Plasma norepinephrine in the evaluation of baroreceptor function in humans

The value of plasma norepinephrine measurement in assessing baroreceptor-mediated changes in sympathetic vasomotor activity was studied in seven healthy normotensive volunteers. Blood pressure was decreased by graded steady-state infusions of sodium nitroprusside (25-100 μg/min) and increased by infusions of phenylephrine (25-100 μ/min) at rates producing a 10% to 20% change in diastolic blood pressure. Sodium nitroprusside produced significant decreases in diastolic blood pressure (p < 0.01) and calculated mean arterial blood pressure (p < 0.005), and increases in heart rate (p < 0.001) and plasma norepinephrine (p < 0.001). Phenylephrine administration produced increases in systolic (p < 0.005), diastolic (p < 0.005), and mean blood pressure (p < 0.001). Heart rate (p < 0.001) and plasma norepinephrine (p < 0.05) fell. The absolute changes in diastolic and mean pressure and heart rate were not significantly different for the two drugs, but were of opposite sign; however, the increase in plasma norepinephrine during hypotension was greater than the decrease during hypertension (p = 0.02). We conclude that plasma norepinephrine changes appropriately in response to altered blood pressure and that the response is greater to a given fall than to a rise in blood pressure, consistent with known changes in sympathetic vasomotor outflow.