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The effects of variation in dietary sodium intake on biochemical indices of sympathetic activity were investigated in normal subjects. Sodium restriction for 7 days (mean urinary sodium excretion 21 mmol/day, s.e.m. = 4) increased supine plasma noradrenaline concentration significantly (1.78 nmol/l, s.e.m. = 0.23) compared to levels during medium sodium intake (1.31 nmol/l, s.e.m. = 0.14) and high sodium intake (1.35 nmol/l, s.e.m. = 0.25), when urinary sodium excretion averaged 186 (s.e.m. = 24) and 310 (s.e.m. = 43) mmol/l respectively. Plasma adrenaline concentrations did not vary significantly. During sodium restriction, the apparent release rate of noradrenaline increased significantly; noradrenaline clearance remained unchanged. No changes were observed in the heart rate response after beta-adrenoceptor stimulation with isoprenaline, suggesting that increased sympathetic activity during sodium restriction did not necessarily alter cardiac beta-receptor sensitivity. The responses to cardiac autonomic blockade with propranolol and atropine did not vary with sodium intake. These observations suggest that the sympathetic response to sodium restriction may not be generalized, excluding, in particular, the sympathetic nerves to the heart.
Watson et al. (Sun,) studied this question.