Short-circuit Current Response Research Articles

Enterocyte Functional Adaptation Following Intestinal Resection

Following massive small bowel resection, the remaining intestine undergoes compensatory adaptation to maintain absorptive capacity. The purpose of this study was to determine the relative importance of mucosal hyperplasia and functional adaptation by individual enterocytes in this process. Distal ileum was harvested from rats 2 weeks following 70% small bowel resection or transection with reanastomosis. Transport parameters were determined in Üssing chambers. Short-circuit current (Isc) responses to additions of 3-O-methylglucose were measured to assess Na+/glucose cotransporter kinetics. Microvillus absorptive surface areas were calculated with computer-assisted morphometric modeling. These surface area values were used to normalize transport parameters. Ileal absorptive surface area was 70% greater in resection tissues than in transection tissues (P< 0.05). Na+and Cl−fluxes were generally lower in the resection group. Na+/glucose cotransporter ΔIscmax (an index of cotransporter quantity) for resection and transection tissues were 0.3 ± 0.1 and 1.8 ± 0.3, respectively (P< 0.05).Km(an index of cotransporter affinity for substrate) did not differ significantly. Following intestinal resection, ileal surface area increases; however, transport parameters, when normalized to absorptive surface area values, diminish. During early postresection adaptation, expansion of ileal absorptive surface area due to hyperplasia seems to play a greater role than upregulation of enterocyte Na+, Cl−, and glucose absorption.

P-204 - Effects of YM060 (ramosetron) on the short-circuit current (Isc) responses to 5-HT receptor agonists in rat colonic mucosa.

P-204 - Effects of YM060 (ramosetron) on the short-circuit current (Isc) responses to 5-HT receptor agonists in rat colonic mucosa.

Berberine inhibition of electrogenic ion transport in rat colon.

1. The effects of the alkaloid berberine on basal and stimulated ion transport were investigated in voltage-clamped rat colonic epithelia. 2. Berberine (100-500 microM) reduced basal short circuit current (SCC) when applied basolaterally but not when applied apically. 3. SCC responses to mast cell activation by anti-rat IgE were significantly attenuated in the presence of berberine. 4. Berberine, applied to the basolateral bathing solution, also reduced SCC responses to the following agents which stimulate chloride secretion in rat colon: carbachol, forskolin, sodium nitroprusside, dibutyryl cyclic-AMP, heat-stable E. coli enterotoxin, 8-bromo-cyclic GMP and thapsigargin. Calcium mediated ion transport responses appear to be more sensitive to berberine inhibition than those which are cyclic GMP-mediated, which in turn are more sensitive than cyclic AMP-mediated responses. 5. Berberine added apically was without effect upon forskolin-stimulated ion transport. Cytochalasin D treatment of the lumenal surface of rat colon conferred apical-side sensitivity to berberine. 6. Berberine (at concentrations up to 500 microM) was without effect on generation of cyclic AMP by forskolin or on generation of cyclic GMP by sodium nitroprusside in isolated mucosal segments. Protein kinase A activity stimulated by dibutyryl cyclic AMP was unaffected by berberine (at concentrations up to 500 microM). 7. The precise mechanism of action of berberine remains to be elucidated. However, its site of action appears to be distal to second messenger production and may be at a level common to all stimuli of colonic chloride secretion.

Investigation of the 5-hydroxytryptamine receptor mediating the ?transient? short-circuit current response in guinea-pig ileal mucosa

5-Hydroxytryptamine (5-HT) stimulated an increase in short-circuit current (ISC) in guinea-pig isolated ileal mucosa over a wide concentration range (0.1 nM-0.1 mM). The concentration-response relationship was biphasic, consisting of a high potency phase (0.1 nM-1 microM) and a low potency phase (3-10 microM). Stimulation of ISC observed at the high potency phase tended to be sustained while responses at the low potency phase (3-10 microM) contained two components, an initial "transient" response followed by a "maintained" response. Both the high potency phase (maximum stimulation approximately 30 microA cm-2) and the low potency phase (maximum stimulation approximately 80 microA cm-2) 5-HT response were antagonized by tetrodotoxin (TTX, 0.3 microM) and atropine (1 microM). However, another low potency (3 microM-0.1 mM, maximum stimulation approximately 30 microA cm-2) component of the 5-HT response was revealed in the presence of TTX or atropine. In the presence of methysergide (1 microM), the concentration-response relationship of 5-HT was still biphasic and tropisetron (0.1 and 10 microM) antagonized both phases of the 5-HT response. In the presence of methysergide, the high potency phase 5-HT response was mimicked by 5-methoxytryptamine (5-MeOT) and the selective 5-HT4 agonist SC-53116 but not by BIMU 8. The potent 5-HT4 antagonist GR 113808 antagonized the response to 5-MeOT in a surmountable manner with an affinity estimate of 9.6 +/- 0.3 (n = 4).(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of ion transport by histamine in human colon

