Structure of tight junctions during Cl secretion in the perfused rectal gland of the dogfish shark

Mechanism of interdigitation formation at apical boundary of MDCK cell

It has been reported that the MDCK cell tight junction shows stochastic fluctuation and forms the interdigitation structure, but the mechanism of the pattern formation remains to be elucidated. In the present study, we first quantified the shape of the cell-cell boundary at the initial phase of pattern formation. We found that the Fourier transform of the boundary shape shows linearity in the log-log plot, indicating the existence of scaling. Next, we tested several working hypotheses and found that the Edwards-Wilkinson equation, which consists of stochastic movement and boundary shortening, can reproduce the scaling property. Next, we examined the molecular nature of stochastic movement and found that myosin light chain puncta may be responsible. Quantification of boundary shortening indicates that mechanical property change may also play some role. Physiological meaning and scaling properties of the cell-cell boundary are discussed.

Morphology of the rectal gland of the spiny dogfish (Squalus acanthias) shark in response to feeding

The morphology of the rectal gland was examined in spiny dogfish ( Squalus acanthias L, 1758) sharks fasted (1 week) or 6 and 20 h postfeeding. The morphology of the fasted gland showed a pattern reflecting a dormant physiology, with thick gland capsule, thick stratified epithelium, and secretory parenchyma with tubules of small diameter and lumen. The secretory cells of the tubular epithelium were enlarged and irregularly shaped with abnormally condensed or highly vacuolized cytoplasm containing numerous lysosomes. Early-stage apoptotic cells were not uncommon. Secretory cells showed signs of low activity, e.g., mitochondria with weakly stained matrix and small cristae, poorly branched infoldings of basolateral membranes, and microvesicle-free subapical cytoplasm. All characteristics examined changed significantly upon feeding, consistent with increased salt and fluid secretion: the outer capsule muscle layer and the stratified epithelium decreased in diameter; the tubules enlarged; the secretory cells showed extensive development of the basolateral membrane, more mitochondria, and abundant apical microvesicles. Secretory cell apical surface was increased. The minor differences between morphology in 6 and 20 h postfeeding indicated that changes took place rapidly and were complete by 6 h. Our results are discussed in the context of prior studies of metabolism, proteomics, and cellular pathways of gland activation.

Real time measurements of water flow in amphibian gastric glands: modulation via the extracellular Ca2+-sensing receptor

The mechanisms for the formation of the osmotic gradient driving water movements in the gastric gland and its modulation via the extracellular Ca(2+)-sensing receptor (CaR) were investigated. Real time measurements of net water flux in the lumen of single gastric glands of the intact amphibian stomach were performed using ion-selective double-barreled microelectrodes. Water movement was measured by recording changes in the concentration of impermeant TEA(+) ions ([TEA(+)](gl)) with TEA(+)-sensitive microelectrodes inserted in the lumen of individual gastric glands. Glandular K(+) (K(+)(gl)) and H(+) (pH(gl)) were also measured by using K(+)- and H(+)-sensitive microelectrodes, respectively. Stimulation with histamine significantly decreased [TEA](gl), indicating net water flow toward the gland lumen. This response was inhibited by the H(+)/K(+)-ATPase inhibitor, SCH 28080. Histamine also elicited a significant and reversible increase in [K(+)](gl) that was blocked by chromanol 293B, a blocker of KCQN1 K(+) channels. Histamine failed to induce net water flow in the presence of chromanol 293B. In the "resting state," stimulation of CaR with diverse agonists resulted in significant increase in [TEA](gl). CaR activation also significantly reduced histamine-induced water secretion and apical K(+) transport. Our data validate the strong link between histamine-stimulated acid secretion and water transport. We also show that cAMP-dependent [K(+)](gl) elevation prior to the onset of acid secretion generates the osmotic gradient initially driving water into the gastric glands and that CaR activation inhibits this process, probably through reduction of intracellular cAMP levels.

