The biochemical assessment of sympathoadrenal activity in man

Sympathoadrenal activity in man can be assessed by measuring catecholamines in plasma or by recording impulses in sympathetic nerves to skin and muscles by microneurography. Several studies have indicated that forearm venous plasma noradrenaline concentration and muscle sympathetic nerve activity are closely correlated in normal subjects at rest as well as during various conditions with increased or decreased sympathetic activity. Both parameters are influenced by baroreceptors and increase with age. Plasma adrenaline should preferably be measured in arterial blood because the extraction of adrenaline in organs and tissues may increase considerably when plasma adrenaline increases. The problem of studying the metabolic clearance rate of noradrenaline but not of adrenaline is discussed. It is emphasized that sympathetic activity is highly differentiated and it should therefore be measured in specific organs and tissues. Sympathetic activity in internal organs can be studied by measuring the release of noradrenaline from these organs. Imaging technique may, however, prove useful in future studies. The significance of microdialysis, measurements of plasma catecholamine metabolites, dopa and dopamine, plasma neuropeptide Y, catecholamines in urine and in the cerebrospinal fluid is discussed. Furthermore, it is emphasized that adrenergic agonist and antagonist drugs are important tools to study sensitivity and responsiveness to catecholamines preferably in specific organs and tissues. Finally, a few examples are given of the values in human research of the techniques described.