Measurements of plasma norepinephrine concentrations in human primary hypertension. A word of caution on their applicability for assessing neurogenic contributions.

The relationship between plasma levels of norepinephrine (NE) and sympathetic neural activity is discussed with special reference to human primary hypertension. Since sympathetic discharge is differentiated, neural activity to a given target organ will contribute variably to plasma NE levels in different situations. Hemodynamically, early primary hypertension is often characterized by a mild defense reaction-like pattern with signs of increased sympathetic activity to the heart and vasoconstriction in the renal and splanchnic vascular beds. Although important hemodynamically, these organs seem to be of less importance as contributors to peripheral plasma NE levels. In contrast, muscle sympathetic activity and muscle vascular resistance is unchanged or reduced. Since this organ mass contributes importantly to plasma NE levels, especially in peripheral venous blood, it is not surprising that most patients with primary hypertension have normal NE levels. It is concluded that NE concentrations in forearm or mixed venous blood are unreliable indicators of sympathetic neural contributions to essential hypertension, tending to underestimate this element, and that regional measurements of NE overflow are needed for a reliable analysis.