PK: PNa is 1-26: 1, and is probably higher for the serosal membrane. cells of rat jejunum together with transmural potentials were recorded

Transcription

1 J. Phygiol. (1972), 227, pp With 8 text-figure8 Printed in Great Britain IONIC BSIS OF MEMBRNE POTENTILS OF EPITHELIL CELLS IN RT SMLL INTESTINE BY R. J. C. BRRY ND JCQUELINE EGGENTON From the Department of Physiology, University of Sheffield, S1 2TN (Received 8 May 1972) SUMMRY 1. Potentials across the mucosal and serosal membranes of the epithelial cells of rat jejunum together with transmural potentials were recorded using everted sac preparations. 2. Ionic changes in either mucosal or serosal fluids affect mucosal or serosal membrane potentials respectively with comparable changes in the transmural potential. The contralateral membrane potential is relatively unaffected. 3. Replacement of mucosal sodium chloride by potassium chloride or lithium chloride had little effect on potentials, but its replacement by mannitol or Tris chloride increased the negativity of the mucosal potential, giving linear relationships against loglo[na]m with slopes of 41-4 and 37 mv respectively for tenfold change in [Na]m. 4. t constant [Na]m, potassium or lithium increased the mucosal potential by 25-7 and 19-8 mv respectively for tenfold concentration changes. 5. Qualitatively similar changes occurred in the serosal potential when the ionic composition of the serosal fluid was varied. 6. Mucosal potential changes in response to modifications of the ionic composition of the mucosal fluid were the same in the presence and absence of galactose. 7. Sodium and potassium diffusion potentials largely determine both the mucosal and serosal membrane potentials. For the mucosal membrane, PK: PNa is 1-26: 1, and is probably higher for the serosal membrane. Chloride makes no significant contribution to membrane potentials. 8. Potentials generated by the electrogenic sodium pump are superimposed on diffusion potentials across the serosal membrane.

2 218 R. J. C. BRRY ND JCQUELINE EGGENTON INTRODUCTION In the previous paper the mechanisms involved in the production of the transfer potential across the small intestine in the presence of actively transferred sugars and amino acids were investigated by measuring the changes in the membrane potentials induced by these substances (Barry & Eggenton, 1972). The small intestine also maintains a potential difference in the absence of actively transferred solutes and this potential is dependent upon the ionic environment of the tissue, a sodium diffusion potential appearing to be an important component of this transmural potential (Baillien & Schoffeniels, 1961; Wright, 1966; Barry, Eggenton, Smyth & Wright, 1967). The effects of changes in the ionic environment on the potentials across the mucosal and serosal membranes ofthe epithelial cell were measured to assess the role of sodium and other ions in the generation of these potentials. METHODS The experimental technique employed has been described in the previous paper (Barry & Eggenton, 1972). Since the serosal volume is small (.5 ml.) transfer and metabolic products could rapidly alter the composition of the serosal fluid. When the effects of changes in the ionic constitution of the serosal fluid were investigated the serosal fluid was perfused at.8 ml./min through a modified cannula (Fig. 1), using an injection apparatus (Palmer Ltd). Samples of perfusate could be collected for direct estimation of ion concentrations. In order to accommodate the longer cannula a larger glass vessel, containing 2 ml. mucosal fluid, had to be used. In experiments in which the serosal fluid was changed, the diffusion barrier due to the layers of tissue behind the epithelial cell was reduced by removing the external muscle layers before everting the intestine. Physiological 8aline8 The physiological salines used were based on Krebs bicarbonate saline (Krebs & Henseleit, 1932) and were gassed with 95 % 2/5 % C2. Changes in sodium ion concentration were accomplished by replacing all or part of the sodium chloride of the saline with an equivalent amount of an isotonic solution of the replacement substance. Potassium chloride, lithium chloride, mannitol and Tris chloride were used to replace sodium chloride. If a sodium concentration of less than 25 m-equiv/l. was required Tris bicarbonate was used in place of sodium bicarbonate. In some experiments the effects of various concentrations of potassium chloride and lithium chloride were studied whilst maintaining the sodium concentration constant at 25 m-equivfl. Isotonic sodium chloride of Krebs bicarbonate saline was replaced by an equal volume of a mixture of isotonic mannitol with either isotonic potassium or isotonic lithium chloride. The ratios of the volumes of potassium or lithium chloride to mannitol were adjusted to give the required range of potassium or lithium concentrations. sulphate saline was produced by replacing sodium chloride and potassium chloride with sodium and potassium sulphate. If, in addition, the sodium concentration was to be changed Tris sulphate was used to replace sodium sulphate. Con-

