Tissue osmolality in intestinal villi during luminal perfusion with isotonic electrolyte solutions.

A cryoscoptic technique has been developed that makes it possible to determine tissue osmolality in the core of the intestinal villi. During absorption from an isotonic electrolyte solution containing glucose an osmolality gradient was demonstrated from tip to base of the villi in both the jejunum and the ileum. The tissue osmolality at the villous tips was measured to 1 000-1 200 mOsm/kg H2O while the osmolality at the villous base was approximately isotonic with plasma. Increasing intestinal blood flow by i.a. administration of a vasodilator drug, or making the intestine ischemic by clamping the intestinal vascular supply while supplying the mucosa with oxygen, markedly decreased tissue osmolality. Substituting all sodium ions with choline in the luminal perfusate abolished almost completely the tissue hyperosmolality and the intestine became a secretory organ. These observations are consistent with the view that the observed villous tissue hyperosmolality was created by a countercurrent multiplication of sodium chloride. The physiological implications of this mechanism is discussed and it is, among other things, proposed that the hyperosmolar region represents the hyperosmotic compartment necessary for explaining intestinal water absorption.