Noradrenaline release and sympathetic nervous system activity.

Measurements of the plasma concentration of noradrenaline, or more specifically the rate at which noradrenaline enters plasma, provide a useful guide to sympathetic nervous system function in humans. The overall rate of release of noradrenaline to plasma gives an overview of sympathetic nervous system activity (integrated nerve firing rate), detecting generalized changes, whether occurring as a reflex response, produced by drugs, or accompanying disease processes. The pattern of sympathetic nervous activation, however, is not delineated, only the net change in neurotransmitter release. Measurement of regional rates of noradrenaline release allows the clinical assessment of organ-specific sympathetic nervous tone, and consequently more penetrating analysis of sympathetic nervous system pathophysiology in disease states. The major problem in interpreting regional noradrenaline spillover measurements lies in the difficulty in differentiating those changes in noradrenaline spillover due to altered nerve firing, from those due to extraneous factors which might also affect spillover, such as the possible influence of blood flow on noradrenaline washout.