phosphate concenrtration, ph and tonicity of the fluid circulating in

Transcription

1 THE ABSORPTION OF WATER AND SALT FROM THE SMALL INTESTINE OF THE RAT. By G. J. R. McHARDY and D. S. PARSONS. From the Department of Biochemistry, University of Oxford. (Received for publication 24th July 195) WE have previously reported the results of experiments on the absorption of inorganic phosphate from the small intestine of the rat under controlled conditions [McHardy and Parsons, 195]. During these experiments the net absorption rates of water and some electrolytes were also measured and these findings are described in this paper. METHODS The experimental technique and the methods of determining the phosphate concenrtration, ph and tonicity of the fluid circulating in the intestinal segment have been described previously [McHardy and Parsons, 195]. Analyses.-Sodium and potassium were estimated with a directreading flame photometer [Parsons and Cox, unpublished] and chloride by the method of Sendroy [1937]. Glucose was estimated by the method of Hulme and Narain [1931], and urea by the Kjeldahl procedure on protein-free filtrates of the circulating fluid. At the end of each experiment the circulated segment was removed from the animal and its length and wet weight were measured; after six hours drying in an oven at 15-1 C. the dry weight was measured and the water content found by subtraction. At the end of certain experiments blood was taken from the heart with anaerobic precautions and the plasma obtained was used for electrolyte determinations. Calculation and Expression of Results.-In calculating the absorption rate of water, phosphate was used as a "volume indicator" as described for sugars by Fullerton and Parsons [195], the initial and final volume of circulating fluid being determined from the initial and final phosphate content and concentration. The absorption rates of the various solutes were calculated from the initial and final concentrations and the initial and final volumes of circ ulating fluid, due allowance being made for the quantities and volume s removed in sampling. The net absorption rates were expressed as microlitres of water, or as microequivalents or micromoles of solute absorbed per milligram dry weight of intestine per hour (,ul.,,ueq, or,um/mg./hr.). Mean values were derived from VOL. XLII, NO

2 34 McHardy and Parsons experiments on at least six animals, including at least twelve one-hour absorption periods. The detailed composition of the fluids used for circulation is given elsewhere [McHardy and Parsons, 195]. RESULTS Effects of Hydrogen-ion Concentration.-In the first sets of experiments (Series I), fluids having mean initial ph values of 4.4, 5-, -7, 7-2 or 7-9 were circulated through jejunal segments. Solutions at ph 7-2 and 7-9 were also circulated through ileal segments. All the solutions contained 1 meq/l. Na and were buffered with 5 mm/l. phosphate. The experimental results are set out in fig. 1 and Table I. H.O Na 1d. I.Eq O P E ~~~~~~~~~~~~~ 1 / SODIUM WATER PHOSPHATE ph FIG. 1.-Showing the mean absorption rates of water and sodium found when solutions of differing initial ph were circulated in jejunal segments. For comparison the absorption rates of phosphate [McHardy and Parsons, 195] are also shown. It can readily be seen that the more alkaline the fluid circulated the greater was the net absorption rate of water, which increased fivefold in jejunal segments from 4-4ul./mg./hr. at mean initial ph 4-4 to 24-4,ul./mg./hr. at ph 7-9. In ileal segments the rate increased slightly from 18- ul./mg./hr. at ph 7-2 to 2-3,ul./mg./hr. at ph

3 Water and Salt Absorption 35 P. Z. p r-q w Z 1 C~1 - Q Q ~P4 O ci2 ;s V ;.o P. Z CO to C. (,~4 Cl Cl CC Cl O P + CO Cl Cl4 CO (o CD co 1- Cl 4. cl Cl Cl cq CO CO m 1 Clr cq CA CO Cl 1 - COm 1 Ca (3) 1:I- H1o PT, s P-4 Ls Qb L: :- - E4 z p O oi. pz O vb4 H p. ZE-1 S z.e H = )~ 1D m C.) C.) Gi) C,) * * P4E z E ) s o CO IC _ m ee x N * -

