scholarly journals Changes in the structural components of the mucous membrane and plasma proteins of small intestine enterocytes in fetuses of cattle

2021 ◽  
Vol 23 (102) ◽  
pp. 24-28
Author(s):  
D. M. Masiuk
Keyword(s):  
Alkaline Phosphatase ◽  
Mucous Membrane ◽  
Large Intestine ◽  
Plasma Proteins ◽  
Apical Membrane ◽  
Morphological Changes ◽  
Basolateral Membrane ◽  
Intestinal Wall ◽  
Morphometric Parameters ◽  
Enzymatic Systems

The interrelationships of morphological changes of the mucosa of the large intestine with the dynamics of enzyme activity at different poles of enterocytes of cattle during the fetal period of ontogenesis are shown and analyzed. The work was performed on Holstein cattle aged two to nine months. The activity of hydrolytic and transport enzymatic systems in the basolateral membrane of cattle enterocytes depends less on the morphometric parameters of the large intestine than in the apical membrane. It is proved that during the fruitful period of ontogenesis there are dynamic transformations of enzyme systems of enterocytes of the large intestine of cattle which are connected with morphological changes of the mucous membrane of the large intestine. The activity of hydrolytic and transport enzymes in different domains of the plasma enterocytes of fetal cattle is interrelated with the morphometric parameters of the colon, in particular. the activity of alkaline phosphatase, γ-glutamyltransferase, Na+, K+-ATPase, Ca2 +, Mg2 +-ATPase and Mg-ATPase in the apical membrane of enterocytes is inversely related to the thickness of the intestinal wall with villi and the mucous membrane of the cavity (P ≤ 0.01–0.001). A significant correlation was found between the activity of alkaline phosphatase, γ-glutamyltransferase, Na+, K+-ATPase, Ca2 +, Mg2 +-ATPase and Mg-ATPase in the apical membrane of enterocytes with villi height (P ≤ 0.05). The activity of hydrolytic and transport enzymatic systems in the basolateral membrane of bovine fetal enterocytes depends to a lesser extent on the morphometric parameters of the large intestine than in the apical membrane. In particular, only the activity of alkaline phosphatase and γ-glutamyltransferase is inversely related to the thickness of the intestinal wall with villi and the mucous membrane of the jejunum with villi (P ≤ 0.01–0.001). The activity of transport enzymes is directly related to the morphometric parameters of the colon (P ≤ 0.01). The activity of these enzymes in the basolateral membrane does not significantly depend on the width of the villi, but is dependent on their height.

scholarly journals The morphological changes relationship in the small intestine mucous membrane with the dynamics of the enterocyte’s membranes polypeptide composition in cattle during the fetal period of their ontogenesis

2021 ◽  
Vol 9 (1) ◽  
pp. 52-56
Author(s):  
D. M. Masiuk
Keyword(s):  
Molecular Weight ◽  
Mucous Membrane ◽  
Apical Membrane ◽  
Morphological Changes ◽  
Basolateral Membrane ◽  
Intestinal Wall ◽  
Current Data ◽  
Polypeptide Composition ◽  
Fetal Period ◽  
Molecular Weights

