Evidence of the role of the vagal nerves as a monitor in the gastrointestinal-renal axis of natriuresis in human: Effects of vagotomy

- During a low Na diet vagotomy leads to less weight loss & drop in pulse pressure, less Na deficit & earlier Na equilibrium
- On Na load replenishment vagotomized subjects exhibit less overall natriuresis and later than that of those without vagotomy
- Oral Na with intact vagal nerves exhibits the greatest natriuresis compared to any other vagal status or administration route
- Vagotomy abolishes the mechanism of natriuresis of a Na load in subjects previously on a low Na diet
- Vagal nerves importantly regulate renal sodium excretion directly or indirectly via a neural reflex & a natriuretic response

This study aimed to investigate the mechanism of gastrointestinal regulation of natriuresis. Sixteen subjects without (group I) and sixteen subjects with a truncal vagotomy (group II), were given a daily diet of 18 mmol of sodium for 5 days (D1-D5). The sodium deficit for this period was calculated for each subject and on the morning of day-6 (D6), their cumulative deficit (E) was given as 3% NaCl. In both groups the subjects were divided to receive the hypertonic saline either orally (Ior, IIor) or intravenously (Iiv, IIiv). During the period of low sodium diet when compared to group II subjects of group I (1) had a greater weight loss (p < 0.005), (2) demonstrated a larger drop in pulse pressure (p < 0.005), (3) achieved a positive sodium equilibrium later (D5 vs D4) and (4) developed a greater sodium deficit (p < 0.005). During the two 12 h periods of D6, both Ior and Iiv exhibited greater natriuresis during the first 12 h period (p < 0.0001) whereas both IIor and IIiv did so during the second 12 h period (p < 0.0001). On D6 Ior excreted the greatest percentage of E (E%; 35.63% ± 3.12%, p < 0.0001) compared to Iiv (17.06% ± 1.78%), IIor (16.03% ± 3.54%) and IIiv (15.39% ± 2.77%) whereas E% was not different between the other subgroups. These results indicate that the differential natriuresis between oral and intravenous sodium loading in previously sodium deprived subjects, is due to a mechanism in which the vagal nerves play a significant role as part of neural reflex or via a natriuretic hormone.