4 3 McHardy and Parsons Over the ph range 4-4 to 7-2 a similar marked increase in the net absorption rate of sodium, from -5,uEq/mg./hr. at ph 4*4 to 3*11,uEq/mg./hr. at ph 7-2, was found in jejunal segments. In contrast to the behaviour of water absorption and also of phosphate absorption [McHardy and Parsons, 195] no further increase in sodium absorption was found between ph 7-2 and 7-9; in both jejunal and ileal segments the rate declined slightly. The absorption rate of chloride wan nearly twice as great at ph *7 as at ph 4 4, though the concentration of Cl was 3 meq/l. less at the higher ph level; but as there was a different concentration of Cl in each of the five fluids, it is impracticable to comment on the absorption rates of this ion in terms of the hydrogen-ion concentration alone. Effects of Tonicity and Sodium Concentration.-In a further series of experiments in which the effect of reducing the sodium concentration was studied, the circulating fluids were buffered at a nominal ph of.9 with 25 mm/l. phosphate; the absorption rates observed were compared with those found from a solution containing 1 meq/l. Na, 12 meq/l. Cl, and having tonicity -92 units (the reference solution). The results of the experiments are set out in Table II. In jejunal segments, when a solution having half the tonicity of the reference solution and containing 8 meq/l. Na and 4 meq/l. Cl was circulated, there were considerable changes in the absorptive pattern. The absorption rate of water increased by an amount not statistically significant, but the absorption of sodium and chloride was replaced by a marked net entry of both substances into the intestinal lumen. In ileal segments, when the solution of half-tonicity and salt concentration was circulated water absorption increased (p <.5), there was no net movement of sodium but absorption of chloride occurred, although at about a quarter of the rate found from the reference solution. Effects of Non-electrolytes.-It will also be seen from Table II that when solutions containing 8 meq/l. Na and 4 meq/l. Cl but with the tonicity restored to 94 units by the addition of 148 mm/l. mannitol, were circulated through jejunal segments, the mean absorption of water was zero (-2 ±-9 1l./mg./hr.), the entry of sodium into the intestinal contents practically ceased and the Cl entry was greatly reduced. In ileal segments the water absorption ceased, sodium absorption began to occur, but the chloride absorption rate was the same as from the half-isotonic solution. In further experiments with jejunal segments, additions of 37 mm/l. of glucose, urea or mannitol, chosen as substances absorbed at different rates, were made to the solution. The results of these experiments are shown in Table III, together with the absorption rates of glucose and urea. The rate of absorption of mannitol was not measured. When the solution containing 37 mm/l. mannitol was circulated, the absorption rate of water compared with that from the reference solution

5 Water and Salt Absorption 37 E-~ ~ ~ ~ Z ~~ _ Go 1* ~1 E- ~ ~~ ~ ~ ~ ~ ~ ~ H-fl H -+v H -fi ~ z Z h v SO _ o o O o o H H C: X o aq C' + z HCi)O~~~~C 4p ~~~~~~~~~~~ N Ca Cs Z~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~a ^ C)zz~~~~~~~~~~~~~~~~~~a HO ; X - EvX nn ;X