3 IONS ND INTESTINL POTENTILS centrations of ions in mucosal and serosal fluids are denoted by square brackets with the subscripts m and s respectively. Estimation8 Sodium and potassium concentratiods were estimated by flame photometry using an EEL flame photometer (Evans Electroselenium Ltd). Micro-electrode Salt bridges.-..eo Serosal perfusate -5% 2/5% C2 219 Mucosal. _ d -_. - fluid.*dm Flow of serosal fluid - I~~~~O- Intestine Cannula Fig. 1. Diagram to show the arrangement of the apparatus used for recording membrane potentials in rat jejunal epithelial cells when the ionic composition of serosal fluid was changed. block halfway along the cannula forced the fluid to flow directly over the 'serosal' surface of the gut as shown in detail in lower diagram. RESULTS Changes in the ionic composition of the mucosal fluid When mucosal sodium was partially replaced by potassium or lithium there were no dramatic changes in the membrane or transmural potentials (Fig. 2a and b). s the sodium ion concentration decreased the mucosal potential also decreased slightly. The serosal potential was lowered as lithium replacement rose but was unaffected on potassium replacement. However, in this type of experiment the concentrations of two cations are changed simultaneously, in that the decrease in the sodium concentration is matched by an equivalent increase in potassium or lithium concentration. The contribution of one individual cation to the membrane potential could be masked by concomitant potential changes resulting from variations in the concentration of a second cation.

4 22 R. J. C. BRRY ND JCQUELINE EGGENTON E u C 2 r- 1 F a 2 r b 1 - CL a- -1 I L T I I 25 so 1 15 [Na]m (m-equiv/1.) Fig. 2. Effect on membrane potentials of replacing sodium chloride in the mucosal fluid with potassium chloride (a) or lithium chloride (b). The potentials are plotted against log1o[na]m and each point represents the mean of twelve observations taken from four preparations. The mucosal potential is denoted by the symbol, the serosal potential by, and the transmural potential by *. 2r- 1_~ 1 1 i/-i 25 5 [Na]m (m-equi )/ 15 C (U t- -o -2 _ a -4 Fig. 3. Effect on membrane potentials of replacing sodium chloride in the mucosal fluid with mannitol. The potentials are plotted against log1o[na]. and each point represents the mean of twelve observations taken from four preparations. The mucosal potential is denoted by the symbol, the serosal potential by, and the transmural potential by *. To investigate the individual effects of cations experiments were first carried out in which- sodium chloride was replaced by mannitol (Fig. 3). The effects of mannitol replacement were completely different from the

5 IONS ND INTESTINL POTENTILS 221 effects of potassium or lithium replacement. s the sodium concentration in the mucosal fluid was reduced the mucosal potential increased in negativity and a linear relationship between the mucosal potential and loglo [Na]m was obtained, with a slope of 414 mv for a tenfold change in the mucosal sodium concentration. s the serosal potential was little affected, the changes in the transmural potential follow the pattern observed for the mucosal potential. 21- E U C. C E- -2 _- I 5 1 I [Na]m (m-equiv/1.) / 5 I)i. -- M -4 _- Fig. 4. Effect on membrane potentials of replacing sodium chloride in the mucosal fluid with Tris chloride. The potentials are plotted against loglo[na]m and each point represents the mean of twelve observations taken from four preparations. The mucosal potential is denoted by the symbol, the serosal potential by. and the transmural potential by *. The replacement of sodium chloride by mannitol leaves open the question that altering the chloride concentration may affect the measured membrane potentials. This was examined in experiments in which sodium chloride was replaced by Tris chloride (Fig. 4) and the results were similar to those obtained with mannitol replacement over the same range of sodium concentrations ( m-equiv/l.). gain a straight line relationship was found between logl[na]m and the mucosal potential, but its slope

