Effects of adrenal steroids on Na transport in the lower intestine (coprodeum) of the hen

The amiloride-sensitive Na+ channel: from primary structure to function

Three homologous subunits of the amiloride-sensitive Na+ channel, entitled alpha, beta, and gamma, have been cloned either from distal colon of a steroid-treated rat or from human lung. The alpha, beta, and gamma subunits have similarities with degenerins, a family of proteins found in the mechanosensory neurons of the nematode Caenorhabditis elegans. All these proteins are characterized by the presence of a large extracellular domain, located between two transmembrane alpha-helices, and by short NH2 and COOH terminal cytoplasmic segments. They constitute the first members of a new gene super-family of ionic channels. The epithelial Na+ channel is specifically expressed at the apical membrane of Na(+)-reabsorbing epithelial cells. Its activity is controlled by several distinct hormones, especially by corticosteroids. These hormones act either transcriptionally (such as aldosterone in distal colon, or glucocorticoids in lung) and/or post-transcriptionally (such as aldosterone in kidney). Recent works have provided new insights in the function of that important osmoregulatory system.

Experimental studies of intestinal ion and water transport

A major advance in transport physiology was H. H. Ussing's development of the voltage-clamp method, and later the Koefoed-Johnsen-Ussing model for Na+ transport. In the same decade, J. C. Skou identified the Na(+)-K(+)-ATPase, which maintains the Na+ and K+ gradients that drive most epithelial transport processes. With this foundation, Danish scientists have pursued the mechanism of ion transport and the resulting solute-linked water flow. Recent contributions have been on isosmotic transport, suggesting solute recycling, and KCl-water cotransport in the basolateral epithelial cell membrane. Efficient small intestinal nutrient absorption is dependent on coupling to the Na+ gradient. Cotransport of Na+ and glucose is quantitatively the most important absorptive mechanism in the small intestine, as illustrated by the success of oral rehydration solutions in diarrhoea. The majority of amino acids are likewise transported by Na+ dependent carriers, but recent experiments have identified a concomitant Cl- dependency for some. Regulation of intestinal secretion, both under normal digestive processes, and in response to enterotoxins, has turned out to be very complex. It involves local and central neuronal regulation through an array of neurotransmitters and local actions of gastrointestinal hormones. Major effectors are the submucosal neurons and the main transmitters serotonin, vasoactive intestinal peptide, acetylcholine, substance P, and neurotensin. Development of antisecretagogues is impeded by the existence of several receptor subtypes and significant species differences. The Na+ and water-conserving properties of the large intestine have been shown to be regulated by adrenocortical hormones, with aldosterone as a potent stimulator of colonic Na+ absorption. A major colonic function is the symbiosis with the anaerobic bacterial population. The fermentation of carbohydrate to short-chain fatty acids, which can be absorbed, supplements small intestinal digestive function.

Quinidine-sensitive K+ channels in the basolateral membrane of embryonic coprodeum epithelium: regulation by aldosterone and thyroxine

Basolateral K+ channels and their regulation during aldosterone- and thyroxine-stimulated Na+ transport were studied in the lower intestinal epithelium (coprodeum) of embryonic chicken in vitro. Isolated tissues of the coprodeum were mounted in Ussing chambers and investigated under voltage-clamped conditions. Simultaneous stimulation with aldosterone (1 mumol.l-1) and thyroxine (1 mumol.l-1) raised short-circuit current after a 1- to 2-h latent period. Maximal values were reached after 6-7 h of hormonal treatment, at which time transepithelial Na+ absorption was more than tripled (77 +/- 11 microA.cm-2) compared to control (24 +/- 8 microA.cm-2). K+ currents across the basolateral membrane were investigated after permeabilizing the apical membrane with the pore-forming antibiotic amphotericin B and application of a mucosal-to-serosal K+ gradient. This K+ current could be dose dependently depressed by the K+ channel blocker quinidine. Fluctuation analysis of the short-circuit current revealed a spontaneous and a blocker-induced Lorentzian noise component in the power density spectra. The Lorentzian corner frequencies increased linearly with the applied blocker concentration. This enabled the calculation of single K+ channel current and K+ channel density. Single K+ channel current was not affected by stimulation, whereas the number of quinidine-sensitive K+ channels in the basolateral membrane increased from 11 to 26.10(6).cm-2 in parallel to the hormonal stimulation transepithelial Na+ transport. This suggests that the basolateral membrane is a physiological target during synergistic aldosterone and thyroxine regulation of transepithelial Na+ transport for maintaining intracellular K+ homeostasis.