6 38 McHardy and Parsons was greatly reduced (from 18-1 to 2-3,ul./mg./hr.) and that of Cl virtually abolished. The sodium absorption rate was also considerably reduced below the reference level. The absorption rates from the solution containing glucose did not differ significantly from those found with the reference solution, though the sodium absorption rate was slightly increased. With the ureacontaining solution the water and chloride absorption rates were reduced to less than half the reference level, but the Na absorption was almost identical with that from the reference solution though less than that TABLE III. THE MEAN NET ABSORPTION RATES FROM JEJUNAL SEGMENTS OF WATER, SODIIUM AND CHLORIDE FOUND WHEN ADDITIONS OF 37 MM/L. GLUCOSE, UREA OR MANNITOL WERE MADE TO THE REFERENCE SOLUTION CONTAINING 1 MEQ/L. Na, 12 MEQ/L. Cl AND 25 MM/L. PHOSPHATE AT NOMINAL PH.9. EACH VALUE THE MEAN OF OBSERVATIONS ON 12 EXPERIMENTAL PERIODS. THE TONICITY OF FLUID 31 WAS -92 UNITS, AND OF THE OTHER FLUIDS 1-4 UNITS. Absorption rates, per mg. dry wt. per hour Fluid Additions No. H2O:,l Na:,uEq C1:,Eq Glucose or H2 yi.na ~Eq l:,eeq urea /MM 31 None 18-1 ± ± ± mm/l. glucose 1-7 ± ± ± mm/l. urea 8- ± ±-1-54 ± mm/l. mannitol 2-3 ± ±-14 from the glucose-containing solution. The absorption rate of urea itself was about one-third of the rate of glucose absorption found with an identical concentration of glucose. Comparison of Jejunum and Ileum.-Jejunal and ileal absorption rates could be compared in five sets of experiments, and the results of these comparisons are set out in Table IV. It can be seen that under most circumstances there was not any tendency for greater absorption of water from either region. With sodium the important finding was a greater tendency towards entry into the jejunum when the concentration in the circulating fluid was halved to 8 meq/l., the difference between the two regions being significant (p < 5) under these conditions. In contrast to this was the finding of consistently greater chloride absorption rates in the ileum, under all conditions studied. There was, for example, marked absorption from solutions containing about 4 meq/l. Cl, whereas net entry of Cl into the jejunum took place. Plasma Electrolyte Concentrations.-The concentrations of sodium and chloride were measured in the plasma at the end of many of the experiments, and it was thus possible to correlate these values with the concentrations in the intestinal lumen and the absorption rate of each electrolyte. The results of these comparisons are set out in Table V.

7 Water and Salt Absorption 39 z Z P4 H Z H ~ZH Q H IY C i: z C) 1 Ci) ~v Ci). C - O~ V = * b + 4 M pq Z -E v t. 4 * rl * O ed o C~1 * P r aq * H +t * eq xo - 4 * * + -l + C C\ R t * -o H Q ZH 4 t C)O OsC) = - -- O _- _- es s 4 z4-..~. - Qz H ZHq H C) EH ae cx e o cs csi I X C>c e c: - i- - c - - z - -* cd x I"

8 4 McHardy and Parsons M t~m ^ E- m ; a * : s > s PẠ,. z I z 3o R4 sp Z Z pq S CZ ~4 9 E- =.++ S; eeqro c Pi p -4 5q ~~ ooi- ct~~4- - z E-1~ ~ ~ N.t OD oo ~ ~ ~ ) z v 4 P- E-

9 Water and Salt Absorption Control determinations of plasma Na and Cl, made on animals which were anaesthetized but not circulated, are also shown in the table. It was of interest to record the conditions under which significant absorption of Cl or Na took place when the concentration in the lumen was less than that in the plasma, the so-called "absorption against a gradient ". It will be seen from the table that this occurred from ileal segments, with Cl when fluids containing nominally 4 meq/l. Cl (Nos. 32 and 33) were circulated, and with Na when fluid containing nominally 8 meq/l. Na and 148 mm/l. mannitol was circulated. In the experiments with fluids containing around meq/l. Cl (Nos. 14 and 15, Table I) no plasma chloride determinations were made, but there was considerable absorption from both jejunum and ileum, the lumen concentration being between 7-8 meq/l. This is lower than the lowest plasma concentrations found under unfavourable circumstances when chloride was entering the lumen, so there must be a strong presumption that Cl absorption against a gradient was taking place, not only in the ileum but also in the jejunum. Hydration of the Intestinal Wall.-In a set of control experiments with animals which had been anaesthetized but not circulated, the water content of intestinal segments was found to be linearly related to the dry weight. The parameters of this relation are given in Table VI, TABLE VI.-THE PARAMETERS OF THE LINEAR RELATIONS BETWEEN THE WATER CONTENT AND THE DRY WEIGHT, AND BETWEEN THE WATER CONTENT AND WATER ABSORBED. Jejunum A. Control Segments Water content and dry weight Number of segments Mean dry weight: mg./cm ±4 Mean water content: mg./cm ± 1 Slope of regression line of closest fit: mg. water/mg. dry weight Intercept of line: mg. water/cm Result of F test for linearity.. F =2-15 (1, 8) p < *1 B. Absorbing Segments Water content and water absorbed Number of segments Slope of line of closest fit: mg. increase inwater content/ml. absorbed in 2 hours 12* ± 1 7 Intercept of line: mg. water content/mg. dry weight ±4 Result of F test for linearity.. F =48-2 (1, 4) p < 1 Ileum ± ±1* 2-51 ± ±.1 F=12-3 (1, 7) p < * ± ± F = 4-1 (1, 23) p <.1

