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 micrograms/min) and increased by infusions of phenylephrine (25-100 micrograms/min) at rates producing a 10% to 20% change in diastolic blood pressure. Sodium nitroprusside produced significant decreases in diastolic blood pressure (p less than 0.01) and calculated mean arterial blood pressure (p less than 0.005), and increases in heart rate (p less than 0.001) and plasma norepinephrine (p less than 0.001). Phenylephrine administration produced increases in systolic (p less than 0.005), diastolic (p less than 0.005), and mean blood pressure (p less than 0.001). Heart rate (p less than 0.001) and plasma norepinephrine (p less than 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.