6 222 R. J. C. BRRY ND JCQUELINE EGGENTON was rather less, being 3-7 mv for a tenfold change in sodium concentration. This linearity did not hold at sodium concentrations below 25 m- equiv/l., there being little further change in the mucosal potential under these conditions. s before there was little variation in the serosal potential and the transmural potential again reflected alterations in the mucosal potential. 2 ~~~~~~ ~~~~~ E [K]m (m-equiv/i.) C W -2-4 Fig. 5. Effect on membrane potentials of mucosal potassium concentration when the sodium concentration was maintained at 25 m-equiv/l. The potentials are plotted against loglo [K]m and each point represents the mean of twelve observations taken from four preparations. The mucosal potential is denoted by the symbol, the serosal potential by, and the transmural potential by *. The differences in the slopes of the lines obtained with mannitol and Tris chloride replacement of sodium chloride are unlikely to be due to the presence of chloride in the latter case. In the presence of this ion it would be expected that the slope of the line would be increased, rather than decreased, as chloride diffused down its concentration gradient into the cell. However, to test this further the replacement of sodium with Tris was repeated using a sulphate saline. Linearity was again obtained, the slope of 29-2 mv for the mucosal potential being close to that found using chloride saline. Thus it appears that chloride does not make a significant contribution to the mucosal potential and the discrepancy between the slopes obtained with mannitol and Tris chloride replacement is due to the Tris ion being able to enter the cell to a limited extent and thereby lowering slightly the mucosal potential.

7 IONS ND INTESTINL POTENTILS 223 The results obtained with mannitol or Tris chloride replacement ofsodium chloride both point to a significant role of sodium in the production of the mucosal potential. Since the replacement of sodium with potassium or lithium has little effect on the potential across the mucosal membrane it is possible that these two ions can act as effective substitutes for sodium in this respect. To investigate the specific effects of these ions experiments were undertaken in which the potassium or lithium concentration in the mucosal fluid was altered without the added complication of a changing - 2 o o E I [Li]m (m-eq uiv/i.) / ^ ' -2-4 Fig. 6. Effect on membrane potentials of mucosal lithium concentration when the sodium concentration was maintained at 25 m-equiv/l. The potentials are plotted against loglo[li]. and each point represents the mean of twelve observations taken from four preparations. The mucosal potential is denoted by the symbol, the serosal potential by. and the transmural potential by *. mucosal sodium concentration which was maintained constant at 25 m- equiv/l. Under these conditions it was found that as the mucosal potassium or lithium concentration increased the mucosal potential decreased and this change was linearly related to log1 [K]m or log1 [Li]m respectively (Figs. 5 and 6). In both cases the serosal potential was not significantly changed. tenfold increase in mucosal potassium concentration decreased the mucosal potential by 25-7 mv and lithium gave a decrease of 19-8 mv. Changes in the ionic composition of the serosal fluid The situation at the serosal membrane is complicated by two factors. One is the considerable barrier to diffusion represented by the layers of tissue behind the epithelial cell and the other is the postulated existence of 8 PHY 227