10 42 McHardy and Parsons and it will be seen that the water content and dry weight were the same in the jejunum and ileum and that the dry weight accounted for about 25 per cent of the total weight. In segments which had been circulated there was a well-marked linear relation in both jejunum and ileum between the water content of the segment at the end of two hours' circulation and the volume of water absorbed during that time. This relation is set out in Table VI and illustrated in fig. 2. It will be seen that in both the jejunum and 4 3r3 3 E z 2 _ X NON-PERFUSED CONTROL SEGMENTS O FLUID CONTAINED 8 meq/1. Na u a , mm/i. MANNITOL $,,,, 1 mm/i. Na I mm/i. UREA C..,,,,,,..+37 mm/i. MANNITOL * , +37 mm/i. GLUCOSE O I I WATER ABSORBED:,I./mg. dry wt./2 hrs. FIG. 2.-Showing the linear relation between water content of jejunal segments and the volume of water absorbed by or entering the segments during two consecutive absorption periods. The regression line of closest fit is shown. the ileum about 1-5 per cent of the volume absorbed appeared in the intestinal wall as an increase in water content at the end of two hours. It is also apparent that the water content was highest after circulation with the half-isotonic fluid (No. 32), and it will be recalled that there was considerable absorption of water from this fluid but that Na entered the jejunum and was not absorbed in the ileum. The water content of the whole intestinal wall would thus seem to be related to the net water absorption rather than to the net absorption of Na. If it is assumed that the control segments absorbed little water, it will be seen that the vertical scatter of the control points on the graph is comparable to that of the circulated segments. DIsCussIoN As we have pointed out previously [McHardy and Parsons, 195], it is impossible to prepare a series of solutions differing only in a single

11 Water and Salt Absorption variable, if a buffer such as phosphate is used. The various absorption rates should therefore be thought of in terms of the solution circulated, rather than of any one variable. Effects of Hydrogen-ion Concentration.-The marked increases in the absorption rates of water and sodium with increasing alkalinity are immediately apparent. If the results are considered in terms of tonicity also, it is noticeable that large increases in absorption rate coincided with large differences in ph and small differences in tonicity (e.g. between solutions 11 and 12, ph difference 1 2 units; tonicity difference 1 tonicity units). It would seem probable that the absorption rates were more closely influenced by the ph of the circulating fluid than by the tonicity. There was no evidence, in the range studied, for an optimum ph for water absorption. This is similar to the findings for phosphate absorption. The absence of any increase in Na absorption between ph 7*2 and 7-9 suggests that an optimum ph may lie in this range, but in the absence of results with solutions at still higher ph values it is impossible to state this categorically. Ponz and Larralde [195] have reported an optimum for glucose absorption by rat jejunum at ph 7. The effect of the hydrogen-ion concentration on chloride absorption is obscured by the differences in chloride concentration between the various solutions, but the finding that the absorption rate was nearly twice as great at ph.7 as at ph 4.4, though the concentration was about 3 meq/l. less at the higher ph, suggests that an effect was present. The magnitude of the ph effect is striking. Parsons [195] has suggested that part of the sodium absorption in the intestine may take place through a sodium-hydrogen ionic exchange, and it might be surmised that such a process would be inhibited by increasing the H+ concentration. A comparable finding in the case of sodium is that of Schoffeniels [1955], who has shown clearly that both the influx and net transport of Na ions across frog skin are dependent upon the ph of the bathing solution on either side, both the net transport and the inward flux being reduced in the presence of increasing H-ion concentrations. The effect of decreasing ph on water movement in the present experiments could be explained as a secondary consequence of the reduced salt movement if, as is discussed below, net water absorption occurs as a result of a primairy salt absorption. Effects of Non-electrolytes: Water Absorption.-Water absorption from solutions containing different non-electrolytes varied greatly. The presence of mannitol at a concentration of 148 mm/l. almost completely inhibited water absorption from isotonic solutions in both jejunum and ileum, and at a concentration of 37 mm/l. in the presence of 1 meq/l. Na there was again almost negligible water absorption from the jejunum. These effects and the differing water absorption rates from solutions of 43

