Ion transport in rabbit ileal mucosa. I. Na and Cl fluxes and short-circuit current

Electrical potential profile in rabbit ileum: role of rheogenic Na transport

The electrical potential profile of rabbit ileum was investigated in vitro with the microelectrode technique. The transmural electrical potential difference (PD), designated psims, was immediately reduced by 60% upon cooling the tissue from 37 to 7 degrees C; the PD across the mucosal membrane (transmucosal PD, psimc) was simultaneously reduced by 37%. These electrical changes could not be attributed to alternations in either transmembrane ion concentration gradients or total tissue conductance. The psimc and psims may have substantial values even after the concentration gradients of Na and K across the cell membane are eliminated, provided that active transport mechanisms are still operative. Conversely, in the presence of approximately normal transmembrane ion concentration gradients, but when active transport mechanisms have been inhibited. psimc is reduced by 45% and psims is zero. These observations are consistent with a model of electrolyte transport in which psims and the normal transmembrane cation concentration gradients are established by rheogenic active transport of Na out of the cell. The psimc is generated both by rheogenic active Na transport and by cation concentration gradients which exist across the cell membrane. The Koefoed-Johnsen and Ussing model (Acta Physiol. Scand., 1958, vol. 42, p. 298) of electrolyte transport by epithelial cells does not adequately describe the electrical properties of ileum.

Sugars and sugar derivatives which inhibit the short-circuit current of the everted small intestine of the rat

1. The short-circuit current of everted rat intestine supported on a perforated cannula proved to be stable for up to 3 hr and has been used to study competition between transportable and non-transportable sugars. 2. 4,6-O-Ethylidene-alpha-D-glucopyranose (ethylidene glucose) and 4,6-O-benzylidene-e alpha-D-glucopyranos (benzylinene glucose), two nontransportable inhibitors of the hexose transfer system in human erythrocytes, were found to reduce the short-circuit current generated by transportable sugars such as galactose or 3-O-methyl glucose. 3. These compounds were also found to reduce the basal short-circuit current established by the everted intestine in a sugar-free Krebs solution. Both types of inhibition approached saturation at the higher concentrations used. 4. Similar inhibitory properties were shown by mannose, a non-actively accumulated monosaccharide, and by the beta-disaccharides lactose and cellobiose. 5. It is suggested that this common pattern of behaviour is due to the ability of these compounds to react with the sites for active hexose transfer but without translocation by the system. The significance of the inhibition of the basal short-circuit current is briefly discussed in this context.

Ion transport by rabbit colon. I. Active and passive components

Descending rabbit colon, stripped of muscularis externa, absorbs Na and Cl under short-circuit conditions and exhibits a residual ion flux, consistent with HCO3 secretion, whose magnitude is approximately equal to the rate of active Cl absorption. Net K transport was not observed under short-circuit conditions. The results of ion replacement studies and of treatment with ouabain or amiloride suggest that the short-circuit current ISC is determined solely by the rate of active Na transport and that the net movements of Cl and HCO3 are mediated by a Na-independent, electrically-neutral, anion exchange process. Cyclic AMP stimulates an electrogenic Cl secretion, abolishes HCO3 secretion but does not affect the rate of Na absorption under short-circuit conditions. Studies of the effect of transepithelial potential difference on the serosa-to-mucosa fluxes Jism of Na, K and Cl suggest that JNasm,JIsm and one-third of JCl-sm may be attributed to ionic diffusion. The permeabilities of the passive conductance pathway(s) are such that Pk:PNa:PCl= 1.0:0.07:0.11. Electrolyte transport by in vitro rabbit colon closely resembles that reported from in vivo studies of mammalian colon and thus may serve as a useful model for the further study of colonic ion transport mechanisms.

Electrophysiological study of isolated perfused human collecting ducts: Ion dependency of the transepithelial potential difference

Cortical and outer medullary collecting duct segments were dissected from human kidneys and perfused in vitro. The transepithelial potential difference was measured and found to be lumen positive +6.8 +/- 0.6 mV (n= 20). This lumen-positive potential difference was inhibited by ouabain and furosemide but not by acetazolamide. Replacement of chloride in bath and perfusion fluids caused a reversible decrease of the potential difference to near zero. We conclude from these studies: (a) the lumen-positive potential difference is dependent upon the presence of chloride ion suggesting the existence of an active electrogenic chloride reabsorptive process in the human collecting duct and (b) it is possible to examine human renal physiology directly using in vitro microperfusion of tubule segments.