8 224 R. J. C. BRRY ND JCQUELINE EGGENTON an electrogenic sodium pump at this membrane. To reduce the diffusion barrier stripped sacs (with the outer muscle layer removed) were used and the serosal fluid was perfused through the gut in order to minimize alterations in the serosal ion concentrations. sample of perfusate was collected immediately after the membrane potentials were recorded and the relevant ion concentrations were measured. 2 2 b > ~~~~~ E 1,._ \ W ~~~ X [Na]5 (m-equiv/i.) -1 \ -1 i Fig. 7. Effect on membrane potentials of replacing sodium chloride in the serosal fluid with potassium chloride (a) or mannitol (b). The potentials are plotted against log14[na]. and each point represents the mean of twelve observations taken from three preparations. The mucosal potential ig denoted by the symbol L, the serosal potential by, and the transmural potential by *. The electrogenic sodium pump which is thought to exist at the serosal membrane makes interpretation of the results difficult because it contributes to the potential recorded across this membrane. However, it is possible to show the relative importance of various ions in the production of the serosal potential. The effect of the serosal sodium concentration on the membrane potentials was studied by replacing varying amounts of the sodium chloride of the serosal fluid with an equivalent amount of potassium chloride (Fig. 7a). In these experiments the mucosal potential was unaffected'and changes in the transmural potential reflected changes in the serosal potential. s the sodium ion concentration was lowered the serosal potential was reduced slightly, but sufficient to reverse the polarity of the transmural potential at low sodium concentrations. Since in these experiments a fall in the serosal sodium concentration

9 225 IONS ND INTESTINL POTENTILS was accompanied by a rise in the serosal potassium concentration the effect of sodium alone was studied by replacing varying amounts of sodium chloride in the serosal fluid with an equivalent amount of isotonic mannitol (Fig. 7b). s the serosal sodium concentration decreased there was a rise in the serosal potential which was correlated with an increased transmural potential. The mucosal potential remained unchanged. When 2 ~ ~ ~~ 2!, 'El 1 - U C f15 [K], (m-equiv/i.) -1 _ Fig. 8. Effect on membrane potentials of serosal potassium concentration when the sodium concentration was maintained at 25 m-equiv/l. The potentials are plotted against loglo[k]8 and each point represents the mean of twelve observations taken from three preparations. The mucosal potential is denoted by the symbol, the serosal potential by, and the transmural potential by *. plotted against log1o [Na]s the increase in the serosal potential was linear and for a tenfold change in serosal sodium concentration there was a potential change of 13-2 mv. In order to study the effect of chloride these experiments were repeated using a sulphate saline and replacing sodium sulphate with mannitol in the serosal fluid. Under these conditions a tenfold change in sodium concentration produced a change of 16-6 mv in the serosal potential. The effect of potassium alone was studied by altering the serosal potassium concentration while keeping the serosal sodium concentration constant at 25 m-equiv/l. (Fig. 8). The serosal potential decreased as the 8-2

10 226 R. J. C. BRRY ND JCQUELINE EGGENTON serosal potassium concentration increased and above 17-4 m-equiv/l. was linearly related to log1o [K]i with a slope of 17-3 mv for a tenfold change in potassium concentration. The effect of galactose on the permeability characteristics of the mucosal membrane When incubated in normal Krebs bicarbonate saline the transfer potential induced by hexoses such as galactose results entirely from changes in the serosal potential, the mucosal potential being unaffected (Barry & Eggenton, 1972). The possibility that this pattern might be changed when the ion concentrations in the incubating fluid were varied was investigated. TBLE 1. Mucosal potential changes obtained for tenfold alterations in the concentration of individual cations in the mucosal fluid in the absence and presence of galactose (28 mm) Galactose Galactose Replacement P.d. related absent present substance to (mv) (mv) Mannitol [Nalm TrisCl [Na]m KCl (Na _ 25) [K]m LiCl (Na = 25) [Li]m The relationship between the mucosal potential and the ionic composition of the mucosal fluid in the presence of galactose was compared with that obtained in the absence of this hexose (Table 1). Straight line relationships between the mucosal potential and loglo[na]m, loglo [K]m or loglo [Li]m, were also obtained in the presence of galactose, having the same slopes as were obtained in the absence of galactose. The effect of galactose on the permeability characteristics of the serosal membrane cannot be assessed as the active transport of this sugar stimulates the activity of the electrogenic sodium pump at this membrane. Thus galactose alters the serosal potential by increasing the contribution of the sodium pump to the potential recorded. It is not possible to assess the magnitude of the contribution of the pump which may vary as the ionic composition of the serosal fluid is altered. DISCUSSION Changes in the mucosal and serosal membrane potentials and transmural potentials resulting from modifications ofthe ionic gradients between the cell and the bathing fluids enable a number of suggestions to be made about the ionic basis of these membrane potentials. The small changes in both mucosal and serosal potentials on replacing

