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

The secretory action of barium chloride in rat colon

BaCl2, applied serosally, caused a rise in the p.d. and short-circuit current (s.c.c), and a decrease in tissue resistance in stripped sheets of rat colon. This response was dose dependent. Mucosal application of BaCl2 was without effect. The BaCl2-induced rise in s.c.c. was inhibited by reducing the serosal Na+ concentration to 25 mM. Lowering the mucosal Na+ concentration was without effect. Ouabain (10(-3) M in serosal fluid) and furosemide (10(-3) M in serosal fluid) both reduced the rise in s.c.c. induced by BaCl2. Flux determinations indicated that BaCl2 inhibited Na+ absorption and stimulated Cl- secretion by the colon. In vivo, BaCl2 increased fluid accumulation within the colonic lumen, an effect that was associated with a rise in the transcolonic p.d. Increasing the serosal K+ concentration to 20 mM reduced the responses to BaCl2, acetylcholine and theophylline, and this could not be entirely accounted for by the concomitant reduction in the serosal Na+ concentration. As high serosal K+ did not mimic the secretory response it would appear that BaCl2 does not act by blocking K+ channels. The rise in s.c.c. induced by BaCl2 was not reduced by Ca2+-free conditions, but it was inhibited by 8-(N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate hydrochloride (TMB-8) and trifluoperazine. BaCl2 did not alter cyclic AMP production by colonic scrapes. It is concluded that BaCl2 induces colonic secretion by the release of intracellular Ca2+, which then combines with calmodulin to activate the secretory process.

The involvement of calcium in the intestinal response to secretagogues in the rat

The involvement of Ca2+ in the regulation of intestinal secretion was investigated in stripped sheets of rat mid-intestine. Removal of serosal Ca2+ together with the addition of EGTA at concentrations of 0.5 and 1 mM inhibited the rise in short-circuit current (s.c.c.) induced by both acetylcholine and theophylline, a similar degree of inhibition being observed with both secretagogues. Ca2+-free serosal fluid with 0.5 mM-EGTA added reduced significantly the rises in s.c.c. induced by A23187, acetylcholine, 5-hydroxytryptamine, theophylline, dibutyryl cyclic AMP and prostaglandin E2, but not the increased s.c.c. associated with glucose absorption. The Ca2+ channel blocker verapamil produced similar results. The calmodulin antagonist trifluoperazine inhibited secretagogue action while its sulphoxide derivative was without effect at the same concentration. The intracellular Ca2+ antagonist TMB-8 reduced the increased s.c.c. observed with acetylcholine and dibutyryl cyclic AMP. The net Cl- secretion, but not the decreased mucosal-to-serosal Na+ flux, induced by acetylcholine was abolished in Ca2+-free conditions. There was no consistent effect on the reduction in the residual ion flux caused by acetylcholine. Absence of Ca2+ converted the stimulation of Cl- secretion induced by dibutyryl cyclic AMP observed under control conditions to an enhancement of net Na+ and Cl- absorption. It is concluded that intestinal secretagogues, whether they act through cyclic AMP or not, require both internal and external sources of Ca2+ if they are to produce their full effects. Moreover, it appears that the nature of the response to dibutyryl cyclic AMP depends on the prevailing Ca2+ concentration.

Chloride and sodium transport across bovine tracheal epithelium: effects of secretagogues and indomethacin

The regulation of ion transport in bovine tracheal epithelium was studied in vitro. In the absence of exogenous modifiers of ion transport, average values for transepithelial electrical potential difference (psi t), short-circuit-current (Isc) and tissue resistance (Rt) were 35.4 mV (lumen negative), 5.4 muEq X h-1 X cm-2 and 187 omega X cm2 respectively; net Cl secretion (3.2 muEq X h-1 X cm-2) and net Na absorption (1.3 muEq X h-1 X cm2) accounted for 82% of the Isc. Amiloride reduced psi t and Isc, and increased Rt. The values of psi t, Rt and Isc obtained following addition of theophylline, epinephrine or prostaglandin E1 (PGE1) were not different from control values. Theophylline also did not alter Na and Cl fluxes but it increased tissue cAMP content 3-fold. Indomethacin did not affect psi t but it increased Rt and net Na absorption, and decreased Isc and net Cl secretion; it did not significantly reduce tissue cAMP. When added to indomethacin-treated tissues, epinephrine restored Isc, Rt and Na and Cl fluxes to control levels and increased tissue cAMP 3-fold. Similarly, when PGE1 was added to indomethacin-treated tissues, Isc and Rt were restored to control levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for size and charge permselectivity of rat ascending colon. Effects of ricinoleate and bile salts on oxalic acid and neutral sugar transport

We have measured unidirectional transmural fluxes of oxalate and neutral sugars across rat ascending colon in vitro, under short-circuit conditions, to characterize permeability barriers selective for size and charge. Ionic oxalate appears to be transported preferentially to sodium oxalate. Mucosal addition of taurocholate (1 mM), deoxycholate (1 mM), or ricinoleate (1 mM) increased bidirectional oxalate fluxes, and the ricinoleate effects were independent of medium calcium. Bidirectional fluxes of uncharged sugar molecules fell sharply at molecular weights above 76 (molecular radius above 3 A), and oxalate transport was retarded relative to that of uncharged molecules of similar size, suggesting that there is both size and charge permselectivity. Ricinoleate increased fluxes of all neutral molecules tested but changed neither the exclusion limits nor the cation selectivity of the epithelium. Bile salts and ricinoleate increase oxalate transport, probably by making more channels available, but do not alter size and charge selectivity.

