised mean arterial pressure from 115 ± 18 to 148 ± 21 mm Hg (p<0.001) and heart rate from 76 ± 9 to 95 ± 13 beats/min (p<0.001) but not plasma norepineph-rine (683 ± 253 to 741 ± 253 pg/ml). Bicycle exercise increased mean arterial pressure from 114 ± 20 to 146 ± 26 mm Hg (p< 0.001), heart rate from 77 ± 9 to 128 ± 19 beats/min (p< 0.001), and plasma norepinephrine from 645 ± 228 to 1151 ± 462 pg/ml (p<0.001). Both the maximum mean arterial pressure and the peak heart rate attained during bicycle exercise were related to the exercise plasma norepinephrine level (r = 0.33, p<0.02 and r=0.28, p<0.03, respectively). Increases in plasma norepinephrine with exercise were not greater in older or more hypertensive subjects. In fact, the relative rise in mean arterial pressure and the absolute increase in plasma norepinephrine during cycling were greater in those with lower resting blood pressures. As only bicycle exerdse increased plasma norepinephrine, and as there was no direct relationship between plasma norepinephrine and arterial blood pressure in subjects studied supine, seated, and during these three activities, we conclude that plasma norepinephrine concentrations do not consistently reflect variations in sympa-thetic neuronal activity and therefore are relatively insensitive measures of sympathetic tone in humans. (Hypertension 8: 641-649, 1986) KEYWORDS • essential hypertension • sympathetic nervous syste