11 IONS ND INTESTINL POTENTILS 227 mucosal sodium with either potassium or lithium (Fig. 2a and b) suggest that the mucosal membrane has a similar permeability to these monovalent cations. The slight decrease in the serosal potential on replacing mucosal sodium with lithium could result from a lower ability of the serosal sodium pump to move lithium out of the cell, analogous to the lithium effect in frog skin (Zerahn, 1955). When sodium chloride in the mucosal fluid was replaced with mannitol, the increase in the mucosal potential bore a linear relationship to the loglo[na]m giving a potential change of 41-4 mv for a tenfold change. If the membrane had behaved as a sodium electrode, a potential change of 61-5 mv would have been predicted from the Nernst equation. It therefore appears that asodium diffusionpotential is a major component ofthe mucosal membrane potential, although it is not solely responsible for it. similar dependence of the mucosal potential on the [Na]m has been observed in rat ileum (McKenney, 1969) and rabbit ileum (Rose & Schultz, 1971). The discrepancy between the experimental observations and the theoretical potential changes for a sodium electrode suggests that other ions may be involved in the generation of the mucosal potential. ny significant contribution of chloride ions to the mucosal potential seems to be excluded since the slope obtained with Tris sulphate replacement (29.2 mv) was close to that with Tris chloride replacement (3.7 mv). The difference in the slopes obtained in the mannitol and Tris chloride replacement experiments must therefore be attributed to the ability of the Tris ion itself to enter the intestinal epithelial cell to a limited extent, hence causing a reduction in the mucosal potential. Reducing the mucosal sodium concentration to less than 25 m-equiv/l. caused little further reduction in the mucosal potential. The reason for this is not clear but it is possible that the permeability characteristics of the mucosal membrane may be altered when the sodium concentration is low. It has been observed that decreasing the sodium concentration causes the rat small intestine to become less permeable to acetamide and thiourea (Esposito, Faelli & Capraro, 1969). Under normal conditions the intracellular sodium concentration is lower than that in the mucosal fluid so that if the movement of sodium was entirely responsible for the mucosal potential the cell contents would be expected to be positive with respect to the mucosal fluid. Since, this is not the case the contribution of other ions must be postulated. When the sodium concentration was held constant at 25 m-equiv/l. the mucosal -potential was found to be linearly related to log1o[k]m (Fig. 5) giving a potential change of 25-7 mv for a tenfold change. It is suggested that diffusion potentials arising from the unequal distribution of both sodium and potassium ions between the intracellular compartment and the

12 228 R. J. C. BRRY ND JCQUELINE EGGENTON mucosal fluid contribute to the mucosal membrane potential. Their relative contributions will depend on the activities of the two ions inside the cell and in the mucosal fluid and the relative permeability of the mucosal membrane. This relationship can be described by the Hodgkin & Katz (1949) modification of the Goldman (1943) constant field equation: Em RT1 [Na]m+b[K]m Em = -F oge [Na]i+b[K]j where m and i are mucosal and intracellular respectively and b is the permeability coefficient of K relative to Na. R, T, z and F have their usual significance. Using values for intracellular sodium and potassium concentrations of 44 and 129 m-equiv/l. respectively (P. G. Kite, personal communication), with normal Krebs bicarbonate saline as the mucosal fluid and a mucosal potential, Em, of - 92 mv, b is calculated to be Thus the mucosal membrane of the intestinal epithelial cell is permeable to both sodium and potassium ions, being slightly more permeable to potassium. This relatively high permeability of the mucosal membrane is not consistent with the low potassium fluxes which have been observed across the dog small intestine (Phillips & Code, 1966). This discrepancy could be explained if a mechanism for the uptake of potassium into the cell exists at the mucosal membrane. The presence of a sodium, potassiumdependent TPase in the brush border of intestinal epithelial cells (Berg & Chapman, 1965) suggests that a sodium/potassium exchange pump could be present here. The relationship between the mucosal potential and the ionic composition of the mucosal fluid was not affected by the presence of galactose (Table 1). Thus actively transported hexoses do not seem to alter the permeability characteristics of this membrane. The effects of altering the ionic concentrations on the serosal side of the intestine are more difficult to evaluate. There are two main reasons for this. The first is the lack of direct accessibility of the serosal face of the cell due to the layers of tissue behind the epithelial cells. Removing the outer muscle layers may reduce this diffusion barrier but certainly does not eliminate it as considerable subepithelial tissue still remains. The second factor is the presence of an electrogenic ion pump at this membrane. By definition this pump must contribute to the potential across the serosal membrane and the magnitude of its contribution may change as the ionic composition of the serosal fluid is altered. Thus the relative contributions of diffusion potentials and the sodium pump to the actual potential recorded are difficult to assess under these conditions. Changing either the sodium or potassium concentration of the serosal fluid independently caused a smaller change in the serosal potential (Figs. 7 b and 8) than the