Immunocytochemical localization of amiloride-sensitive sodium channels in the lower intestine of the hen

We have used polyclonal antibodies generated against purified bovine renal amiloride-sensitive Na+ channels to localize amiloride-sensitive Na+ channels within the lower intestine (colon and coprodeum) of the hen. These antibodies cross-reacted with two polypeptides exhibiting M(r)'s of 235 and 150 kDa on immunoblots of detergent-solubilized apical membrane fractions from both the colon and coprodeum. The apparent molecular masses of theses polypeptides are in agreement with the M(r)'s of 2 of the subunits of the renal high amiloride-affinity Na+ channel, namely the alpha and the beta (= amiloride binding) subunits. The cellular distribution of Na+ channels was determined by immunoperoxidase and indirect immunofluorescence cytochemical techniques. The apical (luminal) membrane and cytoplasm of villar principal cells in both colon and coprodeum exhibited immunoreactivity, whereas goblet cells were negative. Both principal and goblet cells of the crypts were also negative. We conclude that the amiloride-sensitive Na+ channels are localized to the principal cells of the intestinal villi and that these cells are responsible for intestinal Na+ absorption.

Effects of corticosteroids on short-circuit current across the cecum of the domestic fowl, Gallus domesticus

Both avian corticosteroid hormones, aldosterone and corticosterone, increased short-circuit current across the wall of the ceca of the domestic fowl (Gallus domesticus) in vitro. About 80% of this short-circuit current was inhibited by the Na-channel blocking drug amiloride. Corticosterone was about ten times less potent than aldosterone in increasing short-circuit current and it exerted a similar maximal effect. Cortisol (an endogenous corticosteroid hormone in mammals but not birds) was about ten times less potent than corticosterone and this difference appeared to reflect the presence of the 17 alpha-OH group in cortisol. Carbenoxolene, which inhibits 11 beta-hydroxysteroid dehydrogenase, increased the effect of corticosterone. This effect is consistent with inhibition of the metabolism of corticosterone to 11-dehydrocorticosterone. The latter was found to be about 100 times less potent than corticosterone. The effects of both aldosterone and corticosterone (also dexamethasone) were abolished by the mineralocorticoid receptor antagonist spironolactone. The results suggest that corticosterone has an effect similar to aldosterone but in vivo its action may be depressed by the activity of 11 beta-hydroxysteroid dehydrogenase. The sensitivity of the cecal preparations to corticosterone indicates that this hormone could contribute to the regulation of transcecal Na transport (absorption) in vivo.

Structural and enzymatic studies on the plasma membrane domains and sodium pump enzymes of absorptive epithelial cells in the avian lower intestine

The coprodaeum of the domestic hen maintained on a low-NaCl diet adapts by enhanced sodium transport. This study examines the adaptive response at the single cell and whole organ levels. Surface areas of apical (microvillous) and basolateral plasma membranes of columnar absorptive epithelial cells were estimated by use of ultrastructural stereology. The activities of succinic dehydrogenase (a mitochondrial enzyme) and ouabain-sensitive, potassium-dependent paranitrophenyl phosphatase (a sodium pump enzyme) were determined in tissue homogenates. Sodium, potassium-ATPase (pump enzyme) activity in cell membranes was localized by ultrastructural cytochemistry. Apical and basolateral membranes responded differently. In high-NaCl hens, the membrane signature of the average cell was 32 microns 2 (apical), 932 microns 2 (lateral) and 17 microns 2 (basal). Cells from low-NaCl hens had more apical membrane (49 microns 2 per cell) but essentially the same area of basolateral membrane. However, total surfaces per organ were greater for all membranes. Sodium pump enzymes were localized in basolateral membranes. Enzyme activities per unit mitochondrial volume and per unit basolateral membrane surface were higher in low-NaCl birds. These findings are discussed in the context of known mechanisms of transcellular sodium transport via apical ion channels and basolateral pumps.