Decreased hepatocellular volume and intact morphology of tight junctions in calcium deprivation-induced cholestasis. Stereological and multiple indicator dilution analysis

Cholestasis induced by perfusion of the liver with hypocalcemic media has been ascribed to several defects in bile secretion including increased biliary permeability. To investigate this model of cholestasis further, livers perfused with hypo- and normocalcemic media were examined stereologically using thin sections and freeze-fracture replicas. Organization of tight junctions was not altered by hypocalcemia; neither the number of strands nor the junctional depth were significantly affected. By contrast, the volume of hepatocytes decreased by 11% (p less than 0.001), compensated for by an increase in the space of Dissé and of the sinusoids. The canalicular length decreased by 25% (p less than 0.01), while the canalicular membrane surface was not altered. Multiple indicator dilution studies confirmed a decrease in hepatocellular volume, measured as the water space by 14% (p less than 0.03). This was compensated for by an increase in the extravascular sucrose, but not the albumin space. Immediately after switching from normo- to hypocalcemic perfusate a K+ efflux of 62 mumol/g liver was observed corresponding to approx. 8% of the hepatocellular water space. Our results suggest that hypocalcemia-induced cholestasis is due, at least in part, to a disturbance of the osmotic equilibrium, possibly caused by impairment of an ion transport system involved in hepatocellular volume control.

Direct current electrical measurement in epithelia: steady-state and transient analysis

A method has been presented for the determination of resistance of biological tissues in which the PD response to step currents is determined. The delta PD after the dielectric capacitors are charged, divided by the current, gives the resistance, provided the current density is low enough so that the tissue behaves as a linear-bilateral system. In the gastric mucosa the PD continues to increase after the dielectric capacitors are charged and it is shown that this part of the delta PD is due to polarization of EMFs and should not be used in determining the resistance. It has been shown that (1) resistance measurements have enabled us to demonstrate that during acid secretion there is a neutral mechanism(s) for the movement of HCO3- out of and the entrance of Cl- into the oxyntic cells, (2) the transmucosal resistance varies inversely with the rate of acid secretion, and (3) the low resistance of the secreting frog fundus is due to the low resistance of the lumen-tubular cell pathway--the parallel pathways (the TIC or paracellular and surface cell pathways) have high resistances. The results of both the resistance and PD measurements have recently been analyzed with respect to the problem of whether the proton pump is neutral or electrogenic in the intact tissue.

Ultrastructural features of chloride cells in the gill epithelium of the Atlantic salmon, Salmo salar, and their modifications during smoltification

To elucidate the ultrastructural modifications of the gill epithelium during smoltification, gills of the Atlantic salmon (Salmo salar) were examined by electron microscopy at three stages of this process, which were defined as follows: "parrs" were freshwater fish that had not yet started their transformation; "freshwater smolts" were freshwater fish that were ready to enter seawater; and "seawater smolts" were smolts that had been transferred from fresh water and maintained for 4 days in seawater (35%). In the gill epithelium of parrs, there were two types of chloride cells. The large chloride cells contained deeply stained mitochondria and numerous apical, irregular, dense, membrane-bound bodies that formed 77% of the chloride cell population and were distinguished easily from small chloride cells that have distinctly paler mitochondria and no dense bodies in their apical cytoplasm. In freshwater smolts, the large chloride cells formed 95% of the chloride-cell population. In contrast to the small chloride cells that were not modified, they almost doubled in size. Their tubular system developed extensively to form a tight network with regular meshes significantly smaller than those observed in parr chloride cells. Forty percent of the large chloride cells were associated with a new type of cell, the accessory cell, to which they were bound by shallow apical junctions. Half of these accessory cells were not seen to be in contact with the external medium. In seawater smolts, 80% of the large chloride cells were associated with accessory cells. Most accessory cells reached the external medium and sent numerous cytoplasmic interdigitations within the apical portion of the adjacent chloride cells. As a result, a section through the apical portion of the chloride cells and their associated accessory cells revealed a mosaic of interlocked cell processes bound together by an extended, shallow apical junction. It was concluded that the Atlantic salmon develops in fresh water most of the ultrastructural modifications of the gill epithelium which in most euryhaline fish are triggered by exposure to seawater. The effective transfer into seawater would act only as a final stimulus to achieve some adequacy between the freshwater smolt and its new environment.