12 44 McHardy and Parsons the same tonicity and ionic composition containing glucose, urea or mannitol can be explained as osmotic effects. The important factor is the osmotic pressure of substances which cannot penetrate the mucosal cells, as opposed to the tonicity of the solution as a whole. The less readily a substance is absorbed, the more effective is its osmotic pressure in opposing water absorption. The water absorption rates in Table III can be seen to be roughly in proportion to the absorption of glucose and urea and to that of mannitol if we assume that its absorption rate is very small [Hober and Hober, 1937]. This phenomenon has been described in several animal tissues. Jacobs and Stewart [1947], noting the effect of sucrose in preventing the colloid-osmotic haemolysis of erythrocytes made abnormally permeable to cations, remark that "A substance of this sort may be effective in such low concentrations as at first sight to suggest that it must have some non-osmotic protective effect on the cell-surface". The phenomenon has been observed in kidney slices by Deyrup [1953] and in isolated rat intestine and butanol-treated leucocytes by Wilson [1954]. Both these authors point out that the tissues swell in saline solutions and shrink in solutions of non-penetrating non-electrolytes alone. The theory can be correlated with our observations in which the degree of hydration of the intestinal wall was found to be closely related to the water absorption rate, which in its turn is influenced by osmotic restraint. It is worth noting that the increase in water content in vivo was only 15 per cent of the water absorbed. This may be partly due to the efficient lymphatic drainage of the intestine, which has been shown to increase threefold in rats when isotonic saline was present in the intestine [Borgstrm and Laurell, 1953]. Nevertheless, the fact that the water content of the intestinal wall increased means that water was being absorbed more rapidly than it could be removed by the circulatory system and the lymphatics. Effects of Non-electrolytes: Salt Absorption.-The other striking finding, which can also be seen from the data in Table III, was that in the presence of 37 mm/l. mannitol the absorption of Cl, and also that of Na, was greatly reduced from solutions in the jejunum containing concentrations of these ions greater than those in the plasma. Inorder to offer an interpretation of this effect we must assume one of two possible modes of net absorption. In the first, net water absorption might be consequent upon osmotic differences set up by solute absorption. In the second, solute absorption would follow a primary water absorption process, such as entrainment of solutes in a water stream. If a mechanism of the first type is assumed, the restraint on water absorption by 37 mm/l. mannitol might be sufficient to permit the establishment of local concentration gradients (for example at the lumen border of the cells) due to primary solute uptake. Net solute transport would then