Expression of amiloride-sensitive Na+ channels of hen lower intestine in Xenopus oocytes: electrophysiological studies on the dependence of varying NaCl intake

Epithelial Na+ channels were incorporated into the plasma membrane of Xenopus laevis oocytes after micro-injection of RNA from hen lower intestinal epithelium (colon and coprodeum). The animals were fed either a normal poultry food which contained NaCl (HS), or a similar food devoid of NaCl (LS). Oocytes were monitored for the expression of amiloride-sensitive sodium channels by measuring membrane potentials and currents. Oocytes injected with poly(A)+RNA prepared from HS animals or non-injected control oocytes showed no detectable sodium currents, whereas oocytes injected with LS-poly(A)+RNA had large amiloride-blockable sodium currents. These currents were almost completely saturated by sodium concentrations of 20 mM with a Km of about 2.6 mM sodium. Amiloride (10 microM) inhibits the expressed sodium channels entirely and examination of dose response relationships yielded a half-maximal inhibition concentration (Ki) of 120 nM amiloride. I-V difference curves in the presence or absence of sodium or amiloride (10 microM) indicate a potential dependence of the sodium transport which can be described by the Goldman equation. When Na+ is replaced by K+, no amiloride response was detected indicating a high selectivity for Na+ over K+. These results provide strong evidence that intestinal Na+ channels are regulated by dietary salt intake on the RNA level.

Na-transport during long-term incubation of the hen lower intestine: No aldosterone effect

1. In order to test the aldosterone effect in vitro, Na-transport of the coprodeal epithelium from hens on low-NaCl diet was measured in the Ussing chamber for up to 8 hr. Short-circuit current (SCC, near equal to the amiloride inhibitable Na-transport) was recorded. 2. Incubation media were either Krebs-phosphate or bicarbonate buffer with and without addition of beta-hydroxybutyrate, glutamine and mannose as "metabolic fuels". The media were replaced every hour. The Krebs-phosphate buffer was further tested with and without indomethacin and media replacement. Na-transport was best maintained in this buffer with replacement: SCC at 4 hr: 156 +/- 21 microA/cm2, 8 hr: 73 +/- 14 microA/cm2. 3. The aldosterone experiments were carried out on tissues from hens resalinated for 24 hr. No effects were demonstrated at concentrations up to 10(-5) M. The SCC showed an unexpected raise within 2-4 hr to a very high level (4 hr: 221 +/- 61 microA/cm2) both in the control and in all aldosterone-treated tissues. This SCC decreased slowly to 210 +/- 29 microA/cm2 at 8 hr. It was abolished by amiloride. 4. No increase in SCC was observed in tissues from hens after 48 and 72 hr of resalination either after aldosterone or on chronic high-NaCl diet.

Different modes of electrogenic Na+ absorption in the coprodeum of the chicken embryo: role of extracellular Ca2+

Transepithelial electrogenic Na+ transport (INa) was investigated in the coprodeum of 20-days-old chicken embryos in Ussing chambers. Short circuit current (Isc) and transepithelial resistance (Rt) were 14.7 +/- 4.8 microA.cm-2 (n = 12) and 0.53 +/- 0.09 k omega.cm-2 (n = 12), respectively. INa was calculated from changes in Isc by substitution of mucosal Na+ by (N-methyl-D-glucamine) (NMDG). Isc inversed during Na+ removal, and INa was found to be 27.8 +/- 4.7 microA.cm-2 (n = 12). Amiloride (100 mumol.l-1) inhibited only about 60% of INa. Analysis of Isc fluctuations revealed a Lorentzian component in the power density spectrum with a corner frequency of about 57 Hz. This component was not correlated to INa, and its origin is still unclear. Removal of mucosal Ca2+ increased INa about 2.5-fold due to an increase of the amiloride-insensitive component of INa in additionally investigated adult tissues. The results clearly show that this is due to a non-selective cation channel with an "apparent" order of selectivity Cs+ greater than Na+ = K+ greater than Rb+ greater than Li+. The Ca2+ concentration required to block 50% of the Isc was about 18 mumol.l-1. The IscCa could also be suppressed by other divalent cations such as Mg2+ and Ba2+. Additionally, an INa-linked Lorentzian component occurred which dominated the control spectrum with a significantly higher corner frequency (about 88 Hz). The results indicate that Na+ absorption in the coprodeum of the chicken embryo is more complex than in adult hens.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of sodium absorption and chloride secretion in an intestinal epithelium