Effect of intraluminal bicarbonate and chloride on fluid absorption by the rat renal proximal tubule

In order to study mechanisms of fluid transport in the rat renal proximal convoluted tubule, the effects of large variations in intraluminal HCO3- and Cl- concentrations were measured by microperfusion techniques. No differences in rates of fluid transport were found when intraluminal HCO3- was varied from 4 to 30 mEq/liter and Cl- from 146 to 120 mEq/liter. Inhibition of H+ secretion with benzolamide had no effect on fluid absorption when little or no HCO3- was present in the lumen, but did reduce fluid transport when 25 mEq of HCO3- was present. If several different mechanisms are responsible for proximal fluid transport, such as nonelectrogenic active NaHCO3 transport, passive chloride diffusion and active sodium transport linked to H+ secretion, the above observations imply that they all operate at approximately the same rate, since the dominant driving force would have been different with each perfusion solution. The data seem more compatible with the view that active sodium transport is the major driving force for fluid absorption in the proximal tubule, that this is not linked to H+ secretion and that anions modify the rate of absorption only to the degree that they are able to accompany sodium across the epithelium. An additional observation was that absorption of isotonic NaCl was very slow in short segments of tubule, as compared to HCO3--containing perfusion solutions. Although the mechanism is uncertain, these data suggest that a finite amount of intraluminal HCO3- is necessary for optimal proximal fluid transport.

Active secretion of calcium, sodium and chloride by adult rat duodenum in vitro

Active secretion of Ca2+ is observed from the serosal to the mucosal surface across adult rat duodenum in vitro when absorptive Ca2+ flux is saturated by a high [Ca2+]. Sodium and chloride are spontaneously secreted by this tissue with Cl secretion apparently accounting for about one-third of the short-circuit current when there is no absorptive co-transport of Na+.

Characteristics of sodium flux from serosa to mucosa in rabbit ileum

Sodium flux from serosa to mucosa, J(sm) (Na) in rabbit ileum in vitro has been studied as a function of applied electrical potential at equal sodium concentrations in the bathing solutions. The results indicate that J(sm) (Na) involves two pathways, a diffusional flux through a paracellular shunt pathway and a flux that is independent of applied potential and presumably involves a transcellular pathway. The latter pathway comprises approximately 25 % of J(sm) (Na) in Ringer's solution containing 10 mM glucose and 25 mM bicarbonate. It is stimulated significantly by theophylline unaffected by removal of glucose or addition of ouabain but is reduced to negligible values by anoxia, dinitrophenol, and replacement of all chloride and bicarbonate by isethionate. Thus this component of J(sm) (Na) has a number of characteristics consistent with involvement in a specific secretory process mediating an electrically neutral secretory transport of sodium plus anion from serosa to mucosa. In addition to stimulating this process, theophylline significantly reduced the permeability of the paracellular shunt pathway to sodium.

Vasoactive intestinal peptide stimulation of adenylate cyclase and active electrolyte secretion in intestinal mucosa