Flounder intestinal absorptive cells have abundant gap junctions and may be coupled

We noted that, unlike mammalian intestinal absorptive cells, cells of the winter flounder (Pseudopleuronectes americanus) displayed abundant gap junctions on the lateral plasma membrane. We compared the distribution of gap junctions in winter flounder to that in rabbit intestinal epithelium. We also examined for evidence of gap junction-mediated intercellular coupling by comparing the cell-to-cell variation of electrical potential difference across winter flounder intestinal cell apical membranes with that in rabbit small intestinal epithelium in which gap junctions are rare. Gap junctions were seen in 95% of flounder absorptive cells and were localized largely to the apical third of the lateral membrane. Individual gap junctions often contained several hundred uniform 9-nm intramembrane particles. Gap junction size and structure was independent of the position of individual absorptive cells on mucosal folds. These findings sharply contrasted flounder intestinal absorptive cells with rabbit small intestinal absorptive cells, in which gap junctions were rarely detected and when present consisted of few intramembrane particles. Correlating with this distribution of morphologically detectable gap junctions, rabbit small intestinal epithelial cells demonstrated marked variability in potential difference across their apical membranes, whereas those in flounder small intestine showed little variation in apical membrane potential difference. Thus, in contrast to intestinal epithelium of rabbits, flounder intestinal epithelium demonstrates morphological and functional characteristics, suggesting a substantial degree of electrical coupling.

The effect of barium chloride on intestinal secretion in the rat

The addition of BaCl2 to the serosal solution, at concentrations from 0 X 25 mM to 10 mM, caused increases in potential difference, short-circuit current and resistance across stripped sheets of rat mid-intestine, although mucosal application had little effect. The response to BaCl2 was significantly greater than that obtained with the same concentration of CaCl2. An increase in net Cl- secretion entirely accounted for the rise in short-circuit current induced by BaCl2. BaCl2 inhibited net fluid uptake by everted sacs. It also enhanced the accumulation of fluid by intestinal loops in vivo and this was associated with an increased potential difference. The response to BaCl2 in vitro was not reduced in the absence of serosal Ca2+ ions. The effect of BaCl2 was abolished by trifluoperazine and also by TMB-8 (8-(N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate hydrochloride). BaCl2 did not alter cyclic AMP production by isolated enterocytes. It is concluded that BaCl2 induces intestinal secretion by releasing Ca2+ from intracellular stores which then combines with calmodulin to stimulate the secretory process.

Inhibition of the antisecretory effects of [D-Ala2,D-Leu5]enkephalin in the guinea-pig ileum by a selective delta opioid antagonist

The novel opioid antagonist [diallyl-Tyr1,(CH2)S-Phe4,Leu5]enkephalin (M154,129) was examined for its ability to block the antisecretory effects of [D-Ala2,D-Leu5]enkephalin (DADLE) in isolated mucosal segments of the guinea-pig ileum, actions mediated predominantly through the delta opiate receptor. DADLE reduced transepithelial potential difference and short-circuit current (ED50 18 nM); these effects were competitively antagonized by M154,129 with a Ke value of 746 nM. These results are consistent with others demonstrating M154,129 is a relatively selective although not highly potent blocker of delta opiate receptors.

Influence of vasopressin and calcium on electrolyte transport across isolated colonic mucosa of the rat

Vasopressin enhanced the absorption of water and Na+ across everted sacs of rat colon descendens but had no effect on absorption across the colon ascendens. The short-circuit current (Isc) and open-circuit potential difference (p.d.) across the colon descendens were dose-dependently decreased by vasopressin. Isc and p.d. across the colon ascendens were not altered by vasopressin. In the colon descendens the decrease in Isc and p.d. was significant at 1 microu. vasopressin/ml and reached a maximum at 1 mu./ml. Propranolol and phentolamine or naloxone did not alter the decrease in Isc and p.d. to a submaximal dose of vasopressin. Vasopressin increased the mucosal to serosal flux of Na+ and Cl- and decreased the serosal to mucosal flux of Cl- across short-circuited colon descendens. Consequently these changes increased the net flux of Na+ and Cl-. Adenylate cyclase activity in homogenates of the colon descendens was not altered by vasopressin. Omission of Ca2+ from the serosal bathing solution reversibly decreased Isc and p.d. and increased Na+ and Cl- absorption across the colon descendens in a similar way as did vasopressin. The results suggest that the effect of vasopressin on the colon descendens may be due to a decrease in intracellular Ca2+ activity.

Bradykinin-stimulated electrolyte secretion in rabbit and guinea pig intestine. Involvement of arachidonic acid metabolites

Bradykinin (BK) increases short-circuit current (Isc) when added to the serosal side of rabbit or guinea pig ileum or rabbit colon. Significant effects on Isc are seen at concentrations as low as 10(-10) M. Anion substitution experiments and unidirectional 36Cl flux measurements indicate that this effect of BK on Isc is due to Cl secretion. The effect of BK on Isc can be partially blocked (60-70% inhibition) by cyclooxygenase inhibitors (indomethacin and/or naproxen) and completely blocked by the phospholipase inhibitor, mepacrine. The combined cyclooxygenase/lipoxygenase inhibitors BW 755 and eicosa-5,8,11,14-tetraynoic acid (ETYA) also completely block the effect of BK on Isc but the slow-reacting substance of anaphylaxis (SRS-A) antagonist FPL 55712 has no effect. None of the above inhibitors diminish the effect on Isc of other exogenously added secretory stimuli such as vasoactive intestinal peptide (VIP), theophylline, or prostaglandin E2 (PGE2). Prior desensitization of rabbit ileum to PGE2 blocks the effect on Isc of BK but not those of VIP or theophylline. Conversely, prior desensitization of rabbit ileum to BK greatly reduces the effect of PGE2 on Isc. BK also stimulates the synthesis of PGE2 in rabbit ileal and colonic mucosa and this effect can be blocked by prior addition of either indomethacin or mepacrine. These effects of BK are similar to those of exogenously added arachidonic acid (AA). AA also stimulates Cl secretion and increases PGE2 synthesis and its effect on Isc can be inhibited by prior desensitization to PGE2 or by prior addition of indomethacin. The above results indicate that BK stimulates active Cl secretion in both small and large intestine and suggest that this effect is due to the intracellular release of AA. Although the prostaglandins appear to be the major products of AA metabolism contributing to the secretory response, lipoxygenase products may also play a role.