Hen coprodeum absorbs sodium electrogenically and, when stimulated by theophylline, secretes chloride. In this study the vibrating microprobe technique was used to localize the transport of these ions to intestinal villi/folds and crypts. With the isolated, stretched epithelium, controlled by light microscopy and scanning electron microscopy, in open circuit, currents were inward, 40 +/- 7 microA/cm2, 50 microns vertically above villi, and outward, 36 +/- 7 microA/cm2 above crypts. The currents decayed exponentially to near zero at 300 microns with the same length constant. A physical model simulating the observed loci of current sources and sinks predicts potential profiles consistent with our data. Extrapolation of the currents gives a surface potential of 45 microV, negative on villi and positive above crypts. Short circuiting increased villus current to 86 +/- 27 microA/cm2 at 50 microns, and amiloride treatment reduced it to -8 microA/cm2; in both cases crypt currents were abolished. The inward currents are compatible with sodium absorption. Induction of chloride secretion after amiloride treatment, resulted in current circuits similar to those induced by sodium absorption, with villus currents of 23 +/- 7 microA/cm2. This is in accord with chloride secretion at the villi. Quantitative estimates of crypt number (860/cm2) and opening diameter (15 microns), in conjunction with isotopic measurements of active and electrical potential-driven ion fluxes demonstrate, however, that only 4% of the potential-driven co-ion transport occurs through the crypts. This indicates that nearly all chloride secretion comes from the sodium-absorbing villar area. Were the chloride secretion to occur solely from the crypts, the current should have been in the opposite direction and 10,000-fold larger.

Steady-state sodium absorption and chloride secretion of colon and coprodeum, and plasma levels of osmoregulatory hormones in hens in relation to sodium intake

The plasma levels of four osmoregulatory hormones and their target ion-transport systems in the lower intestines of the domestic fowl were determined in order to elucidate their interrelationship and their setpoints in relation to NaCl intake. White Plymouth Rock hens were adapted to six intake levels of NaCl (0.20 +/- 0.02-24.7 +/- 1.9 mmoles Na+.kg bw-1.day-1) for 6 weeks. The Na+ absorption and the Cl- secretion of colon and coprodeum were characterized in vitro by the effects of hexoses, amino acids, amiloride, and theophylline on the short-circuit current (SCC) and electrical potential difference (PD). The NaCl-conserving system of the adult chicken is set at low intake levels of NaCl as the 80% range (quantized by non-linear, logistic regression analyses) of the change in the plasma [ALDO], the amiloride-inhibitable Na+ absorption of coprodeum and colon (delta SCC), occurred from 0.18 to 2.3, from 0.9 to 4.3, and from 1.2 to 7.3 mmoles Na+.kg bw-1.day-1, respectively. These results demonstrate that the amiloride-inhibitable Na+ absorption of coprodeum is more closely linked to plasma [ALDO] than that of colon. The aminoacid-Na+ coabsorption of colon increased over exactly the same range of Na+ intake as the colonic amiloride-inhibitable Na+ absorption decreased, whereas the hexose-Na+ coabsorption increased at higher levels of Na+ coabsorption increased at higher levels of Na+ intake, from 2 to 11 mmoles Na+.kg bw-1.day-1. Both these Na+ absorption types had reached their maximums at 24.7 mmoles Na+.kg bw-1.day-1, whereas the plasma [AVT] and plasma [PRL], although significantly increased, apparently had not; their 80% range of change occurred from 9.9 to 99 mmoles Na+.kg bw-1.day-1, and the main changes in plasma osmolality were predicted to occur from 5.4 to 107 mmoles Na+.kg bw-1.day-1. These results suggest that these colonic and hormonal variables conserve osmotically-free water and operate at high NaCl intake. The theophylline-induced colonic Cl- secretion did not change with NaCl intake, whereas the stimulation of SCC in coprodeum decreased with increasing NaCl intake: the main change occurred between 0 and 3.2 mmoles Na+.kg bw-1.day-1. Thus, all ion-transport capacity (although the nature of the Na+ transport changes). It is suggested that hormones defending the extracellular volume and composition are regulated close to zero input and output of both NaCl and water, regardless of whether they are NaCl conserving or free-water conserving.(ABSTRACT TRUNCATED AT 400 WORDS)