Histamine, added to the basolateral side of voltage clamped human colon in vitro, induced a rapid onset, transient inward short circuit current which was concentration dependent over the range 0.01–3 mM. This response was largely due to electrogenic chloride secretion since it was virtually abolished by bumetanide or by chloride replacement in the bathing solutions. Responses were unaffected by amiloride or acetazolamide. Neither the histamine H 2 receptor agonist dimaprit (1 mM) nor the histamine H 3 receptor agonist S-(+)-α-methyl histamine (1 mM) altered short circuit current. Responses to histamine were significantly reduced by the histamine H 1 receptor antagonist mepyramine (1–10 μM) but not altered by the histamine H 2 receptor antagonist cimetidine (100 μM) or by the histamine H 3 receptor antagonist thioperamide (1 μM). Short circuit current responses to histamine were not altered by tetrodotoxin (1 μM). Piroxicam (10 μM) and nordihydroguaiaretic acid (100 μM) were without effect when used individually but significantly reduced responses to histamine when used simultaneously. These results indicate that histamine stimulates chloride secretion across human colonic epithelium by a mechanism which is mediated exclusively via histamine H 1 receptors. This action does not involve intrinsic nerves but appears to be dependent upon eicosanoid synthesis.

Severe phenotype in mice with termination mutation in exon 2 of cystic fibrosis gene.

Mice with a termination codon mutation in exon 2 of the cystic fibrosis (CF) gene were generated using homologous recombination in embryonic stem cells. Animals homozygous for the mutant allele display a severe intestinal phenotype similar to that previously reported for CF mutant mice. The null nature of this allele was demonstrated by the absence of detectable wild-type mRNA, by the absence of detectable CFTR in the serous gland collecting ducts of salivary tissues, and by the lack of cAMP-mediated short-circuit current responses in colonic epithelium of mutant animals.

Colitis reduces short-circuit current response to inflammatory mediators in rat colonic mucosa

Inflammatory mediators may contribute to the diarrhea associated with colitis. Although the secretory action of such mediators is reported in normal tissue, there is little information regarding their effects on inflamed tissue. We examined the short-circuit current response (Isc) to these mediators, in mitomycin-C (MC)-induced colitis, a model with histological similarities to colitis in man. Rats were injected once with MC (3.25 mg/kg, intraperitoneally) or vehicle. The colons were removed three and seven days later and mounted, devoid of muscularis, in Ussing chambers for measurement of Isc, potential difference (PD), and resistance (Rt). MC-treated rats had diarrhea after three days, and microscopic studies revealed colonic inflammation. There were no significant differences in Rt, PD, and Isc between control and MC-treated tissues at three and seven days. Maximal increases in Isc to bradykinin, prostaglandin E1, carbachol, substance P, and serotonin were depressed at three and/or seven days after MC. The Isc response to theophylline was not affected. Theophylline activates secretion through an intracellular mechanism; the other agonists act by interaction with epithelial cell membranes. Therefore, the mechanism for the decreased Isc may result from uncoupling of receptors to second-messenger systems or desensitization of receptor-linked secretory mechanisms.

Temporal changes in permeability and structure of piglet ileum after site-specific infection by cryptosporidium parvum

Background/Aims: Cryptosporidiosis is an important enteric infection associated with diarrhea in humans. The structural and functional basis for diarrhea is poorly understood. The aim of the study was to determine the structural and functional basis of diarrhea in cryptosporidiosis during evolving host cell-parasite interactions in the intestine. Methods: We used the piglet model for temporal studies of alterations in intestinal epithelial structure and function that occurred 12–48 hours postinoculation. Segments of intestine were directly inoculated in vivo, harvested, and studied in vitro using Ussing chamber techniques. Results: Villus architectural alterations corresponded to the extent of infection. Increased numbers of lamina propria inflammatory cells were evident at 36 hours postinoculation. Solute and macromolecular permeability was not increased. Glucose-responsive short-circuit current was diminished at 48 hours postinoculation. The short-circuit current response to phlorizin was attenuated. The short-circuit current response to theophylline was the same in control and infected tissues. Conclusions: We conclude that passive solute and macromolecular permeability in infected tissues is not significantly increased during parasite-host cell interactions 12–48 hours postinoculation. Electrogenic glucose stimulated Na + absorption, a function principally of villus absorptive cells, is impaired, and electrogenic CI − secretion, a function of crypt epithelial cells, remains the same. These findings parallel structural observations that include loss of the Na +/glucose-transporting villus epithelium without loss of crypt epithelium.