The article presents current data on the relationship of morphological changes in the jejenum intestine mucous membrane with the dynamics of the polypeptide composition in the cattle enterocytes membranes during the fetal period of ontogenesis. The study was carried out on 80 fetuses of Holstein cattle, aged 2–9 months, with a body weight of 0.6–39 kg. It was found that during the fetal period of ontogenesis, structural transformations took place in the jejunal mucous membrane of cattle, characterized by intense morphofunctional changes in its structural components. The polypeptide composition of the jejunal enterocytes plasma membrane in cattle during the fetal period of ontogenesis was determined, in particular, in the apical and basolateral membranes, respectively, 27 and 25 protein fractions with a molecular weight of 9.6–14.2 kDa to 300 kDa were found, respectively. With the help of the correlation analysis for the received data, reliable interconnections of morphological changes in the jejunal mucous layer with the dynamics of the polypeptide composition of the cattle enterocyte’s membranes during the fetal period of ontogenesis were obtained. It has been proven that the thickness of the intestinal wall with villi and the mucous membrane with villi is directly related to the content of proteins with a molecular weight of 250 kDa and 155 kDa on the apical domain of enterocytes (P ≤ 0.01–0.001). The height of the villi is associated only with the content of polypeptides with a molecular weight of 250 kDa and 155 kDa (P ≤ 0.05–001) in the apical membrane and is inversely related to the content of polypeptides with a molecular weight of 9.6–14.2 kDa, 21 kDa, 22.5 kDa, 26 kDa, 33 kDa, 35 kDa, 170–185 kDa (P ≤ 0.01–0.001). The width of the villi is related to the content on the apical membrane of proteins with a molecular weight of 170–185 kDa and 21 kDa (P ≤ 0.05–0.01). The thickness of the intestinal wall with villi and mucous membrane with villi is associated with the content of proteins with molecular weights 155 kDa, 100 kDa, 87 kDa, 66 kDa, 52 kDa) and 43 kDa (P ≤ 0.05–0.001) on the basolateral domain of enterocytes and inversely dependent on the content of polypeptides with molecular weights of 19 kDa, 21 kDa and 31 kDa (P ≤ 0.05). The height of enterocyte villi is directly related to the content of polypeptides with a molecular weight of 155 kDa and 52 kDa (P ≤ 0,05–0,01) in the basolateral membrane and is inversely dependent on the content of polypeptides with a molecular weight of 24 kDa, 22.5 kDa and 17 kDa (P ≤ 0.05–0.001). The width of the villi of enterocytes is reliably inversely related only to the content of proteins with a molecular weight of 155 kDa on the basolateral membrane (P ≤ 0.05).

scholarly journals The enzyme activity dynamic relationship with the content of structural polypeptides of enterocyte membranes in cattle fetal

2021 ◽  
Vol 4 (2) ◽  
pp. 3-6
Author(s):  
D. M. Masiuk
Keyword(s):  
Alkaline Phosphatase ◽  
Enzyme Activity ◽  
Protein Content ◽  
Apical Membrane ◽  
Basolateral Membrane ◽  
Molecular Weights ◽  
Dynamic Relationship ◽  
Molecular Masses ◽  
Alkaline Activity ◽  
The Relationship

The article performs new data on the relationship between the hydrolytic and transport enzyme activity at different poles of the enterocytes plasmolemma of cow's fetal large intestine with the content of individual fractions of polypeptides. An expressive direct dependence of enzyme activity dynamics on the apical and basolateral membranes of enterocytes containing low molecular weight proteins and an inverse relationship with the concentration of proteins with medium and large molecular weights has been proved. It was found that the alkaline activity of the phosphatase and γ-glutamyltransferase on the apical domain of enterocyte plasmolemma is directly related to the proteins content with molecular masses of 9.6–14.2 kD, 21 kD, 22.5 kD, 26 kD, 33 kD, 35 kD, 170–185 kD, and 205 kD (P ≤ 0.05–0.001). Gamma-glutamyltransferase activity is straightly related to protein quantity with molecular weights of 15.5 kDa and 39 kD (P ≤ 0.05). In contrast, alkaline phosphatase and GGT activity have inverse correlations with the content of polypeptides with molecular masses of 46 kD, 63 kD, and 250 kD in the apical membrane of enterocytes (P ≤ 0.01–0.001). The lactase activity in the cattle enterocytes apical membrane during the test period has significant direct correlations only with the amount of the polypeptide of polypeptides with molecular weights of 31 kD, 39 kD, and 100 kD (P ≤ 0.05–0.01) and inverse relationships containing proteins with molecular masses of 46 kD and 120 kD (P ≤ 0.05). A linear dependence of the different ATPase activity of the apical membrane of red blood cells containing proteins with molecular weights of 9.6–14.2 kD, 15.5 kD, 21 kD, 22.5 kD, 33 kD, 35 kD, 39 kD, and 205 kD (P ≤ 0.05–0.001) was observed. Alkaline phosphatase activity in the apical membrane of enterocytes is only directly related to the number of proteins with molecular weights of 17 kD and 24 kD (P ≤ 0.001) in this domain. It inversely depends on the content of proteins with molecular masses of 9.6–14.2 kD and 52 kD (P ≤ 0.001). G-glutamyltransferase activity is inversely related to protein content with molecular weights of 43 kD, 52 kD, 66 kD, 87 kD, and 100 kD and 155 kD (P ≤ 0.001). The Ca2+, Mg2+-ATPase of the basolateral membrane activity of enterocytes is directly related to the protein amount with molecular weights of 26 kD (P ≤ 0.01), Mg2+-ATPase and Mg2+-ATPase with protein content with the molecular value of 100 kD (P ≤ 0.05).

