Dynamic changes in renal sodium handling during sympathetic stimulation in healthy human males

The temporal response of changes in renal sodium reabsorption during increased renal sympathetic nerve activity has not been investigated. Central hypovolemia by application of lower-body negative-pressure (LBNP) elicits baroreceptor mediated sympathetic reflexes to maintain arterial blood pressure. We hypothesized, that during 90 min LBNP, the renal sodium retention would increase rapidly, remain increased during intervention, and return to baseline immediately after end of intervention.

30 young, healthy, sodium loaded, non-obese males were exposed to -15 mmHg LBNP, -30 mmHg LBNP, -15 mmHg LBNP + renin blockade or time-control (0 mmHg LBNP) for 90 min. Urine was collected every 15 min during 90 min of intervention and 60 min of recovery to identify a possible relation between time of intervention and renal response.

All intervention groups exhibited a comparable reduction in distal sodium excretion at the end of the intervention (P = 0.46 between groups; -15 mmHg: -3.1 ± 0.9 %, -30 mmHg: -2.9 ± 0.6 %, -15 mmHg + aslikiren: -1.8 ± 0.6 %). -15 mmHg+Aliskiren resulted in a slower onset, but all groups exhibited a continued reduction in sodium excretion after 1 h of recovery despite return to baseline of renin, aldosterone, diuresis and cardiovascular parameters.

Sympathetic stimulation for 90 min via LBNP at -30 mmHg LBNP compared to -15 mmHg did not result in a greater response in fractional Na⁺ excretion, suggesting that additional baroreceptor unloading did not cause further increases in renal sodium reabsorption. Changes in distal Na⁺ excretion were linear with respect to time (dose) of intervention, but seem to exhibit a saturation-like effect at a level around 4 %. The lack of recovery after 1 h is also a new finding that warrants further investigation.