In vitro studies of theophylline-induced changes in Na, K and Cl transport in hen (Gallus domesticus) colon suggesting bidirectional, basolateral NaK2Cl cotransport

1. In isolated mucosa from a NaCl-loaded hen theophylline stimulates both unidirectional chloride fluxes (JmsCl and JsmCl). Conductive and electroneutral exchange processes, besides a bumetanide-sensitive, rheogenic process contribute. 2. The bumetanide-sensitive fraction of the theophylline-induced delta JcmCl is sodium-dependent. 3. Incubation in nominally K(+)-free solutions reduces the bumetanide-sensitive fraction delta JsmCl more than treatment with ouabain. 4. With respect to chloride the bumetanide-sensitive fraction of delta JsmCl has a Hill coefficient of 1.93 +/- 0.03, a Jmax of 12.9 +/- 0.2 mumol/cm2.hr and a K 1/2 of 73 +/- 1 mmol/l. 5. After ouabain treatment delta JmsCl and delta JsmCl are equally sensitive to bumetanide, while delta JmsCl is bumetanide insensitive without ouabain treatment.

Molecular genetics of Na,K-ATPase

Researchers in the past few years have successfully used molecular-genetic approaches to determine the primary structures of several P-type ATPases. The amino-acid sequences of distinct members of this class of ion-transport ATPases (Na,K-, H,K-, and Ca-ATPases) have been deduced by cDNA cloning and sequencing. The Na,K-ATPase belongs to a multiple gene family, the principal diversity apparently resulting from distinct catalytic alpha isoforms. Computer analyses of the hydrophobicity and potential secondary structure of the alpha subunits and primary sequence comparisons with homologs from various species as well as other P-type ATPases have identified common structural features. This has provided the molecular foundation for the design of models and hypotheses aimed at understanding the relationship between structure and function. Development of a hypothetical transmembrane organization for the alpha subunit and application of site-specific mutagenesis techniques have allowed significant progress to be made toward identifying amino acids involved in cardiac glycoside resistance and possibly binding. However, the complex structural and functional features of this protein indicate that extensive research is necessary before a clear understanding of the molecular basis of active cation transport is achieved. This is complicated further by the paucity of information regarding the structural and functional contributions of the beta subunit. Until such information is obtained, the proposed model and functional hypotheses should be considered judiciously. Considerable progress also has been made in characterizing the regulatory complexity involved in expression of multiple alpha-isoform and beta-subunit genes in various tissues and cells during development and in response to hormones and cations. The regulatory mechanisms appear to function at several molecular levels, involving transcriptional, posttranscriptional, translational, and posttranslational processes in a tissue- or cell-specific manner. However, much research is needed to precisely define the contributions of each of these mechanisms. Recent isolation of the genes for these subunits provides the framework for future advances in this area. Continued application of biochemical, biophysical, and molecular genetic techniques is required to provide a detailed understanding of the mechanisms involved in cation transport of this biologically and pharmacologically important enzyme.

