Cyclic AMP regulation of active chloride transport in the rectal gland of marine elasmobranchs

The Rectal Gland of the Shark: The Road to Understanding the Mechanism and Regulation of Transepithelial Chloride Transport

Pictured, described, and speculated on, for close to 400 years, the function of the rectal gland of elasmobranchs remained unknown. In the late 1950s, Burger discovered that the rectal gland of Squalus acanthias secreted an almost pure solution of sodium chloride, isosmotic with blood, which could be stimulated by volume expansion of the fish. Twenty five years later, Stoff discovered that the secretion of the gland was mediated by adenyl cyclase. Studies since then have shown that vasoactive intestinal peptide (VIP) is the neurotransmitter responsible for activating adenyl cyclase; however, the amount of circulating VIP does not change in response to volume expansion. The humoral factor involved in activating the secretion of the gland is C-type natriuretic peptide, secreted from the heart in response to volume expansion. C-type natriuretic peptide circulates to the gland where it stimulates the release of VIP from nerves within the gland, but it also has a direct effect, independent of VIP. Sodium, potassium, and chloride are required for the gland to secrete, and the secretion of the gland is inhibited by ouabain or furosemide. The current model for the secretion of chloride was developed from this information. Basolateral NaKATPase maintains a low intracellular concentration of sodium, which establishes the large electrochemical gradient for sodium directed into the cell. Sodium moves from the blood into the cell (together with potassium and chloride) down this electrochemical gradient, through a coupled sodium, potassium, and two chloride cotransporter (NKCC1). On activation, chloride moves from the cell into the gland lumen, down its electrical gradient through apical cystic fibrosis transmembrane regulator. The fall in intracellular chloride leads to the phosphorylation and activation of NKCC1 that allows more chloride into the cell. Transepithelial sodium secretion into the lumen is driven by an electrical gradient through a paracellular pathway. The aim of this review was to examine the history of the origin of this model for the transport of chloride and suggest that it is applicable to many epithelia that transport chloride, both in resorptive and secretory directions.

Sugar uptake, metabolism, and chloride secretion in the rectal gland of the spiny dogfish Squalus acanthias

The rectal gland of the spiny dogfish Squalus acanthias secretes a salt solution isosmotic with plasma that maintains the salt homeostasis of the fish. It secretes salt against an electrochemical gradient that requires the expenditure of energy. Isolated rectal glands perfused without glucose secrete salt, albeit at a rate about 30% of glands perfused with 5 mM glucose. Gradually reducing the glucose concentration is associated with a progressive decrease in the secretion of chloride. The apparent K_m for the exogenous glucose-dependent chloride secretion is around 2 mM. Phloretin and cytochalasin B, agents that inhibit facilitated glucose carriers of the solute carrier 2 (Slc2) family such as glucose transporter 2 (GLUT2), do not inhibit the secretion of chloride by the perfused rectal glands. Phloridzin, which inhibits Slc5 family of glucose symporters, or α-methyl-d-glucoside, which competitively inhibits the uptake of glucose through Slc5 symporters, inhibit the secretion of chloride. Thus the movement of glucose into the rectal gland cells appears to be mediated by a sodium-glucose symporter. Sodium-glucose cotransporter 1 (SGLT1), the first member of the Slc5 family of sodium-linked glucose symporters, was cloned from the rectal gland. No evidence of GLUT2 was found. The persistence of secretion of chloride in the absence of glucose in the perfusate suggests that there is an additional source of energy within the cells. The use of 2-mercapto-acetate did not result in any change in the secretion of chloride, suggesting that the oxidation of fatty acids is not the source of energy for the secretion of chloride. Perfusion of isolated glands with KCN in the absence of glucose further reduces the secretion of chloride but does not abolish it, again suggesting that there is another source of energy within the cells. Glucose was measured in the rectal gland cells and found to be at concentrations in the range of that in the perfusate. Glycogen measurements indicated that there are significant stores of glucose in the rectal gland. Moreover, glycogen synthase was partially cloned from rectal gland cells. The open reading frame of glycogen phosphorylase was also cloned from rectal gland cells. Measurements of glycogen phosphorylase showed that the enzyme is mostly in its active form in the cells. The cells of the rectal gland of the spiny dogfish require exogenous glucose to fully support the active secretion of salt. They have the means to transport glucose into the cells in the form of SGLT1. The cells also have an endogenous supply of glucose as glycogen and have the necessary elements to synthesize, store, and hydrolyze it.