Vasoactive intestinal peptide (VIP), originally isolated from hog small intestinal mucosa, has been shown to cause small intestinal secretion. More recently, this peptide has been identified in the plasma and tumors of patients with the so-called "pancreatic cholera" syndrome. In order to explore the possible role of VIP in the pathogenesis of this syndrome, we examined the effects of this peptide and other hormones on the cyclic AMP levels, adenylate cyclase activity, and ion transport in in vitro preparations of ileal mucosa. In rabbit ileal mucosa, VIP (20 mug/ml) caused a prompt fivefold increase in cyclic AMP level, whereas nine other hormones, which have been postulated to cause intestinal secretion, failed to exert such an effect. Pentagastrin and glucagon also failed to increase cyclic AMP levels in canine ileal mucosa. An increase in mucosal cyclic AMP levels was observed at a VIP concentration of 0.1 mug/ml and appeared to be nearly maximal at 2.0 mug/ml. VIP (100 mug/ml) stimulated adenylate cyclase activity in a membrane preparation from rabbit ileal mucosa. Secretin (6.0 x 10(-5) M) failed to do so. When added to the serosal side of isolated rabbit ileal mucosa clamped in an Ussing chamber, VIP (2 mug/ml) increased short-circuit current (SCC) and caused net secretion of both Cl and Na. Net Cl secretion exceeded net Na secretion. These effects of VIP on mucosal cyclic AMP metabolism and ion transport are similar to those observed with cholera enterotoxin and certain prostaglandins. VIP was also tested with normal human ileal mucosa. At a concentration of 2 mug/ml it caused a fivefold increase in cyclic AMP level and an increase in SCC of the same magnitude as that caused by 5 mM theophylline. Addition of a second 2-mug/ml dose of VIP and addition of theophylline after VIP produced no further change in SCC. We conclude the VIP stimulates adenylate cyclase and active ion secretion in both rabbit and human ileal mucosa. This may be related to the pathogenesis of diarrhea in patients with the pancreatic cholera syndrome.

Effects of dietary calcium restriction and chronic thyroparathyroidectomy on the metabolism of (3H)25-hydroxyvitamin D3 and the active transport of calcium by rat intestine

Previous studies have shown that chronically thyroparathyroidectomized (TPTX) rats, fed a diet with restricted calcium but adequate phosphorus and vitamin D content, have higher levels of intestinal calcium absorption than controls. The results of recent acute experiments have suggested that parathyroid hormone (PTH) may be essential for regulating the renal conversion of 25-hydroxyvitamin D(3) (25-OH-D(3)) to 1,25-dihydroxyvitamin D(3) [1,25-(OH)(2)-D(3)] in response to dietary calcium deprivation. Since 1,25-(OH)(2)-D(3) is the form of the vitamin thought to be active in the intestine, increases in calcium transport mediated by this metabolite would not be expected to occur in the absence of the parathyroid glands if the preceding model is correct. The present study was undertaken to examine the chronic effects of both dietary calcium restriction and the absence of PTH on the metabolism of [(3)H]25-OH-D(3) and duodenal calcium-active transport in rats given thyroid replacement. These relatively long term studies confirm earlier observations which indicated that the adaptation of calcium absorption to a low calcium intake occurs in both sham-operated and TPTX animals. The present studies also demonstrated that despite reduced levels of 1,25-(OH)(2)-D(3) in the plasma of chronically TPTX animals fed a low calcium diet, the accumulation of this metabolite in at least one target tissue, intestinal mucosa, is identical in both the sham-operated and TPTX groups. A reduced, but continued level of 1,25-(OH)(2)-D(3) production, together with its selective accumulation by intestinal mucosa, probably explains the calcium adaptation which is observed inspite of the chronic absence of the parathyroid glands.

Reversal of cyclic AMP-mediated intestinal secretion by ethacrynic acid

Ethacrynic acid (EA) has been reported to reduce cholera toxin-induced intestinal fluid secretion in the intact animal. We explored the nature of this inhibition in vitro by measuring unidirectional, transmural fluxes of (22)Na and (36)Cl across isolated rabbit ileal mucosa. Under control conditions (short-circuited mucosa bathed in bicarbonate-Ringer), there was net absorption of Na and Cl. Theophylline (10 mM), cyclic AMP (5 mM), and cholera toxin (added in vivo) abolished net Na flux and produced net Cl secretion. In the presence of either theophylline or cAMP, addition of 0.1 mM EA to the serosal bathing solution abolished net Cl secretion and restored net Na absorption. Cholera toxin-treated mucosa was exposed to 0.05 and 1.0 mM EA. The lower concentration restored net Na absorption but did not significantly reduce Cl secretion. The higher concentration abolished net transport of both Na and Cl. Short-circuit current and Na flux measurements in the presence and absence of glucose indicated that 0.1 mM EA does not inhibit glucose-coupled Na transport. Short-circuit current measurements in the presence of 1.0 mM EA suggested that even this concentration of EA does not inhibit glucose-coupled Na transport. Thus EA appears to specifically inhibit Cl (or NaCl) secretion without inhibiting the absorptive Na "pump." The anti-secretory effect of 0.1 mM EA does not appear to result from inhibition of adenylate cyclase since secretion stimulated by addition of 5 mM cAMP was abolished. Furthermore, 0.1 mM EA did not significantly reduce theophylline-augmented and cholera toxin-augmented cAMP levels in ileal mucosa. We conclude that EA interacts specifically with the active Cl (or NaCl) secretory mechanism of the small intestine at a step beyond generation of cAMP.

