[Plasma noradrenaline. Significance and practical value of its determination]

The noradrenaline (NA) that has diffused from sympathetic synapses into plasma can be assayed accurately and reliably in plasma, as it is the best index of sympathetic activity. A considerable advance has been achieved by using infusions of tritium-labelled exogenous NA, such infusions being devoid of functional effect. By measuring simultaneously blood NA levels and circulating tritiated NA levels the amplitude of synaptic spillover and therefore of sympathetic activity can be evaluated and the metabolic NA clearance can be measured. Synaptic NA spillover and NA metabolic clearance were measured in 24 physiological, pathological and pharmacological situations, providing an accurate definition of the sympathetic states explored. By combining these techniques with selective vascular catheterization, the sympathetic activity of the organs explored can be assessed, and regional sympathetic activities are currently being mapped. Such regional studies are of great value to understand the mechanisms involved in heart failure, cirrhosis or arterial hypertension. Sympathetic system regulations result from a central influence and from peripheral adjustments that are potentially specific to each organ.