Disruption of colonic electrolyte transport in experimental colitis

The time course of disturbances of distal colonic electrolyte transport during experimental colitis in rats was compared with changes in the severity of inflammation and epithelial disruption. Colitis was induced by intracolonic administration of trinitrobenzenesulfonic acid (TNBS). At time points from 1 day to 8 wk thereafter, Na+ and Cl- transport was studied under short-circuited conditions in Ussing chambers, three indexes of tissue inflammation were measured, and histology was performed. Net absorption of Na+ and Cl- was abolished at 1 and 4 days after induction of colitis, but recovered to control levels by 1 wk. All unidirectional electrolyte fluxes were elevated at 1 day and fell significantly, generally to control levels, by 4 days. The indexes of inflammation, tissue myeloperoxidase, and synthetic capacities for prostaglandin E2 and leukotriene B4 were all significantly elevated from 1 day to 2 wk post-TNBS. At 1 and 4 days, there was widespread epithelial necrosis, and reepithelialization was consistently seen by 2 wk. In additional studies, 2 wk post-TNBS, the short-circuit current response to 3-isobutyl-1-methylxanthine was reduced compared with controls. These data suggest that abolition of electrolyte absorption early in this experimental colitis was largely attributable to epithelial damage. Despite demonstrable tissue inflammation 2 wk post-TNBS, basal electrolyte transport was not impaired, while the colitic epithelium was hyporesponsive to a Cl- secretagogue. The data also suggest that gross barrier function was restored prior to reepithelialization.

The A Adenosine Receptor Mediates cAMP Responses to Adenosine Receptor Agonists in Human Intestinal Epithelia

Adenosine is thought to be a major effector in immunological stimulation of Cl- secretion in intestinal epithelia. Previous studies indicate that both apical and basolateral domains of intestinal epithelial cells possess functionally defined adenosine receptors. However, it is unclear whether the same receptor subclass is expressed, what the receptor subclass(es) is, or how the receptors signal the Cl- secretory response. We now characterize the intestinal epithelial adenosine receptor subtype using the model epithelium, T84. Both apical and basolateral adenosine receptor agonist response profiles revealed a hierarchy (ED50) of 5'-(N-ethylcarboxamido)adenosine > adenosine > CGS-21680. Similarly inhibition studies revealed identical ID50 hierarchies for apical and basolateral antagonism by xanthine amine congener > 1,3-diethyl-8-phenylxanthine > aminophylline. Analyses of both agonist and antagonist pharmacological hierarchies in Chinese hamster ovary cells stably expressing the A2b receptor revealed these same hierarchies. Northern blots performed on RNA extracted from polarized T84 monolayers demonstrated no detectable message for A1 or A2a adenosine receptor, but strong hybridization was detected for the A2b adenosine receptor. Subsequent Northern blots of RNA prepared from human alimentary tract revealed that A2b adenosine receptor message was heavily expressed throughout the colon, in the appendix, and more modestly expressed in the small intestine (ileum). Analyses of cAMP generation in T84 cells in response to adenosine indicated that the basolateral A2b receptor elicits Cl- secretion through this signaling pathway. Stimulation of Cl- secretion through the apical A2b receptor exhibited relatively small but significant increases in cAMP compared with basolateral stimulation. The protein kinase A inhibitor H-89, used at concentrations that did not affect short circuit current responses to the Ca(2+)-mediated agonist carbachol, effectively inhibited short circuit current elicited by either apical or basolateral adenosine. These data suggest that the major intestinal epithelial adenosine receptor is the A2b subclass, which is positively coupled to adenylate cyclase. Such observations have potentially important implications for the treatment of diarrheal diseases.

