Unidirectional water absorption as a measure of intestinal surface area in rats.

Methods to measure expeditiously the anatomic and functional mucosal surface area of the small intestine are needed. We postulated that passive, unidirectional water flux would assess the anatomic mucosal surface area and that the increase in unidirectional flux that occurs in the presence of glucose would correlate with functional surface area. These hypotheses were tested in control and methotrexate-treated rats. A single-pass perfusion technique was used in vivo. Methotrexate treatment reduced anatomic and functional surface area (p less than 0.05) as measured by computerized planimetry, wet mucosal weight, sucrase activity, and sucrase activity normalized to mucosal DNA content. A formula to predict anatomic surface area was derived from control animal data. The best model used passive, unidirectional water flux as the independent variable, but this model overestimated mucosal surface area in the methotrexate-treated animals (p less than 0.01). Mean unidirectional water flux increased by 45% (p less than 0.01) in controls during glucose perfusion, but did not increase in methotrexate-treated animals (p greater than 0.37). Thus, unidirectional water flux cannot be used as a universally applicable method to assess the anatomic mucosal surface area, but glucose-enhanced unidirectional water flux should be evaluated further as an indirect measure of functional surface area.