Steroid C-20 oxidoreductase activity of duck intestinal mucosa: the interrelations of the enzymatic activity with steroid binding

The binding of corticosterone and aldosterone to domestic duck (Anas platyrhynchos)-dispersed colonic mucosal cells at 37 degrees was investigated. It was found that in contrast to experiments using cell-free intestinal preparations, corticosterone was extensively metabolized and it was the metabolite, not the native steroid that became receptor bound and all the bound ligand was in the nuclear fraction. The metabolite turned out to be identical with 4-pregnene-11 beta,20 beta, 21-triol-3-one (20 beta-dihydrocorticosterone, 20 beta-DHB). Binding experiments with [3H]corticosterone yielded the following kinetic parameters: Kd = 87.6 nM, Nmax = 337,900 sites/cell. When synthetic [3H]20 beta-DHB was used as the ligand a curvilinear-binding isotherm was obtained. This could be resolved into a high affinity-low capacity (HA) and a low affinity-high capacity (LA) component with the following binding parameters: Kd,HA = 91 nM, Nmax,HA = 130,800 sites/cell; Kd,LA = 5.4 x 10(-6) M, Nmax, LA = 3.7 x 10(6) sites/cell. Binding of the metabolite to cell-free preparations, at 0 degree, gave the following results: for cytosol, linear-binding isotherm, Kd = 14.0 nM, Nmax = 26.5 fmol/mg protein; and for crude nuclei, curvilinear-binding isotherm, Kd,HA = 45.0 nM, Nmax, HA = 5.33 pmol/mg DNA; Kd,LA = 2.2 x 10(-6) M, Nmax,LA = 286.6 pmol/mg DNA. [3H]Aldosterone was also bound by the dispersed whole cells and again, this binding was only nuclear (Kd = 9.3 nM, Nmax = 10,042 sites/cell). The bound ligand was unchanged aldosterone. Competition experiments have shown that aldosterone did not compete with 20 beta-DHB for binding sites and vice versa. The intracellular 20 beta-hydroxysteroid oxidoreductase responsible for the transformation of corticosterone was found mostly in the cytoplasm. Kinetic studies with the enzyme yielded classical Michaelis-Menten kinetics (Km = 15.7 microM, Vmax = 2.6 nmol/min/mg protein). The enzyme had an apparent Mr of 35 kDa and a Rs of 25.5 A. It is believed that our results might explain the binding of aldosterone to mineralocorticoid-binding sites in the presence of overwhelming concentrations of corticosterone and that experiments with cell-free tissue preparations, performed at 0 degree, do not reflect the true cellular-binding events.

Avian colonic ion transport: effects of corticosterone and dexamethasone

1. Corticosterone, a natural corticosteroid hormone in birds, when injected into domestic fowl (Gallus domesticus) (2000 micrograms.kg-1, 4-5 h before experiment) increases both the basal Isc (short-circuit current) and amiloride-sensitive Isc as well as the PD across the colon in vitro. Dexamethasone, a synthetic analogue (650 micrograms.kg-1, 4-5 h before experiment) also increases the basal and amiloride-sensitive Isc as well as PD in these preparations. 2. In marked contrast, longer term injection or infusion of dexamethasone (650 micrograms.kg-1) for 3 or more days caused a decline in basal Isc and PD (the PD often reversed with the serosal side becoming electronegative) and a drop in resistance. However in these preparations, the amiloride-sensitive Isc was significantly elevated which could be accounted for by an increase in net Na flux. 3. No significant change occurs in net flux of Cl or K although unidirectional fluxes in both directions were increased for both ions in birds given dexamethasone for 3 days. 4. A disparity between the basal Isc and the amiloride-sensitive Isc appeared in these preparations from dexamethasone injected birds reflecting the transport of other ions, possibly HCO3- or H+. The possible role of corticosterone in mineral metabolism of birds is discussed.

Function and regulation of the avian caecal bulb: influence of dietary NaCl and aldosterone on water and electrolyte fluxes in the hen (Gallus domesticus) perfused in vivo