13 IONS ND INTESTINL POTENTILS 229 effect of similar alterations in the mucosal fluid on the mucosal potential (Figs. 3 and 5). These differences could be due to the remaining subepithelial tissue which acts as a diffusion barrier so that the ionic composition of the fluid adjacent to the serosal membrane is not the same as that of the bulk phase. The composition of the fluid at the serosal pole of the epithelial cell will be determined by the rate of diffusion of ions through the subepithelial tissue and also by the activity of the cell itself. lternatively another ion could be making a significant contribution to the serosal potential. However, the fact that the serosal potential is linearly related to log1o [Na]8 and log1o [K]8 suggests that sodium and potassium diffusion potentials make an important contribution to this potential. When serosal sodium was replaced with potassium (Fig. 7a) there was a greater change in the serosal potential than that in the mucosal potential on replacing mucosal sodium with potassium (Fig. 2a). Thus the relative permeability of potassium to sodium must be greater at the serosal side of the cell. This membrane appeared to be more permeable to chloride than the mucosal membrane, although its permeability to this anion was small compared with its permeability to sodium and potassium. The Hodgkin-Katz modification of the Goldman equation is based on the assumption that any active movement of ions across the membrane is non-electrogenic. However, the sodium pump that is thought to exist at the serosal membrane of the epithelial cell is electrogenic and therefore contributes to the potential recorded across this membrane. Thus it is not possible to apply this equation to the serosal potential. Changes in the serosal potential caused by alterations in the ionic composition of the serosal fluid have no effect on the potential across the mucosal membrane. This is in contrast to the situation that exists in the proximal tubule of the kidney where a change in the ionic composition of the peritubular fluid results in an alteration in the potential across the luminal membrane of the cell as well as that across the peritubular membrane (Boulpaep, 1968). Thus it appears on the basis of these observations that rat jejunum does not possess a significant shunt pathway. This is in contrast to the situation that is thought to exist in rabbit ileum (Rose & Schultz, 1971) and bullfrog small intestine (White & rmstrong, 1971). This difference between these species may partially account for the different behaviour observed in the presence of actively transferred sugars and amino acids. From this investigation the following picture emerges. cross both faces of the epithelial cell diffusion potentials exist which involve monovalent cations, and depend only on the permeability of the membrane to a particular ion. The principal ions involved in both mucosal and serosal potentials are sodium and potassium. The mucosal membrane is slightly more

