SODIUM ION MOVEMENT BETWEEN THE INTESTINAL LUMEN AND THE BLOOD

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Abstract
Na24 was employed as a tracer in measuring rates of Na ion movement into and out of the gut at several levels in chronic surgically prepared segments of the dog intestine. The absolute rates of movement of material in either direction were calculated from the net transport rate and the absolute movement rate in one direction. Rates detd. from measurements of net transport and of total movement in either direction showed general agreement under comparable conditions. Na ion moved in both directions across the intestinal epithelium at measurable rates under all conditions studied. Even when there was movement of Na from gut to blood against large conc, gradients, small amts. of that ion simultaneously entered the gut from the blood. There is, in general, a descending aboral gradient in Na movement from blood to gut, the movement being least in the colon. The same general rule holds for movement in the gut to blood direction except that there is probably little difference between ileum and colon. The rates of Na ion movement both out of and into the gut are both positively correlated with Na ion conc, in the small gut under the conditions of the expts. reported. This correlation could not have been predicted for movement into the gut and probably has significance in connection with the dynamics of movement of Na ion. The jejunum and ileum show greater variability in measured rates of Na ion movement than does the colon. It is believed that the variations observed in the first 2 regions result from real changes in state of the animal from day to day. The causes of the variations were not ascertained. The difference between Na movement out of and into the gut is ordinarily much greater for colon than for ileum. It appears therefore that the colon possesses a more efficient absorbing mechanism with a smaller "leak." Calculation showed that the total turnover rate of Na between the intestine and the blood is very large, being approx. equal to the total plasma Na/83 mins. These observations stress the importance of dynamic factors in determining the apparent permeability of living membranes. The fact that Na ion movement in a given direction through a structure readily permeable to it is positively correlated with the simultaneous rate of the univalent cation movement through it in the opposite direction, indicates that permeation under these conditions depends upon dynamic factors more than upon structural ones.

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