scholarly journals Intestinal mucosal lymphocytes in neonatal sepsis

2019 ◽  
Vol 64 (4) ◽  
pp. 52-59
Author(s):  
Kh. S. Khaertynov ◽  
V. A. Anokhin ◽  
G. R. Burganova ◽  
G. O. Pevnev ◽  
M. O. Mavlikeev ◽  
...  
Keyword(s):  
Mucous Membrane ◽  
Large Intestine ◽  
Intestinal Mucosa ◽  
Neonatal Sepsis ◽  
Morphological Changes ◽  
Intestinal Bacteria ◽  
Main Group ◽  
Control Group ◽  
Small Intestinal Mucosa ◽  
Cd4 Lymphocytes

We studied the autopsy material obtained from 7 children who died in the neonatal period in order to evaluate the composition of lymphocytes of the intestinal mucosa against the background of morphological changes in the tissues of the gastrointestinal tract in newborns with sepsis. The main group consisted of 4 children with neonatal sepsis, the control group – of 3 newborns who died from other causes. The research material included the specimen of the small and large intestine.Results. Small intestine: it was found that there were less CD4 + lymphocytes in the small intestinal mucosa in the group of children who died from neonatal sepsis in 75% of cases than in the control group, but this difference was not statistically significant (p=0.1). There were no differences in the number of CD8 + and CD20 + cells in the studied groups. Large intestine: the number of CD4 + lymphocytes of the mucous membrane of the colon was greater in the main group of children than in the control group (p=0.03). An increase in the number of CD4 + cells was registered in 3 of 4 cases of neonatal sepsis. The number of CD8+ and CD20+ lymphocytes in the studied groups was the same (р>0.05).Conclusion. The increase in T-lymphocytes CD4+ in the mucous membrane of the large intestine is probably connected with the antigenic stimulation of opportunistic intestinal bacteria. We found no morphological signs of the suppression of the cells of adaptive immunity associated with the intestinal mucosa. 

scholarly journals MODERN MORPHOLOGICAL DIAGNOSTICS OF ULCERATIVE COLITIS AND IRRITABLE BOWEL SYNDROME: HISTOLOGICAL, HISTOCHEMICAL AND IMMUNOHISTOCHEMICAL CRITERIA

2021 ◽  
Vol 74 (3) ◽  
pp. 381-387
Author(s):  
Pavlo P. Snisarevskyi
Keyword(s):  
Diagnostic Criteria ◽  
Mucous Membrane ◽  
Large Intestine ◽  
Morphological Changes ◽  
Goblet Cells ◽  
Proliferative Potential ◽  
Statistical Research ◽  
Biopsy Material ◽  
Study Materials ◽  
Irritable Bowel