Cell solute composition and potassium effects in slices of the rectal gland of the dogfish shark (Squalus acanthias)

Cell solute composition and apparent intracellular ionic concentrations (subscript i) were determined in rectal gland slices of the dogfish shark (Squalus acanthias). On aerobic incubation in elasmobranch Ringer's at 15 degrees C, the steady state values were: Ki+, 151 mM, close to its apparent electrochemical equilibrium; Cai2+, 1.7 mM; Mgi2+, 7.5 mM; Cl1-, 97 mM (maintained against its electrochemical potential gradient); the sum (Nai+ + Ki+) was about 66 meq/kg cell water lower than in the medium. Of free amino acids, only the effectively impermeable anionic taurine (32 mM) and glutamate (3.7 mM) significantly contribute to the Donnan system. Variations of the Ringer's K+ concentrations, or the presence of 0.5 mM ouabain, demonstrated the permeability of the cell membrane to na+, K+, and Cl-. Cell volume changes: 0.5 mM ouabain produced a gradual swelling and ionic changes consistent with an inhibition of the sodium pump. In high-K+ media the cells swelled massively and the membrane depolarized. A linear relationship between cell volume changes and the Nernst potential for K+ was found. These properties are consistent with a pump-and-leak system for cell volume maintenance. The K+ -induced cell swelling was found to be reversible on transfer of the tissue to regular Ringer's. The extrusion of cell water was associated with an influx of Na+ into, and an efflux of K+ from, the cells (both fluxes down-hill), as well as extrusion of Cl- against its concentration gradient.2+

Mechanism of NaCl secretion in the rectal gland of spiny dogfish (Squalus acanthias). I. Experiments in isolated in vitro perfused rectal gland tubules

Rectal gland tubules (RGT) of spiny dogfish were dissected and perfused in vitro. Transepithelial PD (PDte), resistance (Rte), the PD across the basolateral membrane (PDbl) and intracellular chloride and potassium activities (a cell Cl-, a cell K+) were measured. In a first series, 67 RGT segments were perfused with symmetric shark "Ringers" solution. The bath perfusate contained in addition db-cAMP 10(-4), forskolin 10(-6), and adenosine 10(-4) mol X 1(-1). PDte was -11 +/- 1 (n = 67) mV lumen negative, Rte 27 +/- 2 (n = 47) omega cm2, PDbl -75 +/- 0.4 (n = 260) mV. a cell K+ and a cell Cl- were 109 +/- 22 (n = 4) and 38 +/- 4 (n = 36) mmol X 1(-1) respectively. These data indicate that Cl-secretion across the RGT must be an uphill transport process, whereas secretion of Na+ could be driven by the lumen negative PDte. Intracellular K+ is 14 mV above equilibrium with respect to the basolateral membrane PD and Cl- is 23 mV above equilibrium across the apical membrane. In series 2, the conductivity properties of the apical and basolateral membrane as well as that of the paracellular pathway were examined in concentration step experiments. Decrease of the basolateral K+ concentration led to a rapid hyperpolarization of PDbl with a mean slope of 19 mV per decade of K+ concentration change. Addition of 0.5 mmol X 1(-1) Ba2+ to the bath solution lead to a marked depolarization and abolished the response to K+ concentration steps. In the lumen a Cl- concentration downward step led to a depolarization of the lumen membrane; resulting in a mean slope of 18 mV per decade of Cl- concentration change. When dilution potentials were generated across the epithelium, the polarity indicated that the paracellular pathway is cation selective. In series 3 the equivalent short circuit current (Isc = PDte/Rte) was determined as a function of symmetrical changes in Na+ concentration, with Cl- held at 276 mmol X 1(-1), and as a function of symmetrical changes in Cl- concentration, with Na+ held at 278 mmol X 1(-1). Isc was a saturable function of Na+ concentration (Hill coefficient 0.9 +/- 0.1, K1/2 4.4 mmol X 1(-1), n = 7) and also a saturable function of Cl- concentration (Hill coefficient 2.0 +/- 0.1, K1/2 75 mmol X 1(-1), n = 11).(ABSTRACT TRUNCATED AT 400 WORDS)

Partitioning of paracellular conductance along the ileal crypt-villus axis: a hypothesis based on structural analysis with detailed consideration of tight junction structure-function relationships