Bicarbonate-sensing soluble adenylyl cyclase is present in the cell cytoplasm and nucleus of multiple shark tissues

The enzyme soluble adenylyl cyclase (sAC) is directly stimulated by bicarbonate (HCO₃⁻) to produce the signaling molecule cyclic adenosine monophosphate (cAMP). Because sAC and sAC‐related enzymes are found throughout phyla from cyanobacteria to mammals and they regulate cell physiology in response to internal and external changes in pH, CO₂, and HCO₃⁻, sAC is deemed an evolutionarily conserved acid‐base sensor. Previously, sAC has been reported in dogfish shark and round ray gill cells, where they sense and counteract blood alkalosis by regulating the activity of V‐type H⁺‐ ATPase. Here, we report the presence of sAC protein in gill, rectal gland, cornea, intestine, white muscle, and heart of leopard shark Triakis semifasciata. Co‐expression of sAC with transmembrane adenylyl cyclases supports the presence of cAMP signaling microdomains. Furthermore, immunohistochemistry on tissue sections, and western blots and cAMP‐activity assays on nucleus‐enriched fractions demonstrate the presence of sAC protein in and around nuclei. These results suggest that sAC modulates multiple physiological processes in shark cells, including nuclear functions.

Intracellular accumulation of mercury enhances P450 CYP1A1 expression and Cl- currents in cultured shark rectal gland cells

The effects of acute and subchronic exposure to mercury on the Cl- current (ICl) were investigated in cultured shark rectal gland (SRG) cells. The effects of intracellular accumulation of mercury on cytochrome P450 (P450) were also assessed. Bath perfusion of a cocktail solution containing forskolin, 1-isobutyl-3-methylxanthine, and 8-bromoadenosine monophosphate enhanced ICl. Addition of 10 microM HgCl2 significantly inhibited the cAMP-activated ICl (p < 0.05, n = 11). Intracellular dialysis with ATP gamma S did not prevent the inhibitory effect of mercury on ICl. In contrast, incubation of SRG cells with 10 microM HgCl2 for 48 hrs markedly increased ICl (p < 0.01, n = 12). Dephosphorylation of the channel by intracellular dialysis with phosphatase I and II abolished the mercury-incubated increase in ICl. The P450-mediated metabolite of arachidonic acid, 11,12-epoxyeicosatrienoic acid (11,12-EET), significantly increased ICl. However, application of 11,12-dihydroxyeicosatrienoic acid (11,12-DHT) did not alter ICl. Mercury incubation for 48 hrs did not alter the protein expression of Cl- channels, but caused an induction of CYP1A1 in cultured SRG cells. In addition, co-incubation of SRG cells with mercury and the P450 inhibitor clotrimazole prevented the mercury-incubated increase in ICl. Our results demonstrate that acute and subchronic application of mercury has opposing effects on ICl in cultured SRG cells. The acute effect of mercury on ICl may result from mercury blockade of Cl- channels. The subchronic effect of mercury on ICl may be due to an induction of P450 CYP1A1 and its mediated metabolites, but not due to an over-expression of Cl- channels.