The aim is to establish diagnostic and differential-diagnostic criteria for UC and IBS based on a complex morphological (histological, histochemical, immunohistochemical) study. Materials and methods: In this study, it was used autopsy and biopsy material – fragments of the mucous membrane of the large intestine. The material was divided into 5 groups. The first group (G 1) included autopsy material from 6 cases, in which, during autopsies and microscopic examination, we found no general pathological processes in the gastrointestinal tract. The second group (G 2) included biopsy material from 34 patients with diagnosed UC of the 1st activity degree. The third group (G 3) included the biopsy material of 27 patients with UC of the 2nd degree of activity. The fourth group (G 4) included biopsy material from 19 patients, diagnosed with UC of the 3rd degrees of activity. The fifth group (G 5) included biopsy material from 82 patients with clinically diagnosed IBS. Histological, histochemical, immunohistochemical, statistical research methods were used. Results: There are characteristic morphological changes in the mucous membrane of the large intestine in UC of varying degrees of activity, such as changes in the architectonics of crypts of varying severity; presence of erosive and ulcerative defects. Inflammatory and desquamative-dystrophic changes take place in the epithelial layer adjacent to erosive and ulcerative defects. The number of goblet cells of crypts decreases and the size of vacuoles in goblet cells reduces. The intensity of mucin secretion contained in the vacuoles of the goblet cells lowers; there is a partial or complete loss of pericryptal myofibroblasts; the proliferative potential of the intestinal crypts epithelium activates. Conclusions: Differential diagnostic criteria, revealed by the author, improve the morphological diagnosis of UC and IBS, optimizing the tactics of managing patients with this colorectal pathology.

scholarly journals Intestine wall histostructure peculiarities with peritonitis and mechanical intestine obstruction (experimental study)

2020 ◽  
pp. 27-34
Author(s):  
Т. I. Tamm ◽  
V. V. Nepomnyaschy ◽  
O. А. Shakalova ◽  
А. Ya. Barduck
Keyword(s):  
Small Intestine ◽  
Mucous Membrane ◽  
Morphological Changes ◽  
Abdominal Cavity ◽  
Statistical Processing ◽  
Intestinal Wall ◽  
Control Group ◽  
Mechanical Ileus ◽  
Fecal Suspension ◽  
Dynamic Ileus

Today, the histological criteria for differential diagnosis of dynamic ileus due to peritonitis and mechanical obstruction of the intestine remain undeveloped. In this regard, the aim of the work was to establish the difference in morphological changes occurring in the intestinal wall during dynamic and mechanical ileus in the experiment. The experiment was conducted on 33 sexually mature Wistar rats. In 15 animals of the first group, mechanical ileus was modeled by ligation of the lumen of the small intestine at the middle of the distance between the duodenojejunal junction and the ileocecal angle. In 15 rats of the second group, a dynamic ileus model was formed in the form of peritonitis by introducing fecal suspension into the lumen of the abdominal cavity. The control group included 3 animals who underwent laparotomy without the formation of mechanical ileus and peritonitis. For histological examination, fragments of the intestinal wall were sampled 1 cm above the site of the obstruction with mechanical ileus and the portion of the small intestine with peritonitis. Statistical processing was performed in an Excel package using parametric statistics methods. It was stated that with mechanical ileus purulent inflammation develops in the intestine wall beginning from the mucous membrane spreading over wall thickness which can cause its destruction within 48 hours; with dynamical ileus purulent inflammation develops in the intestine wall, it captures particularly serous and muscle layers without causing violations of mucosa cover structure and without intestine wall destruction within 48 hours. Under experimental dynamic ileus, changes in the mucous membrane were reactive in nature and consisted of manifestations of compensatory-adaptive and regenerative processes in response to a violation of the trophism of various structures of the intestinal wall.