Cholinergic activation of Cl − secretion in rat colonic epithelia

Acetylcholine receptor agonists and antagonists were used in a pharmacological analysis to identify which muscarinic receptor(s) may be involved in cholinergic regulation of Cl − secretion across rat colonic mucosa in vitro. A comparative ligand binding analysis for each of the antagonists was carried out in parallel. Both studies elicited identical rank order potencies (atropine ≥ 4-diphenyl-acetoxy- N-piperidine methiodide (4-DAMP) > pirenzepine > 11-[[2[(diethylamino)methyl]-1-pipiridinyl]acetyl[-5,11-dihydro-6 H-pyrido[2,3- b]]1,4]benzodiazepine-6-one (AF-DX 116). Cholinomimetic-induced Cl − secretion was predominantly mediated by activation of muscarinic receptors in rat isolated colonic mucosa, with only a modest contribution from nicotinic receptors. Short circuit current responses evoked by the selective muscarinic M 1 receptor agonist 4-[[(3-chlorophenyl)amino]carbonyl]- N,N,N-trimethyl-2-butyn-1-aminium chloride (McN-A-343) suggest that this receptor subtype, which is thought to be neuronally sited, also plays a minor role in regulation of intestinal ion transport. The principal epithelial cell receptors responsible for acetylcholine receptor-mediated Cl − secretion appear to belong to the M 3 class.

Metabolic inhibition and chloride transport in isolated toad skin.

1. When added to the Na(+)-containing solution bathing the isolated toad skin, dinitrophenol (DNP, an uncoupler of oxidative phosphorilation) caused decreases in the baseline values of short circuit current (SCC) and transepithelial conductance (G). 2. DNP also inhibited the increases in SCC and G caused by theophylline, whether added prior to the xanthine, or after the effect of the latter was fully developed. 3. In skins exposed to theophylline and bathed in Cl(-)-free (sulfate Ringer's) solution, the changes in SCC and G had a similar time course (t1/2 > 15 min). In the presence of Cl- (skins bathed in Ringer's solution), SCC decreased with a similar rate, whereas the rate of the decrease in G was greater (t1/2 < 15 min). 4. DNP also decreased the SCC induced by a Cl- concentration gradient in skins exposed to theophylline (SCCg) with a time course similar to its effect on the theophylline-increased G in the presence of Cl-. DNP was effective irrespective of the presence of ambient Na+. 5. A similar difference was observed in skins bathed in CIR and exposed to forskolin. In contrast to theophylline, however, forskolin partially overcame the inhibition of G brought about by DNP; no such recovery was observed in SCC. 6. In contrast to its influence on the responses to theophylline and forskolin, DNP failed to prevent either the increase in G or the onset of SCCg in skins exposed to dibutyryl cyclic AMP. 7. Rotenone, an inhibitor of the electron-transport chain, significantly decreased SCC and G in the unstimulated skin. It also prevented the SCC response to theophylline, and decreased it if added after the effects of the xanthine were fully developed, but failed to modify the increase in G brought about by theophylline. The time course of SCC inhibition by rotenone was similar to that caused by DNP. 8. Ouabain, an inhibitor of Na+,K(+)-ATPase, decreased SCC in the theophylline-stimulated skin, without affecting G. 9. We conclude that, whereas integrity of oxidative energy metabolism is necessary to sustain SCC in the isolated toad skin, it is not a strict requirement for the increase of Cl(-)-dependent G activated by cAMP. 10. The effect of DNP on Cl(-)-dependent G activated by cAMP is probably exerted at the cAMP generation step, by inhibition of adenyl cyclase and/or a decrease in the availability of ATP.

Localization of bicarbonate transport along the crypt-villus axis in rabbit ileum

Background/Aims: HC03− can be absorbed as well as secreted in the rabbit ileum. With 25 mmol/L HC03− on the serosal side only, a serosa-to-mucosa flux (Jsm) is found; with 25 mmol/L on the mucosal side only, epinephrine elicits a mucosa-to-serosa flux (Jms). This study aimed to localize these two processes along the crypt-villus axis. Methods: Excised ileal segments were exposed luminally to 2 mol/L Na2S04 (hypertonic treatment) or to isotonic Ringer's solution for 15 minutes. Mucosa was then chamber-mounted, and measurements were made of Jsm or Jms and of short-circuit current (Isc) responses to glucose plus alanine and to either theophylline or epinephrine. Results: With HC03− /C02 added to the serosal side only, hypertonically treated tissues showed a 22% decline in Jsm; a 25% decline in Isc response to theophylline; and a 71% decline in Isc response to glucose plus alanine compared with control. With HC03− /C03 added to the mucosal side only, tissues showed 92% and 87% declines in Jms and Isc responses to epinephrine, respectively, and a 87% decline in Isc response to glucose plus alanine. Histological examination showed destruction of villus caused by hypertonic treatment but sparing of crypt cells. Conclusions: Both HC03− and Clt- are secreted mainly by crypt cells and absorbed mainly by villus cells.