A simple method for the simultaneous measurement of sodium and chloride fluxes across transporting epithelia

A simple technique for the separation of 22Na and 36Cl radioactivity in the same solution is described. Two samples were taken from a solution containing both 22Na and 36Cl. One sample was precipitated with AgNO3 and the other was untreated. Samples were assayed for radioactivity by liquid scintillation spectrophotometry. Precipitation with AgNO3 removed 100% of the 36Cl activity from solution while having no effect on the 22Na radioactivity. Thus precipitated samples contained only 22Na radioactivity. Subtraction of the radioactivity in the supernatants of precipitated samples (i.e. 22Na) from the total radioactivity of untreated samples (i.e. 22Na and 36Cl) yielded the level of 36Cl activity in these samples.

Antidiarrhoeal activity of loperamide: studies of its influence on ion transport across rabbit ileal mucosa in vitro

Loperamide is a well-established antidiarrhoeal agent with effects on gastrointestinal motility. We have now shown that the drug influences ion transport. In isolated rabbit ileal mucosa loperamide caused a dose-related fall in potential difference and short-circuit current and reduced the serosa to mucosa flux of chloride. The electrical effects were inhibited by naloxone (10(-6)M) suggesting that they were mediated by opiate receptors. Loperamide (10(-6)M) inhibited secretion provoked by heat stable and heat labile E. coli toxins and by prostaglandin E2. We conclude that loperamide is able to inhibit secretion mediated by cAMP or cGNP, and that this may be relevant to its antidiarrhoeal properties.

Stimulation of colonic secretion by lipoxygenase metabolites of arachidonic acid

Both 5-hydroperoxyeicosatetraenoic acid (5-HPETE) and 5-hydroxyeicosatetraenoic acid (5-HETE) increased the short-circuit current (Isc) in rabbit colonic mucosa mounted in vitro in Ussing chambers. Measurements of chlorine-36 fluxes indicated that the Isc response to 5-HPETE is due to stimulation of active chlorine secretion. 9-, 11-, and 12-HPETE's and leukotrienes C4 and B4 produced either very small increases in Isc or no increase. In contrast to results in rabbit colon, no HPETE, HETE, or leukotriene was effective in rabbit ileal mucosa. The effects of 5-HPETE in the rabbit colon were unaffected by mepacrine, but could be partially blocked by indomethacin. These results suggest that drugs which block both cyclooxygenase and lipoxygenase may be effective antidiarrheals in patients with colitis.

Characterization of the opiate receptor in the guinea-pig ileal mucosa

Several opioid peptides and narcotic drugs reduced transepithelial potential difference (PD) and short circuit current (Isc) in guinea-pig ileal mucosa measured in vitro in Ussing chambers. [D-Ala2,D-Leu5]enkephalin was the most potent peptide tested. Enkephalin analogues with altered C-terminal amino acids were less potent, as were beta-endorphin and dermorphin. Etorphine produced potent effects whereas morphine and SKF 10,047 were inactive. Ethylketazocine produced a biphasic dose-response curve. When added by themselves diprenorphine and naloxone produced small increases in Isc. This effect was not seen when Cl- and HCO3- in the Ringer were replaced by SO42-. Diprenorphine and naloxone were able to shift the dose response curves for all agonists to the right, with the exception of that for ethylketazocine. Diprenorphine was a more potent antagonist than naloxone. SKF 10,047 also acted as a pure antagonist. Morphine and ethylketazocine had no antagonist effects. It is concluded that the opiate receptor in the guinea-pig ileal mucosa is similar to a delta-opiate receptor as defined by ligand binding studies, but that some differences also exist.

Potassium transport by rabbit descending colon

Unidirectional mucosal-to-serosal (Jm leads to s) and serosal-to-mucosal (Js leads to m) fluxes of potassium and sodium were determined simultaneously on paired sections of descending colon from the same rabbit under short-circuit conditions. In 13-16 pairs of tissues, net potassium secretion and sodium absorption averaged 0.49 +/- 0.08 and 4.0 +/- 0.8 mueq.cm-2.h-1, respectively. Short-circuit current (Jsc) averaged 3.7 +/- 0.4 mueq.cm-2.h-1 and was approximately equal to the algebraic sum of net potassium and sodium fluxes. Treatment of both sides of the colon with 10(-4) M ouabain reduced the Jsc and transmural potential difference to near zero. Ouabain abolished net potassium secretion by reducing JKs leads to m and abolished net sodium absorption by inhibiting JNam leads to s. In the presence of ouabain, net potassium absorption averaging 0.15 +/- 0.07 mueq.cm-2.h-1 (n = 11) was observed. In the presence of 10(-3) M 2,4-dinitrophenol, both net potassium and net sodium fluxes were abolished, primarily as a result of a reduction in JKs leads to m and JNam leads to s without altering JKm leads to s and JNas leads to m. These results suggest that the rabbit descending colon has the capacity to secrete and possibly to absorb potassium by active mechanisms requiring metabolic energy. Comparison of potassium and sodium fluxes suggest that the paracellular pathway in the rabbit colon is not potassium selective.