Dibutyryl cAMP activates bumetanide-sensitive electrolyte transport in Malpighian tubules

1991 ◽  
Vol 261 (3) ◽  
pp. C521-C529 ◽  
Author(s):  
J. L. Hegarty ◽  
B. Zhang ◽  
T. L. Pannabecker ◽  
D. H. Petzel ◽  
M. D. Baustian ◽  
...  
Keyword(s):  
Yellow Fever ◽  
Apical Membrane ◽  
Basolateral Membrane ◽  
Fluid Secretion ◽  
Ringer Solution ◽  
Malpighian Tubules ◽  
Membrane Voltage ◽  
Dibutyryl Camp ◽  
K Secretion ◽  
Transepithelial Voltage

The effects of dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP) and bumetanide (both 10(-4) M) on transepithelial Na+, K+, Cl-, and fluid secretion and on tubule electrophysiology were studied in isolated Malpighian tubules of the yellow fever mosquito Aedes aegypti. Peritubular DBcAMP significantly increased Na+, Cl-, and fluid secretion but decreased K+ secretion. In DBcAMP-stimulated tubules, bumetanide caused Na+, Cl-, and fluid secretion to return to pre-cAMP control rates and K+ secretion to decrease further. Peritubular bumetanide significantly increased Na+ secretion and decreased K+ secretion so that Cl- and fluid secretion did not change. In bumetanide-treated tubules, the secretagogue effects of DBcAMP are blocked. In isolated Malpighian tubules perfused with symmetrical Ringer solution, DBcAMP significantly hyperpolarized the transepithelial voltage (VT) and depolarized the basolateral membrane voltage (Vbl) with no effect on apical membrane voltage (Va). Total transepithelial resistance (RT) and the fractional resistance of the basolateral membrane (fRbl) significantly decreased. Bumetanide also hyperpolarized VT and depolarized Vbl, however without significantly affecting RT and fRbl. Together these results suggest that, in addition to stimulating electroconductive transport, DBcAMP also activates a nonconductive bumetanide-sensitive transport system in Aedes Malpighian tubules.

Luminal flow rate regulates proximal tubule H-HCO3 transporters

1992 ◽  
Vol 262 (1) ◽  
pp. F47-F54 ◽  
Author(s):  
P. A. Preisig
Keyword(s):  
Proximal Tubule ◽  
Flow Rate ◽  
Initial Rate ◽  
Apical Membrane ◽  
Basolateral Membrane ◽  
Rate Dependence ◽  
Unstirred Layer ◽  
Disulfonic Acid ◽  
Luminal Flow

In vivo microperfusion was used to examine the mechanism of luminal flow rate dependence of proximal tubule acidification. Luminal flow rate was acutely changed between 5 and 40 nl/min, while luminal and peritubular capillary composition were held constant. With inhibition of basolateral membrane base transport by peritubular 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), cell pH (pHi) provides a sensitive index of apical membrane H secretory activity. At a luminal perfusate [HCO3] of 25 mM, progressive increases in luminal flow rate (5----15----25----40 nl/min) caused progressive increases in pHi. This effect was of a smaller magnitude with a luminal perfusate [HCO3] of 60 mM and was further decreased at a luminal perfusate [HCO3] of 100 mM. This pattern of diminished flow rate dependence at higher luminal [HCO3] is consistent with the presence of a luminal unstirred layer, whose composition can be modified by luminal flow rate. The activity of the apical membrane Na-H antiporter, assayed as the initial rate of pHi recovery from an acid load in the presence of peritubular DIDS, was faster at 40 compared with 5 nl/min. Basolateral membrane Na-3HCO3 symporter activity, assayed as the initial rate of pHi recovery from an alkali load in the absence of luminal and peritubular chloride, was faster at 40 compared with 5 nl/min. This effect was eliminated by luminal amiloride, suggesting an indirect effect of flow mediated by changes in pHi secondary to flow rate-dependent changes in apical membrane Na-H antiporter activity. In summary, increases in luminal flow rate directly increase apical membrane H secretion, possibly by modification of a luminal unstirred layer.(ABSTRACT TRUNCATED AT 250 WORDS)