Effects of prostaglandins and cholera enterotoxin on intestinal mucosal cyclic AMP accumulation. Evidence against an essential role for prostaglandins in the action of toxin

Both cholera enterotoxin and certain prostaglandins have been shown to stimulate intestinal fluid secretion in vivo, to cause ion flux changes in vitro similar to those caused by addition of cyclic 3',5'-adenosine monophosphate (cyclic AMP), and to activate intestinal mucosal adenyl cyclase. It has been suggested that the effects of the enterotoxin on intestinal cyclic AMP metabolism may be indirect, and that locally synthesized prostaglandins may serve as required intermediates for the effects of the enterotoxin in activating intestinal mucosal adenyl cyclase. In order to clarify certain aspects of the mechanisms by which these two agents alter intestinal mucosal cyclic AMP metabolism and ion transport, their effects on cyclic AMP accumulation in rabbit ileal mucosa were examined in vitro. Addition of 5 mug per ml (75 mug per 150 mg mucosa) of purified cholera enterotoxin produced a peak increase in cyclic AMP level in 3 h but there was a time delay of at least 30 min before any effect was observed. Inhibition of cyclic nucleotide phosphodiesterase with theophylline failed to reduce this time delay. In contrast, addition of prostaglandin E(1) (PGE(1)) increased the cyclic AMP level rapidly, a peak effect being observed in 2 min. The time of the peak prostaglandin-induced changes in cyclic AMP level and short-circuit current correlated closely. A maximal increment in cyclic AMP level was achieved with 5 x 10(-5) M PGE(1). When 10(-4) M PGE(1) was added to mucosa already maximally stimulated with cholera toxin, the resulting cyclic AMP level was equal to the sum of the levels reached when each agent was added alone. Furthermore, the effects of the enterotoxin on mucosal cyclic AMP levels were not influenced by indomethacin under conditions where mucosal prostaglandins synthesis was inhibited. The results suggest that endogenous prostaglandins do not provide an essential link in the activation of intestinal mucosal adenyl cyclase by cholera enterotoxin. The present study also indicates that the effect of cholera enterotoxin on intestinal mucosal cyclic AMP metabolism involves a definite time delay which is not due to cyclic nucleotide phosphodiesterase activity.

Effects of 1,25-dihydroxycholecalciferol on intestinal calcium transport in cortisone-treated rats

The administration of glucocorticoids may decrease intestinal calcium absorption in vivo and the active transport of calcium in rat duodenum in vitro. It has been suggested that this apparent "anti-vitamin D-like" effect of steroid hormones may be related to alterations in vitamin D metabolism. In order to test this hypothesis, vitamin D-deficient control and cortisone-treated rats were given an intraperitoneal injection of 5.5 IU of 1,25-dihydroxycholecalciferol (1,25-DHCC), the probable end-organ active vitamin D metabolite in the intestine, and 16 h later studies of duodenal calcium transport were performed in modified Ussing chambers. In the vitamin D-deficient state, cortisone administration was associated with a diminution in J(MS), J(Net), and the flux ratio (J(MS)/J(SM)). While the magnitude of the increases in J(MS) and J(Net) that resulted from 1,25-DHCC treatment were approximately the same in control and cortisone-treated animals, 1,25-DHCC failed to restore these parameters to "normal levels" in the steroid-treated rats. Furthermore, contrary to the results obtained in the saline-treated controls, 1,25-DHCC failed to reduce J(SM) in the duodenum from cortisone-treated rats. The cortisone-related defect in calcium transport was due to alterations in both unidirectional calcium fluxes (decrease in J(MS) and increase in J(SM)), such that the J(Net) and the flux ratio (J(MS)/J(SM)) were only approximately 50% of the levels achieved in vitamin D-deficient control animals repleted with the same dose of 1,25-DHCC. The administration of 1,25-DHCC was accompanied by a marked increase in the serum calcium levels of control rats, but there was no such response in the cortisone-treated group. The results support the concept that under the conditions of these experiments in the rat the apparent antagonism between glucocorticoids and vitamin D may be due to steroid hormone-related alterations in end organ function that are independent of any direct interaction between the hormone and the vitamin and that cannot be reversed by the vitamin.