13 Water and Salt Absorption be reduced either because the solute absorption processes in the jejunum have a limited capacity for doing work against a gradient, or because of an increased " leak " down the gradient into the lumen. In comparing the jejunum and ileum, we present evidence that the ability of the jejunum to perform net absorption against a concentration gradient is limited. It has already been seen that net Cl absorption occurred "against a gradient" from solutions in the jejunum containing initially about meq/l. Cl, and in which the concentration did not rise above 8 meq/l. (Fluids 14 and 15, Table I). But these fluids were alkaline (ph 7-2 and 7 9), which may increase the capacity of the systems responsible for moving solutes or reduce the leak, and contained no non-electrolytes which might restrain water movement. It may thus be imagined that in this instance the uptake of Cl was accompanied by sufficient water to maintain in the steady state a reasonable gradient across the critical boundary. The high Na absorption rate along a favourable concentration gradient in these experinments might further assist Cl uptake by favouring water movement from the intestinal lumen. In experiments with fluids containing 8 meq/l. Na and 4 meq/l. Cl, the effect of mannitol appeared to be to reduce net entry of Na and Cl and, in the ileum, even to favour net Na uptake. In the absence of measurements of the unidirectional rates of movement to and from the intestinal contents, it is not possible to say whether this effect is due to increased activity of the process leading to salt uptake or not. Comparison of Jejunum and Ileum.-The experiments provide further details of the differing absorptive patterns of the upper and lower small intestine. The variation in activity towards glucose has been expressed in the form of gradients by Fisher and Parsons [195], but as no exact measurements of the position of the segments were made, the present results cannot be expressed in this form. In contrast to the jejunum, the ileum proved markedly superior in the absorption of Cl under all conditions studied, and especially in moving Cl ions against a concentration gradient (Table II, B), a phenomenon which is well known for dog intestine [Visscher et at., 1944]. From hypotonic solutions in the ileum there was no net entry of salt, and when the tonicity was restored with mannitol appreciable sodium absorption occurred (Table II, B). Little difference in water and sodium absorption can be found at the two sites. The large entry of sodium into hypotonic jejunal contents bears out Visscher's dictum that "The jejunum is obviously unfitted for the absorption of sodium against a concentration gradient... and the ileum is able under optimum conditions to accomplish positive net movement" [Visscher et al., 1944]. This statement was based on the results of experiments with 24Na in chronic segments in dogs, which 45

14 4 MeHardy and Parsons showed that the permeability to Na of the intestinal wall from blood to gut decreased from jejunum to colon. A similar state of affairs seems to exist in the rat. Relation between Water and Solute Absorption.-In certain experiments it was possible to estimate approximately the rate of absorption ci Ilv 'I 9 Ei I.,8 7 L L.4 c ' 3 _C -) U I ISI -1 ~~~~I I I I -5 S 1 IS 2 WATER ABSORBED: i./mg./hr. FIG. 3.-Showing the linear relation between the total solutes absorbed and the volume of water absorbed by jejunal segments. Circulated fluid, reference solution with addition of 37 mm/l. glucose (), urea (), or mannitol (o). The calculated regression line is also added. or appearance of all the important solutes in the circulating fluid. Assuming that only sodium, chloride, phosphate, potassium and bicarbonate, with glucose and urea when present, were involved in the exchanges across the intestinal wall and that mannitol was not absorbed, the algebraic sum of the absorption rates could be taken and an estimate obtained of the net gain or loss of solutes to the animal. This was expressed in"jzosm/mg./hr., assuming unit ionic activities.