The function of the caecal bulb, and its adaptation to chronic high- or low-Na+ intake, was investigated by in vivo perfusion of anaesthetised birds. Effects of acute aldosterone injection (125 micrograms.kg-1 body mass) were also measured. Evidence was found for primary active net absorption of Na+, inducing parallel Na-linked absorption of water and Cl- and secretion of K+. Around 20-35% of total Cl- absorption and K+ secretion were independent of Na+ fluxes, and these components appear to be driven by passive processes with apparent conductances of 6.3 X 10(-3) (GCl) and 1.1 X 10(-3) (GK) S.cm-2. Acetate (40 mM) stimulated Na+ fluxes (8.5-9.9 microEq.cm-2.h-1) and Na-linked water fluxes (27-44 microliters.cm-2.h-1). Increased coupling ratios (2.9-4.6 microliters.microEq-1) and other data indicate that these effects may be due to increased osmotic permeabilities of barriers involved in the Na-linked water transfer pathway. Low-Na+ maintenance enhanced EPD (49-69 mV, serosa positive) and all net fluxes: JNa (6.8-11.6); JK (-3.2--4.3); JCl (4.3-5.6 microEq.cm serosal area-2.h-1); Jv (28-43 microliters.cm-2.h-1) (mucosal-serosal fluxes positive). Acute aldosterone enhanced JNa (10.8-14.0 microEq.cm-2.h-1) and EPD (54-66 mV) by 3 h after injection, but had no effect on the Na-linked components of JK or JCl.

Autoregulation of apical sodium entry in the colon of the frog (Rana esculenta)

1. Na transport (INa) in the K-depolarized colon of the frog was investigated by electro-physiological current-voltage analysis. 2. INa and the intracellular Na activity [(Na)c] increased with increasing mucosal Na concentration ([Na]m), whereas the apical Na-permeability (PNam) and the transepithelial resistance (RT) decreased. 3. The results are consistent with a Na self-inhibition mechanism; however, a feedback inhibition of INa by intracellular Na must also be considered.

Noise analysis of cAMP-stimulated Na current in frog colon

The effects of oxytocin and cAMP on the electrogenic Na+-transport in the short-circuited epithelium of the frog colon (Rana esculenta, Rana temporaria) were investigated. Oxytocin (100 mU.ml-1) elevated the short-circuit current (Isc) transiently by 70% whereas cAMP (1 mmol.l-1) elicited a comparable sustained response. The mechanism of the natriferic action of cAMP was studied by analysing current fluctuations through apical Na+-channels induced by amiloride or CDPC (6-chloro-3,5-diaminopyrazine-2-carboxamid). The noise data were used to calculate Na+-channel density (M) and single apical Na+-current (iNa). iNa-Values obtained with amiloride and CDPC were 1.0 +/- 0.1 pA (n = 5) and 1.1 +/- 0.2 pA (n = 6) respectively and unaffected by cAMP. On the other hand, cAMP caused a significant increase in M from 0.23 +/- 0.08 micron-2 (n = 5) to 0.49 +/- 0.17 micron-2 (n = 5) in the amiloride experiments. In our studies with CDPC we obtained smaller values for M in control (0.12 +/- 0.04 micron-2; n = 6) as well as during cAMP treatment (0.19 +/- 0.06 micron-2; n = 6). However, the cAMP-induced increase in M was also significant. We conclude that cAMP stimulates Na+-transport across the frog colon by activating "silent" apical Na+-channels. Thus, the mechanism of regulation of colonic Na-transport in frogs differs considerably from that in other vertebrates as mammals and birds.

Potassium secretion by rat distal colon during acute potassium loading: effect of sodium, potassium intake and aldosterone

1. Potassium secretion by the distal colon before and during intravenous infusion of a potassium load was measured in vivo in groups of rats treated in various ways: A, normal control; B, adrenalectomized; C, sodium depleted; D, on potassium-rich diet for 7 days; E, after 72 h aldosterone (1 microgram/h); F, after 72 h aldosterone (10 micrograms/h). 2. Potassium infusion produced no increase of secretion in the adrenalectomized rats but in all the other groups it increased by 2- to 3-fold. Secretion during infusion correlated well with the basal (pre-infusion) rate and in groups C and D reached 140 +/- 15 and 173 +/- 17 nmol min-1 cm-1 respectively compared with 28 +/- 6 nmol min-1 cm-1 in the controls (A). The passive paracellular pathway for potassium was unaffected by the infusion. Amiloride (100 mumol/l) did not significantly affect potassium secretion rate either before or during the acute potassium infusion. The potassium channel blocker, tetraethylammonium chloride, reduced both basal and the secretion rate during infusion. 3. Transepithelial potential difference (PD), active sodium absorption and sodium fluxes were similar in normal controls and rats fed the potassium-rich diet. However, the PD was partially amiloride sensitive in the latter group although amiloride insensitive in the normal group. In sodium-depleted rats, the PD was elevated and totally amiloride insensitive. 4. In both aldosterone-treated groups (E and F), basal potassium secretion rate was high and similar, and during potassium infusion rose 3-fold to 114 +/- 24 (E) and 105 +/- 5 (F) nmol min-1 cm-1. However, the PD was not elevated significantly in group E and was only partially amiloride sensitive, whereas in those infused at the higher rate (F) the PD was increased and was totally amiloride sensitive. 5. The high potassium secretion rates developed by this epithelium in sodium-restricted and potassium-enriched dietary states appear to depend on the presence of an amiloride-insensitive transcellular potassium pathway which is induced at a lower level of aldosterone stimulation than is the amiloride-sensitive transcellular sodium pathway.