Water permeability of apical and basolateral cell membranes of rat inner medullary collecting duct

1990 ◽  
Vol 259 (6) ◽  
pp. F986-F999 ◽  
Author(s):  
B. Flamion ◽  
K. R. Spring
Keyword(s):  
Water Flow ◽  
Water Permeability ◽  
Cell Membranes ◽  
Apical Membrane ◽  
Collecting Duct ◽  
Basolateral Membrane ◽  
Volume Regulatory Decrease ◽  
Water Permeation ◽  
Inner Medullary Collecting Duct ◽  
Medullary Collecting Duct

To quantify the pathways for water permeation through the kidney medulla, knowledge of the water permeability (Posmol) of individual cell membranes in inner medullary collecting duct (IMCD) is required. Therefore IMCD segments from the inner two thirds of inner medulla of Sprague-Dawley rats were perfused in vitro using a setup devised for rapid bath and luminal fluid exchanges (half time, t1/2, of 55 and 41 ms). Differential interference contrast microscopy, coupled to video recording, was used to measure volume and approximate surface areas of single cells. Volume and volume-to-surface area ratio of IMCD cells were strongly correlated with their position along the inner medullary axis. Transmembrane water flow (Jv) was measured in response to a variety of osmotic gradients (delta II) presented on either basolateral or luminal side of the cells. The linear relation between Jv and delta II yielded the cell membrane Posmol, which was then corrected for membrane infoldings. Basolateral membrane Posmol was 126 +/- 3 microns/s. Apical membrane Posmol rose from a basal value of 26 +/- 3 microns/s to 99 +/- 5 microns/s in presence of antidiuretic hormone (ADH). Because of amplification of basolateral membrane, the ADH-stimulated apical membrane remained rate-limiting for transcellular osmotic water flow, and the IMCD cell did not swell significantly. Calculated transcellular Posmol, expressed in terms of smooth luminal surface, was 64 microns/s without ADH and 207 microns/s with ADH. IMCD cells in anisosmotic media displayed almost complete volume regulatory decrease but only partial volume regulatory increase.

PBA increases CFTR expression but at high doses inhibits Cl−secretion in Calu-3 airway epithelial cells

1999 ◽  
Vol 277 (4) ◽  
pp. L700-L708 ◽  
Author(s):  
Johannes Loffing ◽  
Bryan D. Moyer ◽  
Donna Reynolds ◽  
Bruce A. Stanton
Keyword(s):  
Protein Expression ◽  
Apical Membrane ◽  
Basolateral Membrane ◽  
Airway Epithelial Cells ◽  
Cell Phenotype ◽  
Apical Plasma Membrane ◽  
Airway Epithelial ◽  
High Concentration ◽  
Cycle Enzyme ◽  
Cf Transmembrane Conductance Regulator

Sodium 4-phenylbutyrate (PBA), a short-chain fatty acid, has been approved to treat patients with urea cycle enzyme deficiencies and is being evaluated in the management of sickle cell disease, thalassemia, cancer, and cystic fibrosis (CF). Because relatively little is known about the effects of PBA on the expression and function of the wild-type CF transmembrane conductance regulator (wt CFTR), the goal of this study was to examine the effects of PBA and related compounds on wt CFTR-mediated Cl−secretion. To this end, we studied Calu-3 cells, a human airway cell line that expresses endogenous wt CFTR and has a serous cell phenotype. We report that chronic treatment of Calu-3 cells with a high concentration (5 mM) of PBA, sodium butyrate, or sodium valproate but not of sodium acetate reduced basal and 8-(4-chlorophenylthio)-cAMP-stimulated Cl−secretion. Paradoxically, PBA enhanced CFTR protein expression 6- to 10-fold and increased the intensity of CFTR staining in the apical plasma membrane. PBA also increased protein expression of Na+-K+-ATPase. PBA reduced CFTR Cl−currents across the apical membrane but had no effect on Na+-K+-ATPase activity in the basolateral membrane. Thus a high concentration of PBA (5 mM) reduces Cl−secretion by inhibiting CFTR Cl−currents across the apical membrane. In contrast, lower therapeutic concentrations of PBA (0.05–2 mM) had no effect on cAMP-stimulated Cl−secretion across Calu-3 cells. We conclude that PBA concentrations in the therapeutic range are unlikely to have a negative effect on Cl−secretion. However, concentrations >5 mM might reduce transepithelial Cl−secretion by serous cells in submucosal glands in individuals expressing wt CFTR.