14 23 R. J. C. BRRY ND JCQUELINE EGGENTON permeable to potassium than to sodium, the permeability ratio between potassium and sodium being 1P26: 1. It is probable that this ratio is greater at the serosal side of the cell. Superimposed on the diffusion potentials across the serosal membrane is a potential generated by the electrogenic sodium pump which transfers sodium from the cell to the serosal fluid. The increased activity of this pump induced by the presence of actively transported sugars and amino acids causes the increase in the serosal potential that results in the transfer potential. This view of the rat intestinal epithelial cell is similar to that held by Gilles-Baillien & Schoffeniels (1967), working with the isolated epithelium of the tortoise. They found similar changes on lowering the sodium concentration in either mucosal or serosal fluid to those found in the present study. However, in tortoise intestine chloride appears to be important in the production of both mucosal and serosal potentials whereas in rat the relative permeability of either side of the cell to this ion is low. Gilles- Baillien & Schoffeniels also found that the changes in membrane potentials induced by low sodium concentrations could be reversed by potassium. They concluded that the cells were permeable to potassium if the sodium concentration was low. similar phenomenon was observed in this investigation, but the fact that potassium can replace sodium in maintaining the membrane potential is interpreted as evidence for a high relative permeability to potassium. This is supported in the case of the mucosal membrane by the value for the relative potassium permeability coefficient calculated from the Goldman equation. The intestinal epithelial cell seems to differ from other epithelial cells in that the permeability of its mucosal and serosal membranes to sodium and potassium is very similar. This is probably the factor responsible for the low membrane potentials observed in this tissue under normal conditions. The authors are pleased to acknowledge Miss. M. Comley for her skilled technical assistance. During the course of this investigation one of us (J. E.) was in receipt of a Scholarship for Training in Research Methods from the Medical Research Council. REFERENCES BILLIEN, M. & SCHOFFENIELS, E. (1961). Origine des potentials bio6lectrique de l'epith6lium intestinal de la tortue grecque. Biochim. biophy8. cta 53, BRRY, R. J. C. & EGGENTON, J. (1972). Membrane potentials of epithelial cells in rat small intestine. J. Physiol. 227, BRRY, R. J. C., EGGENTON, J., SMYTH, D. H. & WRIGHT, E. M. (1967). Relation between sodium concentration, electrical potential and transfer capacity of rat small intestine. J. Physiol. 192, BERG, G. G. & CHPMN, B. (1965). The sodium and potassium activated TPase of intestinal epithelium. I. Location of enzymatic activity in the cell. J. cell. comp. Physiol. 65,

15 IONS ND INTESTINL POTENTILS 231 BOuLPEP, E. L. (1968). Ion permeability of the peritubular and luminal membrane of the renal tubular cell. In Symposium fiber Transport und Funktion intracelluldrer Elektrolyte, ed. K.RCK, F., pp Munich: Urban und Schwarzenberg. ESPOSITO, G., FELLI,. & CPRRO, V. (1969). Effect of sodium on passive permeability of non-electrolytes through the intestinal wall. Experientia 25, 63. GILLES-BILLIEN, M. & SCHOFFENIELS, E. (1967). Bioelectric potentials in the intestinal epithelium of the Greek tortoise. Comp. Biochem. Physiol. 23, GOLDMN, D. E. (1943). Potential, impedance and rectification in membranes. J. gen. Physiol. 27, HODGKIN,. L. & KTZ, B. (1949). The effect of sodium ions on the electrical activity of the giant axon of the squid. J. Physiol. 18, KREBS, H.. & HENSELEIT, K. (1932). Untersuchungen uiber die Harnstoffbildung im Tierkorper. Hoppe-Seyler's Z. physiol. Chem. 21, McKENNEY, J. R. (1969). Effects of Na+ concentration on microelectrode voltage measurements with rat intestine mucosa. Physiologist, Wash. 12, 299. PHILLIPS, S. F. & CODE, C. F. (1966). Sorption of potassium in the small and the large intestine. m. J. Physiol. 211, ROSE, R. C. & SCHULTZ, S. G. (1971). Studies on the electrical potential profile across rabbit ileum. Effects of sugars and amino acids on transmural and transmucosal electrical potential differences. J. gen. Physiol. 57, WHITE, J. F. & RMSTRONG, W. McD. (1971). Effect of transported solutes on membrane potentials in bullfrog small intestine. m. J. Physiol. 221, WRIGHT, E. M. (1966). Diffusion potentials across the small intestine. Nature, Lond. 212, ZERHN, K. (1955). Studies on active transport of Li+ in isolated frog skin. cta physiol. scand. 33,