Effect of conjugated dihydroxy bile salts on electrolyte transport in rat colon

The mechanism by which excess quantities of bile salts in the colon produce diarrhea is not known. Therefore, experiments were performed in which the effect of conjugated dihydroxy bile salts on ion transport was evaluated in the in vitro short-circuited rat colon. 2 mM glycochenodeoxycholic acid (GCDC), taurochenodeoxycholic acid (TCDC), or taurodeoxycholic acid caused a prompt increase in short-circuit current (I(sc)) and electrical potential difference (PD). Similar results were obtained when theophylline was added. Removal of HCO(2) and C1 prevented the effects of both bile salts and theophylline. Pretreatment with theophylline blocked the increase in I(sc) and PD produced by TCDC and pretreatment with either TCDC or GCDC inhibited the expected theophylline response. Na fluxes in the presence of both TCDC and theophylline demonstrated a decrease in net absorption; and TCDC decreased net C1 absorption and theophylline caused a reversal of net C1 absorption to net C1 secretion. It is proposed that the diarrhea associated with cholerheic enteropathy is produced by active anion secretion possibly mediated by cyclic AMP.

Effects of cycloheximide on the response of intestinal mucosa to cholera enterotoxin

Prior studies have indicated that effects of cholera enterotoxin (CT) on the small intestine are delayed in onset and involve an interaction with adenyl cyclase in the mucosa. It has also been shown that the administration of cycloheximide to rabbits in doses which inhibit crypt cell mitoses (20 mg/kg), diminishes CT-induced fluid production in jejunal loops. These latter studies have been interpreted as indications that CT-related intestinal secretion is a crypt cell function and that it is mediated by a CT-induced protein. The present study was undertaken to delineate more precisely the nature of the interaction in the intestine between cycloheximide and cholera toxin. Pretreatment of rabbits with cycloheximide reduced by 60% the secretory response to CT in isolated ileal loops with intact blood supply. Sodium and chloride flux measurements on mucosa isolated from these and control loops indicated that this antisecretory effect of cycloheximide persists in vitro. Measurements of radioactive leucine incorporation into mucosal protein indicated that the dose of cycloheximide employed inhibited protein synthesis by 90%. This inhibitory effect was shown to be independent of any effect of cycloheximide on amino acid uptake across the brush border. Measurements of adenyl cyclase activity and cyclic AMP levels in ileal mucosa of cycloheximide pretreated and control animals indicated that cycloheximide did not diminish the CT-induced increases in these parameters. These observations demonstrate that cycloheximide reduces CT-induced intestinal fluid production without interfering with the CT-induced augmentation of adenyl cyclase activity or the consequent rise in cyclic. AMP concentration. Since the antisecretory effect of cycloheximide persists in vitro, it probably involves a direct interaction of the antibiotic with mucosal cell ion transport mechanisms rather than an indirect effect mediated by other humoral or neurogenic factors. The present observations also suggest that the secretory response of the intestine to CT involves neither the synthesis of new adenyl cyclase nor that of a protein modifying its activity.

