Gastric Secretion. Iii. the Absorption of Heavy Water from Pouches of the Body and Antrum of the Stomach of the Dog.

It is held as axiomatic that an understanding of the mechanism of the passage of substances from the blood stream to the gastric lumen must include a knowledge of the passage of these same substances in the reverse direction. In the previous paper (1), the absorption of radioactive sodium (Na24) from the stomach was reported. Tangible amounts of Na24 placed in a stomach pouch were found after 15 minutes in the blood serum. From 1 to 7 per cent of the introduced base was absorbed from the acid bearing area during an hour, the amount depending upon whether the stomach was in a resting or secreting state; as much as 50 per cent was absorbed from the alkaline secreting antrum in the same period. The continuous passage of this basic ion into the body fluids altered significantly the concentrations of base in the gastric contents. Since it is probable that the sodium of neutral sodium chloride secreted into the pouch would be reabsorbed in a manner similar to the sodium introduced from the outside, it was concluded that the contents of an isolated pouch represents not true secretion but the resultant of secretion and absorption. Water is an integral part of any secretion and the study of its exchange across the gastric membrane would seem essential for a comprehension of the control of electrolyte concentration in gastric juice. It has long been established (2, 3) that the contents of a gastric pouch is isotonic with the blood plasma, and it has been assumed, therefore, that the secretion itself is isotonic. It might be that the actual product secreted by a cell is not isotonic but that equilibrium with the internal fluids is rapidly reached by the exchange of water through a neighboring cell. Examples of the ability of the body to handle fluids other than isotonic are not wanting; the intestinal tract can absorb large quantities of water, and the kidney produces urine far to either side of isotonic. The absorption of water from the whole stomach or one of its parts has not been demonstrated. Many years ago Pavlov showed that water was a mild stimulant of gastric secretion (4). Thus, if water is inserted into an isolated pouch of the stomach, with the passage of time there is a net gain in volume. It is possible, however, that as the solutions of electrolytes are flowing into the stomach and increasing the volume of the gastric pouch contents, there is a simultaneous absorption of a lesser volume of water. It was decided to answer this question of the absorption of water from the stomach, using deuterium oxide or so called heavy water (D20). This substance can be distinguished from ordinary water (H20) by physical means, and by its greater density, and offers, therefore, the ideal for such an experiment. Heavy water has been used in other parts of the animal body as a tracer substance to demonstrate the direction and rapidity of shift of water from one compartment to another. Recently Kinsey, Grant, and Cogan (5) have measured by means of D20 the flow of water into and out of the eye. The rapidity and volume of exchange found was surprising. In rabbits, one half of the water of the aqueous humor was replaced every 2.7 minutes. In the monkey, the exchange was slower, taking 7 minutes for one half to shift. The exchange of water out of the vitreous humor of the rabbit was somewhat slower than from the aqueous. Animals have been observed to assimilate D20 much as they do H20. Only in high concentrations and over long periods have organisms shown intolerance to D20 (6). For experiments of short duration or when small quantities are used, D20 can be considered to be handled by the organism much as is H20.