Most knowledge about absorptive colonic function in man is derived from the study of patients with ileostomies (1,2) and the comparison of results with the composition of stools of normal subjects. To obtain more direct and quantitative information concerning water and salt absorption from the intact and healthy human colon, the following studies were performed. METHODSThe subjects were healthy male volunteers aged 20 to 30 years. The cecum was intubated from above with a polyvinyl tube 1.8 mm in internal diameter, with a small rubber bag containing mercury affixed to its distal end. During the 3 to 4 days required for the tube to reach the colon the subject ingested clear liquids only. After the tip was located radiologically in the cecum, a radiopaque water-soluble solution was instilled to ascertain that no reflux into the ileum would take place. Thereafter, the colon was cleaned thoroughly by perfusing it with normal saline until the material expelled rectally was clear and relatively free of mucus. After this cleansing period, 2 hours were allowed for the subjects to expel or absorb the saline remaining in the colon.All studies were performed after an 8 hour fast. The standard procedure consisted of infusing the test solution at a constant rate of 10 ml per minute into the cecum and collecting the perfusate through a rectal tube (size Fr. 24) placed into the rectum approximately 8 cm above the anus. The test solution contained 0.85 per cent sodium chloride, 1 per cent polyethelene glycol (PEG), a nonabsorbable reference substance with a molecular weight of about 4,000 (3-6), and tritiated water (0.5 to 1.0 mc per 1,000 ml). In three studies the design of the experiment was modified slightly by intubating the colon with a double polyvinyl tube and collecting samples from the mid-colon as well as from the rectal tube. Drainage from the collecting tubes was dis-* This investigation was supported by research grant 1 The symbols used in the equations are: Vi, = volume infused, ml per minute; PEG,. = PEG concentration in mg per 100 ml in infused fluid; PEGre = PEG concentration in mg per 100 ml in recovered sample; SA1, = specific activity of the radioactive tracer in the infused fluid, counts per minute per ml; SA,. = specific activity of the radioactive tracer in the recovered sample, counts per minute per ml; E1t = concentration of electrolyte in infused fluid, mEq per ml; Ere = concentration of electrolyte in recovered sample, mEq per ml. 1754

A B S T R A C T The influence of sodium concentration and osmolality on net water and monovalent electrolyte absorption from or secretion into the intact human colon was studied in healthy;volunteers.When isotonic solutions containing NaCl and/or mannitol were infused into the colon: (a) a direct linear relationship between luminal sodium concentration (in the range of 23-150 mEq/liter) and rate of net water, sodium, and chloride absorption was found. No water absorption was found when sodium concentration in the luminal fluid was below 20 mEq/liter; (b) water and sodium absorption from the isotonic test solutions was not enhanced by addition of 80-250 mg/100 ml of glucose; and (c) the rate of water and sodium absorption was decreased markedly when chloride was replaced by bicarbonate in the test solution.When the colon was perfused with hypertonic test solutions containing NaCl and mannitol or urea: (a) water was absorbed from hypertonic NaCl solutions against a lumen-to-blood osmotic gradient of 50 mOsm/ kg; (b) when the osmolality of the mannitol solution was increased, water entered the colonic lumen at a more rapid rate. The relationship between the rate of water entering the colon and the osmolality of the test solution was a parabolic one; (c) sodium and chloride entered the colonic lumen at a rate that was lineraly related to that of water entrance when the lumen-to-blood osmotic gradient exceeded 150 mOsm/kg; (d) water flow into the colonic lumen was identical when equimolar urea or mannitol solutions were infused; (e) neither urea nor mannitol was absorbed in significant amounts from the hypertonic solutions; and (f) our results suggest that the equivalent pore radius of the human colon is smaller than the molecular radius of urea (2.3 A).

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