The action of loperamide in inhibiting prostaglandin-induced intestinal secretion in the rat

1 The mechanisms by which loperamide inhibits the intestinal secretion induced by prostaglandin E2 were investigated in rat jejunum. 2 In vivo loperamide prevented prostaglandin-induced fluid secretion but did not reduce the associated rise in the transintestinal potential difference. 3 In intestinal sheets the electrical response to prostaglandin E2 was enhanced in the presence of loperamide. 4 The ionic basis of these changes was determined by measuring Na+ and Cl- fluxes across intestinal sheets. Loperamide did not reduce the prostaglandin-induced increase in net Cl- secretion, although it prevented the inhibition of mucosal-to-serosal Na+ movement. 5 Loperamide does not alter cyclic adenosine 3',5'-monophosphate (cyclic AMP) levels by a direct action at the enterocyte, since in isolated enterocytes neither basal nor prostaglandin-stimulated cyclic AMP levels were affected by the drug.

Effects of bicarbonate on fluid and electrolyte transport by guinea pig and rabbit gallbladder: stimulation of absorption

The effect of bicarbonate (HCO3) on fluid absorption by guinea pig gallbladder was investigated in vitro. Stimulation of fluid absorption was concentration dependent resulting in a fourfold increase in transport over the range 1 to 50 mM. Phosphate, Tris, glycodiazine and glutamine buffers failed to substitute for HCO3 in stimulating absorption. Unidirectional 22Na fluxes were measured across short-circuited sheets of guinea pig and rabbit gallbladders mounted in Ussing-type chambers. In both species the net Na flux was unaffected by serosal HCO3 alone but was stimulated by addition of HCO3 to the mucosal bathing solution. Transepithelial electrical potential difference in rabbit gallbladder was about 1.4 mV (lumen positive) when HCO3 was present in the mucosal or in both compartments. This fell to 0.2 mV under HCO3-free conditions or when HCO3 was present only in the serosal solution. The respective values for guinea pig gallbladder were -1.6 and -0.6 mV (lumen negative). HCO3 stimulation of Na absorption by guinea pig gallbladder was abolished by increasing the bathing pH from 7.4 to 7.8, an effect resulting mainly from a reduction in JNams. Tris buffer (25 mM) inhibited HCO3-dependent fluid absorption in this species completely at pH 8.5 and partially at 7.5. These results indicate that HCO3 stimulates gallbladder transport in both species by an action from the mucosal side. This effect cannot be attributed to simple buffering of H+ but may be explained by the participation of HCO3 in the maintenance of intracellular H+ for a Na/H-exchange.

Comparison of the biological actions of three purified heat-stable enterotoxins: effects on ion transport and guanylate cyclase activity in rabbit ileum in vitro

The biological activities of three purified preparations of heat-stable enterotoxin (ST), elaborated by different strains of Escherichia coli and known to differ in their amino acid composition and molecular size, were compared in the rabbit ileum. The mechanisms of action and potencies of all three purified STs were similar and resembled those previously demonstrated for partially purified ST. They all increased electrical potential difference and short-circuit current, inhibited active Cl- absorption, increased cyclic guanosine 3',5'-monophosphate production, and stimulated particulate guanylate cyclase activity in ileal mucosa. Their molar potencies were also similar, the concentrations of toxin required for half-maximal response differing less than fourfold in short-circuit current response and twofold in guanylate cyclase activity. However, there were 10-fold differences in potency when activity was expressed in mouse units per milliliter. Thus, heterogeneity in the size of these three ST molecules is not reflected in a difference in their mechanisms of action or potencies.

Non-ionic surfactants and gastric mucosal transport of paraquat

The influence of a range of polyoxyethylated non-ionic surfactants upon the transport of [14C]-paraquat dichloride across rabbit isolated gastric mucosa was investigated. Paraquat was shown to cross the mucosal epithelium by passive diffusion. Certain members of the surfactant series enhanced transmucosal paraquat transfer at low surfactant concentrations (e.g. 0.001%) but the occurrence and magnitude of this effect was not dependent in a simple manner upon surfactant structure of physiochemical properties. At micellar concentrations the increase in paraquat transport was greatest with those surfactants possessing both ethylene oxide chains of 10-20 units and alkyl chains longer than C7-C9. The most effective absorption promoter was found to be Brij 36T (C12E10). At micellar surfactant concentrations the enhancement of paraquat transfer appeared, from histological evidence, to be related to the ability of the surfactants to solubilize membrane components and disrupt epithelial cells.

Physiologic and pharmacologic effects of glucocorticoids on ion transport across rabbit ileal mucosa in vitro