Potential Role for Interleukin-1 in the Pathophysiology of Ulcerative Colitis

1. Biopsies of colonic mucosa from patients with ulcerative colitis liberated more interleukin-1 beta, prostaglandin E2, leukotriene C4 and platelet-activating factor into the medium in which they were cultured than biopsies from patients with irritable bowel syndrome and histologically normal mucosa. 2. Addition of interleukin-1 stimulated release of greater quantities of all these inflammatory mediators, including interleukin-1 itself, from inflamed and normal mucosa. 3. Blockade of cyclo-oxygenase with indomethacin or of lipoxygenase with ICI 207968 or of phospholipase A2 with mepacrine inhibited release of prostaglandin E2 or leukotriene C4 or both of these plus platelet-activating factor, respectively. 4. Interleukin-1 stimulated the short-circuit current across isolated rat colonic mucosa mounted in flux chambers in a dose-dependent manner (Km 2 x 10(-11) mol/l). This stimulation was markedly inhibited by the removal of chloride from the bathing media. 5. Indomethacin or ICI 207968 inhibited the short-circuit current response to interleukin-1 and a combination of these antagonists produced a greater inhibition. Mepacrine caused an even greater inhibition whereas tetrodotoxin plus mepacrine inhibited the current completely. 6. These data indicate that interleukin-1, released in excess from inflamed colonic mucosa, stimulates the release of a range of inflammatory mediators as well as of more interleukin-1. It probably acts by stimulating phospholipase A2 in inflammatory cells, probably lymphocytes, and can do so in normal and inflamed mucosa. Since, in rat colonic mucosa it stimulated an electrical response in very low concentrations, it is feasible that it is involved in the chloride secretion, and hence the diarrhoea, which may occur in inflammatory reactions.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of brefeldin A on ADH-induced transport responses of toad bladder

We have used brefeldin A (BFA) to examine the role of membrane traffic in the short-circuit current (ISC) and water permeability responses of the toad urinary bladder. BFA treatment of 1 or 5 micrograms/ml had a complex effect on the response of the ISC to antidiuretic hormone (ADH) or forskolin stimulation. Although the responses to initial challenges by ADH were not impaired by BFA, subsequent ISC responses were progressively reduced. Similarly, while the response to an initial challenge by forskolin was modestly reduced by BFA, subsequent responses were markedly reduced. Inhibition of protein synthesis with cycloheximide (CHM) affected ISC responses similarly. Neither BFA nor CHM had an effect on water permeability responses. These observations show that although the membrane traffic responsible for the water permeability response is insensitive to inhibition by BFA or CHM, the stimulation of Na+ transport becomes increasingly sensitive to these inhibitors with successive challenges by ADH or forskolin. Although initial increases in Na+ transport utilize preexisting components, subsequent responses appear to require an intact system for membrane biogenesis.

Biphasic short-circuit current response to noradrenaline mediated by Ca2+ and cAMP in cultured rat epididymal epithelium

A study was carried out to investigate the short-circuit current (Isc) response to noradrenaline (NA) and the signal transduction mechanisms involved in cultured rat cauda epididymal epithelium. In normal Krebs-Henseleit solution, NA (10 mumol.l-1) added basolaterally elicited a biphasic Isc response consisting of a transient spike followed by a second sustained response. The biphasic response was almost abolished by removing ambient Cl-. Preloading the tissues with a cell-permeant Ca2+ chelator, 1,2-bis(2-aminophenoxy) ethane-N,N,N',N',-tetraacetic acid acetoxymethyl ester (BAPTA/AM), or pretreating them with thapsigargin (Tg), a microsomal adenosine triphosphatase inhibitor abolished the initial spike in the Isc response to NA, but had little effect on the second component. Pretreating the tissues with a non-selective beta-antagonist, nadolol, reduced the second Isc response in a dose-dependent fashion but the initial spike was not affected. Microfluorimetric studies showed that NA (100 mumol.l-1) elicited single Ca2+ spikes in isolated epididymal cells, which could be abolished by prior treatment with Tg. Biochemical assays showed that NA (10 mumol.l-1) increased intracellular cyclic adenosine monophosphate concentration ([cAMP]i) and the response was abolished by prior treatment with nadolol (50 mumol.l-1). The results showed that NA elicited a biphasic Isc response mediated by a rise in intracellular Ca2+ concentration ([Ca2+]i) followed by a rise in [cAMP]i. The Ca(2+)-mediated Isc response had a faster onset and more transient action than the cAMP counterpart. It is suggested that NA released from noradrenergic nerve endings regulates transepithelial Cl- secretion in the epididymis thereby providing the specialized millieu vital for sperm storage and maturation.