15 Water and Salt Absorption A statistically significant (p <.1) linear relation was found between the total absorption of solutes and the absorption rate of water in each set of experiments. An example of this is shown in fig. 3, in which the absorption rates found in the jejunum with fluids containing 37 mm/l. of added glucose, urea or mannitol are plotted together. It can be seen that the points lie around a straight line passing close to the origin, the position along the line depending on the non-electrolyte added. The slope of the line of closest fit in this relation was 331 ±2 mosm/l. and represents the apparent concentration at which the solutes were absorbed. It agrees well with the nominal concentration of solutes in the lumen (342 mosm/l.). This observation means that the fluid leaving the intestine was approximately isotonic with that in the lumen. We have already described two possible modes of net absorption, in which either solute or water absorption was the primary process. Only if the secondary process lagged appreciably behind the primary process in either instance could the net fluid leaving the intestine have a tonicity different from that in the lumen. It is thus impossible from the data to distinguish the two modes of absorption. Either might occur, with the secondary absorption rapidly following the primary process. Further interpretation must require that the measurements of net absorption be resolved into components representing movement in each direction across the intestinal mucosa. SUMMARY 1. The net absorption rates of water and sodium, measured in vivo in the jejunum and ileum of the rat, were greater from alkaline than from acid solutions, over the ph range 4f4-7f2. 2. Chloride was absorbed more rapidly from the ileum than from the jejunum under all conditions studied. 3. Chloride was absorbed by the ileum from solutions containing between 2-5 meq/l. less than plasma. Sodium was absorbed by the ileum from isotonic solutions containing between meq/l. less than plasma, but no absorption occurred from hypotonic solutions. 4. The decreasing absorption rates of water from similar solutions containing glucose, urea or mannitol are accounted for in terms of relative osmotic restraint on absorption. 5. The water content of the intestinal wall was found to be linearly related to the dry weight of the segment and to the amount of water absorbed by the segment. About 1'5 per cent of this amount remained in the intestinal wall after two hours circulation.. The net uptake of Na and Cl from isotonic fluids and the net entry into hypotonic fluids in jejunal segments were both reduced when mannitol was added to the solutions. 47

16 48 McHardy and Parsons 7. With solutions containing 1 meq/l. sodium in the jejunum, the fluid absorbed was approximately isotonic with that in the lumen. ACKNOWLEDGMENT We wish to thank Christine Wills for skilled assistance in these experiments. REFERENCES BOROSTRM, B. and LAURELL, C. B. (1953). "Studies on lymph and lymph-proteins during absorption of fat and saline by rats", Acta physiol. scand. 29, DEYRUP, I. (1953). "A study on the fluid uptake of rat kidney slices in vitro", J. gen. Physiol. 3, FISHER, R. B. and PARsoNs, D. S. (195). "Glucose absorption from surviving rat small intestine", J. Physiol. 11, FULLERTON, P. M. and PARSONS, D. S. (195). "The effects of anaesthetics on hexose and water absorption from rat intestine in vivo ", Quart. J. exp. Physiol. 41, HBER, R. and HBER, J. (1937). "Experiments on the absorption of organic solutes in the small intestine of rats", J. cell. comp. Physiol. 1, HULME, A. C. and NARAIN, R. (1931). "The ferricyanide method for the determination of reducing sugars. A modification of the Hagedorn-Jensenanes technique", Biochem. J. 25, JACOBS, M. H. and STEWART, D. R. (1947). "Osmotic properties of the erythrocyte. XII. Ionic and osmotic equilibria with a complex external solution", J. cell. comp. Physiol. 3, McHARDY, G. J. R. and PARSONS, D. S. (195). "The absorption of inorganic phosphate from the sinall intestine of the rat", Quart. J. exp. Physiol. 41, PARsONs, D. S. (195). "The absorption of bicarbonate-saline solutions by the small intestine and colon of the white rat", Quart. J. exp. Physiol. 41, PONZ, F. and LARRALDE, J. (195). "La absorcion de azuicares en function del ph intestinal", Rev. esp. Fisiol., SCHOFFENIELS, E. (1955). "Influence du ph sur le transport actif de sodium a travers la peau de grenouille", Arch. Tnt. de Physiol. et Bio. 3, SENDROY, J. (1937). "Microdetermination of chloride in biological fluids, with solid silver iodate", J. biol. Chem. 12, VISSCHER, M. B., VARCO, R. H., CARR, C. W., DEAN, R. B. and ERICKSON, D. (1944). "Sodium ion movement between the intestinal lumen and the blood", Amer. J. Physiol. 141, WILSoN, T. H. (1954). "Ionic permeability and osmotic swelling of cells", Science, 12,