Physiologic and pharmacologic effects of glucocorticoids on ileal ion transport were examined in vitro. Tissues were obtained from the three following groups of rabbits: (a) normal; (b) glucocorticoid deficient, which were treated with aminoglutethimide (AG), 100 mg twice daily for 3 d, with a resulting marked reduction in urinary cortisol excretion but no decrease in urinary aldosterone; and (c) methylprednisolone-treated (MP), 40 mg daily for 2 d. Transileal NaCl fluxes were measured with radioisotopes under short-circuit conditions, and the net HCO(3) flux was assumed equal to that portion of the short-circuit current (I(sc)) not accounted for by Na and Cl. In NaCl Ringer's solution containing 25 mM HCO(3) (pH 7.4), normals absorbed both Na and Cl and secreted HCO(3); the I(sc) was greater in both AG and MP groups than in normals; in the AG group, no Na was absorbed, and Cl as well as HCO(3) was secreted; in the MP group, more Na was absorbed and more HCO(3) secreted than in normals. Addition of glucose to the luminal side caused similar increments in I(sc) in all three groups, suggesting similar rates of Na-coupled glucose absorption. Secretory response was assessed with a maximal secretory simulus (8-Br-cAMP) and also a submaximal, cGMP-related secretory stimulus (Escherichia coli heat-stable enterotoxin). After addition of 8-Br-cAMP, the rates of net Cl secretion were similar in all three groups, suggesting no effect of glucocorticoids on maximal secretory capacity. Because the AG group was already secreting Cl, however, the cAMP-induced change in net Cl flux was least in this group. After addition of heat-stable enterotoxin, there were similar changes in net Cl flux in all three groups. To examine specifically Cl-independent, electrogenic Na transport, we used a 10 mM HCO(3), Cl-free SO(4)-Ringer (ph 7.2) in which net Na absorption was previously shown to be equal to the I(sc). Under these conditions, I(sc) was greatest in the MP group and least in the AG group. In vitro addition of hydrocortisone, 50 mug/ml, to AG tissues had no effect on Cl fluxes or I(sc) over a 3.5-h period. No differences among groups were observed with respect to morphology, electrical resistance, or cGMP concentration. We conclude that (a) the effect of glucocorticoid deficiency is similar to that of a submaximal secretory stimulus in that Na absorption is inhibited and some Cl secretion develops; (b) electrogenic Na absorption is depressed in glucocorticoid deficiency and enhanced in glucocorticoid excess; (c) glucocorticoid excess increases HCO(3) secretion; and (d) glucocorticoid status does not affect maximal secretory capacity.

Mode of action of heat-stable Escherichia coli enterotoxin. Tissue and subcellular specificities and role of cyclic GMP

Some enteric strains of Escherichia coli release a heat-stable enterotoxin which, in contrast to cholera and heat-labile E. coli enterotoxins, stimulates guanylate cyclase (GTP pyrophosphate-lyase (cyclizing), EC 4.6.1.2). We have examined the tissue spcificity of its action and the relation of its action to those of the 8-bromo analogues of cyclic GMP and cyclic AMP. Heat-stable enterotoxin stimulated guanylate cyclase activity and increased cyclic GMP concentration throughout the small and large intestine. It increased transepithelial electric potential difference and short-circuit current in the jejunum, ileum and caecum but not in the duodenum or distal colon. This pattern of electrical responses was mimicked by 8-bromo-cyclic GMP. However, 8-bromo-cyclic AMP produced an electrical response in all intestinal segments. The enterotoxin failed to stimulate guanylate cyclase inliver, lung, pancreas or gastric antral mucosa. In the intestines, it stimulated only the particulate and not the soluble form of the enzyme. Preincubation of the toxin with intestinal membranes did not render it capable of stimulating pancreatic guanylate cyclase. Cytosol factors did not enhance the toxin's stimulation of intestinal guanylate cyclase. This study supports the role of cyclic GMP as intracellular mediator for heat-stable enterotoxin and suggests that the toxin affects a membrane-mediated mechanism for guanylate cyclase activation that is unique to the intestines.

Response of the rat small-intestine epithelium to methotrexate

We studied jejunal epithelial structure and function in rats 24, 48, 96, and 192 hours after a single intravenous injection of methotrexate (MTX) 30 mg/kg. The acute effect of the drug on the gut at 24 and 48 hours was characterised, as expected, by reduced mitoses in crypts, shortened villi, and depressed activity of thymidine kinase (an enzyme normally confined to intestinal crypt cells). At 96 hours, when MTX was no longer detectable in serum, the intestine had entered a proliferative phase characterised by increased crypt mitoses, accelerated migration of enterocytes along villi, and the presence on villi of epithelial cells with the enzyme profile of crypt cells, decreased disaccharidase, alkaline phosphatase, and Na+-K+ATPase activities and increased thymidine kinase activity. Although the enzyme data suggested that enterocyte maturation was defective during this proliferative phase, glucose-stimulated Na+ transport, normally a function of fully differentiated villus cells, was normal at 96 hours. Measured both in Ussing chambers and in suspensions of enterocytes isolated from villi, Na+ transport responded normally to glucose at 96 hours, although the response had been significantly depressed at 24 hours. These findings cannot be attributed to MTX-induced malnutrition, as all comparisons included pair-fed controls. We conclude that, in the MTX-induced malnutrition, as all comparisons included pair-fed controls. We conclude that, in the small intestine under conditions of altered epithelial renewal, some components of enterocyte function may be affected more than others. Comparing the present experimental model with another intestinal disorder, acute viral enteritis, in which proliferative activity is excessive, it is clear that the nature of the original intestinal injury is a significant determinant of the pattern of enterocyte response.

Structural and functional membrane polarity in cultured monolayers of MDCK cells

MDCK cells form monolayers which have many of the properties usually found in transporting epithelia. The present article is devoted to the study of the structural and functional polarization of MDCK cells, which is one of the central features of transporting epithelia. The results show: (i) that MDCK monolayers transport 2.6 mumol hr-1 cm-2 of sodium in the apical to basolateral direction; (ii) the passive flux of this ion is relatively large (20.3 mole hr-1 cm-2), which is a characteristic of leaky epithelia; (iii) a large fraction of the penetration of sodium into the cells proceeds through an amiloride-sensitive channel, and the exit is operated mainly by a ouabain-sensitive pump; (iv) the net transport of sodium from the apical to the basolateral side agrees with the asymmetric labeling of the pumps with 3H-ouabain; (v) this asymmetric labeling agrees, in turn, with a higher concentration of intramembrane particles (IMPs) in freeze-fracture replicas of the basolateral side of the plasma membrane; (vi) the structural polarization of confluent MDCK cells is also revealed by the location of microvilli, occluding junctions, and pinocytotic vesicles; and (vii) the presence of a continuous ring formed by actin microfilaments visualized by immunofluorescence under the lateral aspect of the plasma membrane that may be related to the distribution of the occluding junctions, which act as barriers separating apical from basolateral membrane components.