Lack of effect of three putative vasoactive intestinal peptide receptor antagonists on vasoactive intestinal peptide-induced secretory responses in rat colon

We have assessed the potential usefulness of three vasoactive intestinal peptide antagonists for investigating whether vasoactive intestinal peptide has a functional role as a secretomotor neurotransmitter at the neuroepithelial junction in rat colonic mucosa. Vasoactive intestinal peptide (VIP) increases short-circuit current in muscle-stripped preparations of rat colon. The response is unaffected by tetrodotoxin and can only be obtained when the peptide is applied to the basolateral side of the membrane. Three putative antagonists were tested for their ability to inhibt short-circuit current responses to vasoactive intestinal peptide. Vasoactive intestinal peptide-(10–28) (1 μM and 3 μM), human growth hormone releasing factor (human GRF) analogue [Ac-Tyr 1]human GRF-(1–40)OH (0.1 μM and 1 μM) and [Lys 1,Pro 2,5,Arg 3,4,Tyr 6]VIP (0.5 μM) produced concentration-dependent increases in basal short-circuit current but were ineffective as antagonists to vasoactive intestinal peptide.

Evidence for the involvement of a 5-HT4 receptor in the secretory response of human small intestine to 5-HT.

5-Hydroxytryptamine increases transmucosal short-circuit current across human isolated small intestinal mucosa. The competitive 5-HT4 antagonist, DAU 6285 evoked a concentration-dependent, dextral and parallel shift of the concentration-response curve to 5-HT, with no alteration of the maximum response. Schild analysis of this antagonism produced a Schild regression with a slope of 1.00 and an apparent pA2 estimate of 6.17. It appears that a 5-HT4 receptor may mediate the short-circuit current response of human small intestinal mucosa to 5-HT.

Short-circuit current responses to 5-hydroxytryptamine in human ileal mucosa are mediated by a 5-HT4 receptor.

5-Hydroxytryptamine (5-HT) increases short-circuit current when added to the serosal side of human isolated ileal mucosa; mucosally applied 5-HT was ineffective. Tetrodotoxin reduced both basal short-circuit current and increases in short-circuit current due to electrical field stimulation of mucosal nerves. However, neither tetrodotoxin, ondansetron nor methysergide plus ketanserin affected 5-HT induced increases in short-circuit current. Application of SDZ 205-557 (2-diethylaminoethyl-(2-methoxy-4-amino-5-chloro) benzoate) to the tissue caused a significant increase in the concentration ratio between two successive 5-HT response curves. It is concluded that the effect of 5-HT on short-circuit current of human ileal mucosa appears to be due to stimulation of a 5-HT4 receptor.

Ion transport in rat tongue epithelium in vitro: A developmental study

The responsiveness of the rat gustatory system to monochloride salts changes during development. Neurophysiological recordings in the chorda tympani indicate that a) the taste responses to NaCl and KCl in early postnatal rats are small relative to NH 4Cl, b) both salts become more potent stimuli as the animal matures, and c) the developmental increase is accompanied by an increase in sensitivity of the NaCl response to the sodium transport blocker amiloride. We measured ion transport properties of in vitro tongue epithelia from Wistar rats. When the tissue is mounted in an Ussing chamber, the short-circuit current responses to NaCl and KCl are small in the neonatal rat and increase during development in postweaning and adult animals. Amiloride sensitivity of the NaCl response also increases with age. This study confirms that increased sensitivity of the rat gustatory system to NaCl with age reflects changes in the peripheral membranes. The results support the hypothesis that the increased sensitivity is due to amiloride-sensitive membrane components being added or becoming functional.