scholarly journals Electrophysiological effects of basolateral [Na+] in Necturus gallbladder epithelium.

1992 ◽  
Vol 99 (2) ◽  
pp. 241-262 ◽  
Author(s):  
G A Altenberg ◽  
J S Stoddard ◽  
L Reuss
Keyword(s):  
Apical Membrane ◽  
Basolateral Membrane ◽  
Membrane Resistance ◽  
Membrane Voltage ◽  
Gallbladder Epithelium ◽  
K Channels ◽  
Necturus Gallbladder ◽  
Electrophysiological Effects ◽  
Paracellular Diffusion ◽  
Cl Conductance

In Necturus gallbladder epithelium, lowering serosal [Na+] ([Na+]s) reversibly hyperpolarized the basolateral cell membrane voltage (Vcs) and reduced the fractional resistance of the apical membrane (fRa). Previous results have suggested that there is no sizable basolateral Na+ conductance and that there are apical Ca(2+)-activated K+ channels. Here, we studied the mechanisms of the electrophysiological effects of lowering [Na+]s, in particular the possibility that an elevation in intracellular free [Ca2+] hyperpolarizes Vcs by increasing gK+. When [Na+]s was reduced from 100.5 to 10.5 mM (tetramethylammonium substitution), Vcs hyperpolarized from -68 +/- 2 to a peak value of -82 +/- 2 mV (P less than 0.001), and fRa decreased from 0.84 +/- 0.02 to 0.62 +/- 0.02 (P less than 0.001). Addition of 5 mM tetraethylammonium (TEA+) to the mucosal solution reduced both the hyperpolarization of Vcs and the change in fRa, whereas serosal addition of TEA+ had no effect. Ouabain (10(-4) M, serosal side) produced a small depolarization of Vcs and reduced the hyperpolarization upon lowering [Na+]s, without affecting the decrease in fRa. The effects of mucosal TEA+ and serosal ouabain were additive. Neither amiloride (10(-5) or 10(-3) M) nor tetrodotoxin (10(-6) M) had any effects on Vcs or fRa or on their responses to lowering [Na+]s, suggesting that basolateral Na+ channels do not contribute to the control membrane voltage or to the hyperpolarization upon lowering [Na+]s. The basolateral membrane depolarization upon elevating [K+]s was increased transiently during the hyperpolarization of Vcs upon lowering [Na+]s. Since cable analysis experiments show that basolateral membrane resistance increased, a decrease in basolateral Cl- conductance (gCl-) is the main cause of the increased K+ selectivity. Lowering [Na+]s increases intracellular free [Ca2+], which may be responsible for the increase in the apical membrane TEA(+)-sensitive gK+. We conclude that the decrease in fRa by lowering [Na+]s is mainly caused by an increase in intracellular free [Ca2+], which activates TEA(+)-sensitive maxi K+ channels at the apical membrane and decreases apical membrane resistance. The hyperpolarization of Vcs is due to increase in: (a) apical membrane gK+, (b) the contribution of the Na+ pump to Vcs, (c) basolateral membrane K+ selectivity (decreased gCl-), and (d) intraepithelial current flow brought about by a paracellular diffusion potential.

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