Electrophysiological analysis of sodium-transport in the colon of the frog (Rana esculenta). Modulation of apical membrane properties by antidiuretic hormone

Sodium transport and apical bioelectrical membrane properties were investigated in frog colonic epithelium in the absence and presence of the antidiuretic hormone arginine-vasotocin (AVT). Apical Na-permeability and intracellular Na-activity were evaluated by analysis of current-voltage relationships in the serosally K-depolarized tissue. Tissue- and apical membrane capacitance were measured by voltages step analysis. The frog colon was found to be a tight epithelium with a transepithelial resistance of 2.63 +/- 0.25 k omega.muF (n = 17). 85-90% of short circuit current (11.2 +/- 1.1 microA.microF.l-1; n = 17) was related to electrogenic Na-transport from mucosa to serosa. Graded doses of amiloride (less than 50 mumol.l-1) induced Michaelis-Menten-type inhibition kinetics. Serosal addition of 10(-6) mol.l-1 AVT induced a significant increase in sodium current (25%), apical sodium permeability (19%) and tissue capacitance (4.3%) whereas intracellular Na-activity remained unchanged. There was a good correlation between increased Na-current and apical Na-permeability. No correlation was found between Na-current and membrane capacitance. Our results demonstrate that in contrast to other species the amphibian colon shows a natriferic reaction to AVT. We suggest that the regulation of Na-transport in frog colon is similar to that in the toad urinary bladder. It is caused by an activation of preexisting apical Na-channels and not by fusion of subapical cytoplasmic vesicles with the apical membrane.

A low-salt diet facilitates Cl secretion in hen lower intestine

The regulation of sodium and chloride transport in hen coprodeum by mineralocorticoids was investigated with isolated epithelia under short-circuit conditions. Unidirectional fluxes of Na and Cl were measured by isotopes and modulated by amiloride, theophylline and bumetanide. Hens were maintained either on low-NaCl diet (LS) or on high-NaCl diet (HS). Plasma aldosterone (PA) levels of these groups were measured with radioimmunoassay. A group of HS hens received injections of aldosterone on a 6-hr schedule before experiments. Another group of LS hens was resalinated, and experiments carried out on a 24-hr interval. Salt deprivation stimulated PA levels ninefold, compared to HS hens. Na absorption was stimulated according to previous reports. Electrogenic Cl secretion was elicited by theophylline and partially inhibited by bumetanide. Modulation of PA levels by diet, resalination or aldosterone injection changed the magnitude of electrogenic Cl secretion in parallel between 0.5 mueq/cm2.hr (HS) and 4 mueq/cm2.hr (LS), with pronounced alteration in tissue resistance. The results demonstrate a new action of aldosterone which besides stimulating Na absorption also directly or indirectly elicits Cl secretion. Evidence is presented for a hormonal adaptation of chloride transport in this epithelium. There was a morphological change of the apical plasma membrane and further experiments will have to clarify the exact cellular nature of this process.

Modulation of Na and Cl transport by mineralocorticoids

1. The epithelia of the hen lower intestine show a Na-channel, Na-cotransport, chloride cells, and chloride absorption and secretion. 2. The short circuit current is affected by low salt levels, amiloride, glucose, lysine, leucine, galactose, ouabain, bumetanide, aldosterone, dexamethasone and spironolactone. 3. The properties of the different sodium and chloride channels are described.