Mechanism of electrolyte and water transport in the first 2 minutes of hypotonic water resorption

1.2 ml of water or 1.2-2.4 ml of 1/2-isotonic saline were injected into the duodenum of the unanaesthetized rat. Continuous measurements of conductivity and sodium concentration in whole blood, the last by means of ion-selective electrodes, and of protein and electrolyte concentration in a filtrate of portal vein blood were made. Whereas conductivity and sodium or electrolyte concentration dropped promptly and steeply, the drop of protein concentration (Cprot) was registered with a delay of 1.9 min. A quantitative comparison of the conductivity and sodium concentration curves, showing strictly synchronous courses, suggested that no hematocrit changes (these would have influenced conductivity) had taken place during the period of constancy of Cprot. In rats having thirsted for 36 h, Cprot did not fall during a period of 5-8 min in which they drank up to 5.5 ml of water, despite a great drop of electrolyte concentration. It is concluded that at the beginning of water absorption all the fluid absorbed is transported via the lymph ducts. A gradient of negative pressure from the interstitial tissues to the lymphatic system in the non-absorbing gut is proposed, an additional electrolyte secretory process being taken into consideration.