Control of guinea pig intestinal electrolyte secretion by a delta-opiate receptor

The effects of opioids on transepithelial potential difference and short-circuit current across guinea pig ileum stripped of one muscle layer were measured in vitro in Ussing chambers. Opioid peptides such as [DAla2, DLeu5]enkephalin and [DAla2, DMet5]enkephalin, which are primarily agonists at delta-opiate receptors, were able to reduce transepithelial potential difference and short-circuit current at concentrations as low as 1 nM. The narcotic drug etorphine was also very potent in reducing short-circuit current, but fentanyl and morphine, which are primarily agonists at mu-opiate receptors, were almost completely ineffective. Ketocyclazocine was relatively ineffective, and beta-endorphin had intermediate potency. All opioid effects could be reversed by the opiate antagonist naloxone. Somatostatin also reduced short-circuit current, but its effect was not reduced by naloxone. Chloride flux measurements indicated that the effect of etorphine on short-circuit current is associated with an enhancement of active Cl- absorption. The relative effects of opioids in this system suggest that their actions are being mediated by a specific delta-opiate receptor. In contrast, opioid effects on guinea pig intestinal smooth muscle seem to be primarily mediated by a mu-opiate receptor.

The role of bicarbonate ions and of adenosine 3',5'-monophosphate (cAMP) in chloride transport by epithelial cells of bullfrog small intestine

In an HCO3-free medium, isolated segments of bullfrog small intestine, stripped of their external muscle layers, displayed a small, serosal positive PD that did not, on the average, differ significantly from zero. Similarly, in this medium, the mean values of Isc and of net Na+ and Cl- absorption under short-circuit conditions did not differ significantly from zero. External HCO3- (25 mM) induced a highly significant serosal negative PD and Isc and a large net absorption of Cl-. Net Cl- absorption exceeded Isc, i.e., there was a significant net flux, JR, which was consistent with a net secretion of HCO3-. The ratio of the internal Cl-activity of the absorptive cells (alpha Cli) to its equilibrium value was larger in the presence than in the absence of HCO3-. In the presence of HCO3-, cAMP, added to the serosal medium, reversed the serosal negative PD and Isc, and inhibited, though it did not completely abolish, net Cl- absorption. JR was unchanged; tissue Cl- and alpha Cli were reduced, and tissue Na+ decreased and tissue K+ increased. When HCO3- and Cl- were removed from the bathing medium, the electrical response of the tissue to cAMP, though greatly attenuated, was not completely abolished. Under these conditions, cAMP induced a significant net Na+ absorption. A model for ion transport in the absorptive cells of the small intestine is proposed that is consistent with these findings.

Energetics of coupled Na+ and Cl- entry into epithelial cells of bullfrog small intestine

Na+, K+ and Cl- concentrations (cij) and activities (aij), and mucosal membrane potentials (Em) were measured in epithelial cells of isolated bullfrog (Rana catesbeiana) small intestine. Segments of intestine were stripped of their external muscle layers, and bathed (at 25 degrees C and pH 7.2) in oxygenated Ringer solutions containing 105 mM Na+ and Cl- and 5.4 mM K+. Na+ and K+ concentrations were determined by atomic absorption spectrometry and Cl- concentrations by conductometric titration following extraction of the dried tissue with 0.1 M HNO3. 14C-labelled inulin was used to determine extracellular volume. Em was measured with conventional open tip microelectrodes, aiCl with solid-state Cl-selective silver microelectrodes and aiNa and aiK with Na+ and K+-selective liquid ion-exchanger microelectrodes. The average Em recorded was -34mV. ciNa, ciK and ciCl were 51, 105 and 52 mM. The corresponding values for aiNa, aiK and aiCl were 18, 80 and 33 mM. These results suggest that a large fraction of the cytoplasmic Na+ is 'bound' or sequestered in an osmotically inactive form, that all, or virtually all the cytoplasmic K+ behaves as if in free solution, and that there is probably some binding of cytoplasmic Cl-. aiCl significantly exceeds the level corresponding to electrochemical equilibrium across the mucosal and baso-lateral cell membranes. Earlier studies showed that coupled mucosal entry of Na+ and Cl- is implicated in intracellular Cl- accumulation in this tissue. This study permitted estimation of the steady-state transapical Na+ and Cl- electrochemical potential differences (deltamuNa and deltamuCl). deltamuNa (-7000 J . mol-1; cell minus mucosal medium) was energetically more than sufficient to account for deltamuCl (1000--2000 J . mol-1).

The effects of electrical field stimulation and tetrodotoxin on ion transport by the isolated rabbit ileum

To determine whether intramural nerves affect intestinal ion transport, we studied the effect of electrical field stimulation (EFS) on the movement of ions across isolated rabbit ileum. EFS increased the transmural electrical potential difference and the short circuit current (Isc), caused C1 secretion, and reduced conductance, but did not alter fluxes of Na or the residual current (JRnet). The neurotoxin, tetrodotoxin, prevented all the changes caused by EFS but did not prevent the increase in Isc caused by theophylline (5 mM), carbachol (10 micrometer), or glucose (10 mM), or the reduction in Isc caused by norepinephrine (10 micrometer), implying that tetrodotoxin prevented responses to EFS by affecting electrically excitable cells rather than epithelial cells. Tetrodotoxin also enhanced the mucosa to serosa fluxes of Na and C1, reduced the potential difference and Isc, and increased conductance. The site of tetrodotoxin action is uncertain because it may affect the release of at least four neuro-transmitters and the release of peptides from endoctine cells. The Isc response to EFS was not affected by atropine (10 micrometer), physostigmine (10 micrometer), or by hemicholinium (1 micrometer). The mechanism by which EFS causes C1 secretion remains to be determined.

