Renal medullary blood flow (MBF) is considered of pivotal importance for sodium homeostasis. Nevertheless, the experimental data are conflicting. Studies in anaesthetized animals generally support the concept, but chronic measurements in conscious animals do not. Using a new a method for quantification of MBF in humans, we tested the hypothesis that the natriuresis of acute saline loading is associated with changes in MBF.

Methods: 

Healthy young men were given a low sodium diet (40 mmol/day, 4 d). Compliance was assed by 24-hour urinary sodium excretion. The following day the subjects were exposed to s low intravenous saline loading (12 mmol/kg/min) for 4 hours. Renal medullary and cortical blood flow were determined by positron emission tomography (PET) before and after the loading procedure. Blood pressure was measured oscillometrically every 10 min. Cardiac output (CO), heart rate (HR) and total peripheral resistance (TPR) were recorded continuously by impedance cardiography.

Results: 

Urinary sodium excretion increased 3.7-fold during the loading procedure (21.7 to 78.0 mmol/min, p< 0.01). Renal medullary blood flow was 2.73 ml/g tissue/min in the baseline setting and 2.89 ml/g tissue/min following saline loading (not significant). Cortical blood flow was also unaltered. There was no significant change in mean arterial blood pressure (90.1 vs 90.7 mmHg).

Conclusion: 

The results seem to reject the hypothesis that changes in medullary perfusion precede the natriuresis associated with mild volume expansion. The volume-mediated natriuresis can take place independent of changes in arterial blood pressure and medullary blood flow and therefore seems to be regulated by neurohumoral factors.