Plasma Noradrenaline as an Index of Sympathetic Activity
Plasma noradrenaline represents the small fraction of neurotransmitter noradrenaline released from peripheral sympathetic nerve endings which is not taken up into neuronal or extraneuronal tissue or metabolised by methylation or deamination and which overflows into the circulation. Plasma adrenaline in contrast is derived from the adrenal medulla and exerts its action as a blood borne hormone on metabolic, endocrine and vascular adrenergic receptors. In man, under most circumstance, noradrenaline levels in plasma are several fold higher than adrenaline unlike some animal species. In several studies, using different approaches, there is very little evidence, except during heavy physical exercise, that concentrations of noradrenaline achieved in plasma would significantly and directly influence arterial pressure or heart rate. When noradrenaline was infused intravenously to normal volunteers plasma concentrations had to increase by several fold to cause any change in systolic blood pressure (Fitz-Gerald et al., 1979). During Long-term intravenous infusions of noradrenaline to conscious dogs, an increase in circulating noradrenaline from 3.0 to 10.0 nmol/1 was not associated with significant changes in blood pressure (Casals-Stenzeletal., 1979).
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