Ion transport across the isolated intestinal mucosa of the winter flounder, Pseudopleuronectes americanus. I. Functional and structural properties of cellular and paracellular pathways for Na and Cl

The isolated intestinal mucosa of the flounder, Pseudopleuronectes americanus, when bathed in a 20 mM HCO3-Ringer's solution bubbled with 1% CO2 in O2, generated a serosa-negative PD and, when short-circuited, absorbed Cl at almost 3 times the rate of Na. Reducing HCO3 to 5 mM decreased the net Cl flux by more than 60%. The following results suggest that, despite the PD, Na and Cl transport processes are nonelectrically coupled: replacing all Na with choline abolished both the PD and net Cl flux; replacing all Cl with SO4 and mannitol abolished the PD and the net Na flux; and adding ouabain (to 0.5 mM) abolished the PD and the net Cl flux. Nearly all of the unidirectional serosa-to-mucosa Cl flux (JClsm) seemed to be paracellular since it varied with PD and Cl concentration in a manner consistent with simple diffusion. JClsm was only about one-fourth of JNasm, suggesting that the paracellular pathway is highly cation-selective. The data can be explained by the following model: (i) Na and Cl uptake across the brush border are coupled 1 : 1; Na is pumped into the lateral space and Cl follows passively, elevating the salt concentration there; (ii) the tight junction is permeable to Na but relatively impermeable to Cl; and (iii) resistance to Na diffusion is greater in the lateral space (considered in its entirety) than in the tight junction. If these assumptions are correct, the serosa-negative transmural PD is due mainly to a salt diffusion potential across the tight junction and, under short-circuit condition, most of the Na pumped into the lateral space diffuses back into the luminal solution, whereas most of the Cl enters the serosal solution. Morphological features of the epithelium support this interpretation: the cells are unusually long (60 micrometer); there is little distension of the apical 12 micrometer of the lateral space during active fluid absorption; and distension distal to this region is intermittently constricted by desmosomes.

Increased tissue conductance and ion transport in guinea pig ileum after exposure to Staphylococcus aureus delta-toxin in vitro

Prior studies had shown that Staphylococcus aureus delta-toxin was able to inhibit water absorption in guinea pig ileum and to elevate the cyclic AMP content of this tissue, but was unable to elicit certain cyclic AMP-mediated changes in Y-1 adrenal or Chinese hamster ovary cells. Because water movement passively follows the net movement of electrolytes in the gut, this study investigated the effect of delta-toxin on ion transport in guinea pig ileum maintained in vitro. The transmural potential difference (PD) of guinea pig ileum was measured and nullified with an automatic voltage clamp. The short circuit flowing under these conditions (I(sc)) was measured, and the conductance was calculated (I(sc)/PD). Unidirectional (22)Na(+) and (36)Cl(-) fluxes were measured. In a glucose-free Ringer solution, delta-toxin caused an immediate spike in PD and I(sc), and the extent and duration of the spike generally increased with increasing toxin concentration. The conductance of ileum was increased by delta-toxin, and this effect on conductance could be blocked by lecithin, a known inhibitor of delta-toxin. Tissue in the presence of glucose did not exhibit a spike in PD or I(sc) when exposed to delta-toxin. In a glucose-free medium, delta-toxin caused a 1.5- to 2.5-fold increase in both the unidirectional absorption and secretion of Na(+) and Cl(-), whereas the net secretion of Na(+) increased above basal levels. The observation that delta-toxin causes a prompt increase in intestinal ion flux lends credence to the concept that the elevation in cellular cyclic AMP, which occurs later, is a secondary response to the toxin. The rapid increase in ion flux may reflect the ability of delta-toxin to augment intercellular movement of ions across the mucosa rather than the stimulation of transcellular processes.

Effects of ethanol on sodium, 3-O-methyl glucose, and L-alanine transport in the jejunum

Effects of ethanol on Na+, Cl-, 3-O-methyl glucose (3-O-MG), and L-alanine fluxes were studied in the isolated rattit jejunal mucosa. Ethanol (3% v/v present on both sides of the mucosa) decreased electrical potential difference (PD), short-circuit current (Isc) and inhibited active transport of Na+, 3-O-MG, and L-alanine. This concentration also increased the permeability of the mucosa for Cl-, 3-O-MG, and L-alanine. Ethanol at 5.4% potentiated the effects on PD, Isc, and the permeability for electrolytes and organic substances. These effects of ethanol could not be fully explained by an osmotic action.

Activity of chloride in absorptive cells of Amphiuma small intestine

Double-barreled chloride-selective microelectrodes constructed by a new method have been used to determine intracellular chloride activity (aic1) in the absorptive cells of isolated, stripped proximal and distal segments of Amphiuma small intestine. Chloride was passively distributed across the mucosal membrane in fall and winter animals and aic1 is about 20 mM. In contrast summer animals actively accumulated chloride in both proximal and distal segments. Parallel measurements of intracellular chloride concentration (Cic1) reveal a very low apparent chloride activity coefficient (aic1/CiC1) indicating that all of the chloride is not free in the cytoplasm. The chloride activity in the bath immediately adjacent to the mucosa is higher than in the bulk solution.

Alterations in electrophysiology of isolated amphibian small intestine produced by removing the muscle layers

Isolated segments of Amphiuma small intestine bathed in chloride or sulfate buffer generate a greater short-circuit current and a larger change in current in response to galactose when the serosal muscle layers are stripped from the mucosa. Intact (unstripped) segments are not apparently anoxic since stripped segments exposed to serosal N2 for 3 h display normal short-circuit currents but a reduced potential response to galactose, while the presence of muscle layers tends to reduce the short-circuit current but does not alter the potential response to galactose. Bullfrog small intestine also generates greater short-circuit current following removal of the muscle layers. The enhancing effect of stripping appears to be related to removal of a resistance to ion flow across the tissue.

