Effect of mannitol on sodium retention during whole-gut perfusion

To undergo whole-gut perfusion, normal Chinese subjects drank an isotonic saline solution at a rate of 40 ml/min for 3-6 h. In spite of a diuresis, expansion of the extracellular fluid was evidenced by an increase of body weight and a decrease of haematocrit and plasma specific gravity at the end of perfusion. This fluid expansion resulted from sodium retention at a rate of about 140 mmole/h. When the NaCl of the perfusion solution was gradually replaced by isomotic amounts of mannitol, sodium retention was gradually reduced. Addition to the perfusion solution of up to 144-152 millimolar mannitol reduced the sodium retention rate to 6.5 mmole/h and thus prevented fluid expansion. We found that the sodium retention rate was inversely related to the difference between sodium concentration in the plasma and that in the perfusion solution. Thus, our results suggest that addition of mannitol with a decrease of sodium concentration in the perfusion solution creates a sodium concentration gradient between plasma and intestinal lumen which is unfavourable for intestinal absorption of sodium.