OSMOTIC AND ELECTROLYTE CONCENTRATION RELATIONSHIPS DURING ABSORPTION OF SALT SOLUTIONS FROM ILEAL SEGMENTS

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Abstract
Results of studies on osmotic and electrolyte conc, relationships during absorption of isotonic Na2SO4 and NaCl solns. from ileal segments in nembutal anesthetized dogs are reported. The influence of additions of 0.001-0.003 M HgCl2 or NaAsO2 was also studied. The osmotic activity was studied by the thermoelectric vapor tension technique. Special attention was paid to the CO2 tension in the osmotic activity measurements. When originally approx. isotonic solns. of NaCl and Na2SO4 in equiosmotic proportions are placed in ileal segments they invariably become hypotonic to the blood plasma. The avg. hypo-tonicity was found to be equivalent to 5.9 mM. NaCl/kg. water. When 0.001 M HgCl2 was added to such solns. the tendency to hypotonicity was virtually abolished. In the presence of this poison the impoverishment of chloride otherwise found was also abolished. When originally isotonic NazSO4 solns. were introduced into ileal segments such solns. were on the avg. 2.3 mM./kg. H2O hypotonic after 40 min., a figure significantly less than that found when NaCl was present. The difference is believed to be related to the greater mobility of chloride than sulfate across the ileal epithelium. The net increase in chloride conc, in Na2SQt solns. was very small, on the av. 4 mE./l. in 40 min. From isotopic tracer data it is concluded that this represents a small fraction of the chloride that entered in that time. The remainder must have returned to the blood by some active transport mechanism. The addition of 0.001-0.002 M HgCl2 or NaASO2 to isotonic Na2SO4 solns. in ileal segments caused those solns. to become significantly hypertonic, the mean value for osmotic activity being 3.4 mM./kg. water above the plasma. This was associated with a great influx of chloride, the av. rate of increase in chloride conc, being 11 times that found in the unpoisoned solns. Net water movement and osmotic activity studies in the above mentioned expts. show that in every case in solns. from which there was a significant water absorption there was definite hypotonicity of the gut soln. as compared with the plasma at 40 min. after insertion of the fluid in the gut segment. It is concluded that when water is absorbed from approx. isotonic spins, of NaCl and/or Na2SO4, the intestinal mechanism is such that invariably the solns. become hypotonic within a short time. However, there is not a significant positive correlation between the magnitude of the osmotic gradient between gut and blood and the rate of water absorption. It is believed that although a developing hypotonicity is a regular concomitant of water absorption from such solns., the osmotic gradient is not the driving force for water transfer. When the net water transfer was zero, or in the direction of gut volume increase, the gut fluid osmotic activity was sometimes lower and sometimes higher than that of the plasma. Entrance of water into the intestine occurred frequently when the fluid in it was hypotonic to the plasma, a result that could not occur from the action of normal osmotic forces. This observation constitutes further evidence that forces other than those of normal osmosis bring about water transport across the intestinal epithelium.

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