Electrical properties and active solute transport in rat small intestine. II. Conductive properties of transepithelial routes

The transepithelial resistance, the cell membrane resistance and the ratio of resistances of the serosal (baso-lateral) to the mucosal (brush border) cell membrane were measured in rat duodenum, jejunum and ileum by means of microelectrode techniques. These measured values were not affected in the presence of actively transported solutes in the mucosal bathing fluid. Contribution of an electrical conductance through the extracellular shunt pathway to the total transepithelial conductance was quantitatively estimated using an electrically equivalent circuit analysis. These values estimated in respective tissues of small intestine were approx. 95% of the total transepithelial conductance, remaining unaffected by an active solute transport. From these data, the changes in emf's of the mucosal and serosal membrane induced by D-glucose or glycine were separately evaluated.

Na and Cl transport and short-circuit current in rabbit ileum

Na and Cl fluxes and short-circuit current (Isc) in rabbit ileum have been studied as a function of ionic concentrations in HCO3-free solutions. Both net Na flux (JNanet) and Isc show similar saturation functions of [Na] at fixed [Cl]. They show no significant difference between zero and 112 mM Na but at 140 mM Na Isc is significantly greater than the JNanet. Net Cl transport, secretion, is observed only at 140 mM Na and is approximately equivalent to the difference between the Isc and JNanet. The transcellular mucosa-to-serosa Na fluxes measured at 140 and 70 mM Na do not differ significantly from the corresponding Isc. The net Cl flux varies with [Cl] at fixed [Na] while Isc is virtually not affected by [Cl]. These results suggest that the absorptive Na transport process is electrogenic and responsible for the Isc and that the secretory fluxes of Na and Cl are coupled, require high [Na], vary with [Cl], and do not contribute to Isc. K-free solution abolishes the Isc after a prolonged lag. Finally, the effect of a low resistance shunt pathway on active Na absorption is examined with a four-compartment model.

A new solid-state microelectrode for measuring intracellular chloride activities

Solid-state microelectrodes from measuring intracellular Cl activity (alphaiCl) were made by sealing the tips of tapered glass capillaries (tip diameter 0.3 mum), coating them under vacuum with a 0.2-0.3 mum thick layer of spectrscopic grade silver, and sealing them (except for the terminal 2-5 mum of the tip) inside tapered glass shields. 106 microelectrodes had an average slope of 55.0+/- 0.6 m V (S,E,) per decade c hange in alphaCl. Tip resistance was (77.1+/- 3.1) x 10(9) omega(n=30). Electrode response was rapid (10-20 s), was unaffected by HCO3, H2PO4, HPO42 or protein, and remained essentially unchanged over a 24-h period. AlphaiCl in frog sartorius muscle fibers and epithelial cells of bullfrog small intestine was measured in vitro. In both tissues, alphaiCl significantly exceeded the value corresponding to equlibrium ditribution of Cl across the cell membrane.

Ileal mucosal cyclic AMP and Cl secretion: serosal vs. mucosal addition of cholera toxin

Changes in ion transport and cyclic AMP (cAMP) concentration produced by addition of cholera toxin to the serosal side of isolated rabbit ileal mucosa (CTs) were compared to the changes produced by addition to the mucosal side (CTm). CTs increased short-circuit current (SCC) as did CTm but it did so more slowly. CTs, unlike CTm, did not significantly decrease electrical conductance. Inhibition of the SCC response to theophylline, a measure of preexisting secretion, was almost complete 180 min after CTm but was not yet significant 180 min after CTs. Longer (280 min) after CTs, the SCC response to theophylline was reduced by 59%, a significant reduction but less than that caused by CTm. A statistically significant change in net Cl flux could not be demonstrated after CTs, although at 280 min the measured flux was halfway between the fluxes for control and CTm tissues. Cyclic AMP concentrations were determined at 190 min, 10 min after addition of theophylline. CTs, despite little or no effect on ion transport, increased cAMP to the same level as did CTm, and the effect on cAMP of adding toxin to both sides was additive. We conclude that 1) active secretion is probably stimulated by cholera toxin added on the serosal side, although more slowly than after addition to the mucosal side and 2) much of the toxin-stimulated cAMP content of the mucosa is not coupled to secretion.

Transport across epithelia. A kinetic evaluation

Compartmental analysis of three models for solute transport across epithelial tissue is presented. The simplest model describes only one tissue compartment, the second incorporates the notion of a pore as a parallel pathway and the third model introduces a serial compartment corresponding to non-epithelial portions of the tissue. Experimental data were obtained, using a modified Ussing and Zerahn technique ((1951 Acta Physiol. Scand. 23, 110-127), for salicylate transport across rat jejunum in vitro and analyzed in terms of these three models. The conclusions based solely on the mathematical analysis of this rather simple experiment are: the tissue is not a homogeneous penetration barrier as often considered. Transport is not limited by unstirred layers either at the tissue surfaces or within the tissue itself. Salicylate is not passively transported. Parallel transport pathways do exist.

Chloride transport across isolated opercular epithelium of killifish: a membrane rich in chloride cells

The opercular epithelium of Fundulus heteroclitus contains typical gill chloride-secreting cells at the high density of 4 X 10(5) cells per square centimeter. When isolated, mounted as a membrane, and short-circuited, it actively transports chloride ions from the blood side to the seawater side of the preparation. This preparation offers a useful approach to the study of osmoregulation in bony fishes.