Current models of intestinal transport suggest cells which absorb ions are located on the villus while secretory cells are located in the crypt and putatively have paracellular pathways which are highly conductive to Na+. One approach to assess possible variation in small intestinal paracellular conductance along the crypt-villus axis is to morphometrically analyze the structural aspects of crypt and villus tight junctions (TJs) which relate to paracellular resistance. Such detailed analysis of junctional structure in this heterogeneous epithelium would permit one to compare intestinal TJ structure-function relationships with those in a structurally simpler epithelium such as that of toad urinary bladder. This comparison would also be of considerable interest since previous similar comparisons have failed to consider in detail the geometric dissimilarity between these two epithelia. We applied light, electron microscopic, and freeze-fracture morphometric techniques to guinea pig ileal mucosa to quantitatively assess, for both crypts and villi, linear TJ density, relative surface contributions, and TJ strand counts. Mean linear TJ densities were 76.8 m/cm2 for crypt cells and 21.8 m/cm2 for villus absorptive cells. Mean TJ strand counts were 4.45 for undifferentiated crypt cell TJs and 6.03 for villus absorptive cell TJs. The villus constituted 87% and the crypt 13% of total surface. We utilized these data to predict paracellular conductance of crypts vs. villi based on equations derived from those of Claude (P. Claude, J. Membrane Biol. 39:219-232, 1978). Such analysis predicts that 73% of ileal paracellular conductance is attributable to the crypt. Furthermore, we obtained literature values for paracellular resistance in mammalian ileum and toad urinary bladder and for toad bladder TJ structure and linear density and constructed a relationship which would allow us to more accurately compare TJ structure-function correlates between these two epithelia. Such a comparison, which considers both surface amplification and TJ structure and distribution in these epithelia, shows that one would predict in vitro measured values for paracellular resistance should be approximately two orders of magnitude less in mammalian ileum than in toad urinary bladder. This predicted discrepancy (115-fold) correlates well with the observed difference (100-fold).(ABSTRACT TRUNCATED AT 400 WORDS)

Pardaxin increases solute permeability of gills and rectal gland in the dogfish shark (Squalus acanthias)

The action of the ichthyotoxic secretion of the Red Sea flatfish Pardachirus marmoratus and its derived toxin, pardaxin, was examined in the dogfish shark (Squalus acanthias). Pardaxin was more toxic when administered to the bathing medium than when injected into a dorsal artery and it transiently diminished the spiracular rate and caused a severe struggling response in the adult shark only when administered to the head region of the shark. Pardaxin caused a transient leakage to urea and sodium between the shark and the seawater. In the isolated perfused rectal gland pardaxin irreversibly reduced the rate of chloride secretion and concentration gradient of urea between perfusate and rectal gland fluid. In addition, ultrastructural studies on the rectal gland showed that ionic lanthanum penetrated the tight junctions and foci of cell necrosis were observed. These studies indicate that in shark the gills are the most probable target of the toxicity of pardaxin.

Perfusion of isolated tubules of the shark rectal gland. Electrical characteristics and response to hormones

Both the mammalian thick ascending limb of Henle's loop and the shark rectal gland actively transport Cl against an electrochemical gradient by mechanisms involving hormone-sensitive NaCl transport. In contrast to mammalian renal tubules, individual tubules of the shark rectal gland previously have not been perfused in vitro. Using a combination of renal slice and microdissection techniques we were able to isolate and perfuse single rectal gland tubules without the use of enzyme treatment. Single tubules consistently generated lumen-negative transepithelial voltages (Vt) of -1.8 mV when perfused and bathed with identical shark Ringer's solution. The addition of cyclic AMP, vasoactive intestinal peptide (VIP), and adenosine to the bath increased Vt to -7.5, -9.0, and -4.3 mV, respectively (all P less than 0.02 compared with paired controls). Each stimulation could be reversed by addition by furosemide to the bath. The adenosine response was inhibited by theophylline, a specific inhibitor of adenosine receptors. The tubules had a low transepithelial electrical resistance of 12-26 omega X cm2 and exhibited a transepithelial permselectivity for small cations. These results indicate that tubules of the rectal gland can be perfused in vitro and have receptors for VIP and adenosine. Cyclic AMP and secretagogues hyperpolarize the membrane consistent with electrogenic chloride transport, and these effects are reversed by furosemide, an inhibitor of coupled sodium-potassium-chloride co-transport. The response of Vt to cyclic AMP and furosemide, the transepithelial electrical resistance, and the cation selective permeability of tubules are remarkably similar to measurements in perfused mammalian thick ascending limbs.