Effects of sugar and amino acid transport on transepithelial fluxes of sodium and chloride of short circuited rat jejunum

1. Using the Ussing Zerahn (1951) technique the relation between short circuit current (I(sc)) and unidirectional ion fluxes across the short circuited rat jejunum has been examined.2. These parameters of jejunal function have been measured in absence of sugars and amino acids and at 10 mM glucose, 28 mM glucose, 28 mM galactose and 20 mM proline. Sulphate has been used as substituent for chloride on both sides of the gut wall and potassium has been used on the serosal side to substitute for sodium. With regard to the effects of proline on I(sc) and ion fluxes, investigations were made to see whether sulphate and potassium substitutions significantly affected proline transport.3. The purpose of the study was to examine the I(sc) seen in sugars and amino acid free conditions and the Delta I(sc) induced by these substances. Further, it was our purpose to examine the validity of extending to amino acids and sugars in general the suggestion that jejunal transport of these substances stimulates an electroneutral secretion of sodium chloride.4. In the absence of sugars and amino acids at least two-thirds of an I(sc) of 50-70 muA/cm(2) are accounted for by a process of chloride secretion located in the deep parts of the epithelium. The remaining one third or less of the I(sc) can be accounted for in terms of net flux of sodium. The results further indicate that a state of hypoxia in the deep part of the epithelium is the cause of the very low I(sc), p.d. on electrical resistance usually found for sacs of everted rat jejunum.5. The increase in I(sc) induced by proline and glucose is best accounted for by the simultaneously induced increase in mucosa to serosa flux of sodium. There is no compelling evidence that proline should increase I(sc) by a stimulation of chloride secretion.6. Proline, glucose (and galactose) stimulate unidirectional fluxes of sodium and chloride. The effects on serosa to mucosa fluxes equal or exceed those on mucosa to serosa fluxes. The effect of proline on mucosa to serosa flux of thiourea exceeds by far that on serosa to mucosa flux. This and the effects of potassium and sulphate substitution lead to the conclusions in (7) and (8).7. Transport of sugars and amino acids induces or stimulates in rat jejunum in vitro an electroneutral secretion of sodium chloride.8. This effect suffices to explain why, in spite of marked increases in I(sc), increments in net flux of sodium are not induced by transport of proline and glucose. It also explains why the effects of sugar and amino acid transport on water transport in vitro are much less than in vivo.

Effect of cholera enterotoxin on ion transport across isolated ileal mucosa

The effects of cholera enterotoxin on intestinal ion transport were examined in vitro. Addition of dialyzed filtrate of Vibrio cholerae (crude toxin) to the luminal side of isolated rabbit ileal mucosa caused a delayed and gradually progressive increase in transmural electric potential difference (PD) and shortcircuit current (SCC). A similar pattern was observed upon addition of a highly purified preparation of cholera toxin, although the changes in PD and SCC were smaller. Na and Cl fluxes across the short-circuited mucosa were determined with radioisotopes 3-4 hr after addition of crude toxin or at a comparable time in control tissues. The toxin caused a net secretory flux of Cl and reduced to zero the net absorptive flux of Na. Similar flux changes were observed when either crude or purified toxin was added in vivo and tissues were mounted in vitro 3-4 hr later. Additon of D-glucose to the luminal side of toxin-treated mucosa produced a large net absorptive flux of Na without altering the net Cl and residual ion fluxes. Adenosine 3',5'-cyclic phosphate (cyclic AMP) and theophylline had previously been shown to cause a rapid increase in SCC and ion flux changes similar to those induced by cholera toxin. Pretreatment of ileal mucosa with either crude or purified cholera toxin greatly reduced the SCC response to theophylline and dibutyryl cyclic AMP, which, together with the flux data, suggest that both cyclic AMP and cholera toxin stimulate active secretion by a common pathway. Inhibition of the SCC response to theophylline was observed after luminal but not after serosal addition of toxin. In vitro effects of cholera toxin correlated closely with in vivo effects: heating toxin destroyed both; two V. cholerae filtrates which were inactive in vivo proved also to be inactive in vitro; PD and volume flow measurements in isolated, in vivo ileal loops of rabbit revealed that the PD pattern after addition of toxin is similar to that seen in vitro and also correlates closely with changes in fluid movement. The results suggest that stimulation by cholera toxin of a cyclic AMP-dependent active secretory process of the intestinal epithelial cells is a major cause of fluid loss in cholera.