Role of guanylyl cyclase receptors for CNP in salt secretion by shark rectal gland

The role of C-type natriuretic peptide (CNP) and its guanylyl cyclase-linked receptors in mediating salt secretion by the rectal gland of the spiny dogfish shark (Squalus acanthias) was investigated using HS-142-1, a competitive inhibitor of the binding of natriuretic peptides to their guanylyl cyclase receptors. CNP binds to receptors and activates guanylyl cyclase in rectal gland membranes in a way that is inhibited by HS-142-1. Guanylyl cyclase activation in rectal gland membranes is far more sensitive to CNP than to atrial natriuretic peptide, whereas the reverse is true for membranes derived from mammalian (rabbit) renal collecting duct cells. HS-142-1 inhibited the stimulatory effect of CNP on ouabain-inhibitable oxygen consumption by rectal gland tubules. In explanted rectal glands continuously perfused with blood from intact donor sharks, HS-142-1 inhibited the increase in salt secretion normally provoked by infusing isotonic saline solutions into the donor animal. These results strongly support the view that CNP released into the systemic circulation in response to volume expansion mediates the secretion of chloride by the rectal gland via receptors linked to guanylyl cyclase.

The rectal gland of Squalus acanthias: a model for the transport of chloride

The rectal gland of the spiny dogfish shark, Squalus acanthias, secretes chloride by a furosemide sensitive process that has been termed "secondary active." Chloride enters the cell across the basolateral cell membrane via the sodium:potassium:2 chloride cotransporter. The energy for this electroneutral uptake step is provided by the electrochemical gradient for sodium directed into the cell. This is maintained by Na-K-ATPase present in the basolateral cell membrane. Present as well in the basolateral cell membrane is a potassium conductance that permits potassium to exit passively. Chloride leaves the cell across the luminal membrane via a chloride conductance closely similar to CFTR. The rectal gland is thus a model for the mechanism of secondary active chloride transport utilized by various epithelial organs throughout the vertebrate kingdom. This report reviews the humoral agents that regulate the secretion of chloride by the rectal gland and the intracellular mechanisms that mediate it. CNP, released from the heart in response to a volume stimulus, causes the release of VIP from nerves within the gland and together with VIP directly activates the rectal gland cell.

cAMP-activated Cl- channels in primary cultures of spiny dogfish (Squalus acanthias) rectal gland

Whole cell and single-channel patch-clamp techniques were used to identify and characterize the Cl- currents responsible for adenosine 3',5'-cyclic monophosphate (cAMP)-mediated Cl- secretion in the rectal gland of the spiny dogfish (Squalus acanthias). During whole cell recordings, in cultured rectal gland cells forskolin (10 microM) and 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (400 microM) stimulated a 28-fold increase in Cl- conductance (n = 10). This cAMP-activated conductance pathway had a linear current-voltage (I-V) relationship that was time and voltage independent. Substitution of 235 meq Cl- with I- in the bath inhibited the cAMP-activated current at both positive and negative voltages (64%). Glibenclamide (60 microM) abolished the cAMP-stimulated current, and its effect was irreversible (n = 3). During cell-attached recording, increased cellular cAMP activated single Cl- channels in nine previously quiet patches. These channels had a linear I-V relationship with an average single-channel conductance of 5.1 +/- 0.2 pS (n = 6). Similar properties were observed in excised inside-out patches, permitting further characterization of the single-channel properties. Excised quiescent patches could be activated by the addition of ATP and protein kinase A. Replacing bath Cl- with I- inhibited both inward and outward currents (n = 3). In three inside-out patches, glibenclamide (300 microM) reversibly reduced open probability by 74%, with no effect on single-channel current amplitude. Similar results were obtained in four outside-out recordings. These results suggest that increased cellular cAMP in dogfish rectal gland activates a small linear Cl- channel that resembles human cystic fibrosis transmembrane conductance regulator in its biophysical and pharmacological properties.

Secretion of nitrate by rectal gland of Squalus acanthias

1. Rectal glands secrete nitrate at 30% of their capacity to secrete chloride. 2. Nitrate secretion is directly related to its concentration at constant chloride concentrations. 3. Chloride has a biphasic effect on nitrate secretion. 4. Hill coefficients at chloride < 100 mM are equal to 1, while at 100 mM indicate inhibition of nitrate by chloride. 6. Lineweaver-Burk plots at chloride < 100 indicate a single site, while at 100 mM indicate inhibition of nitrate by chloride. 7. Bumetanide inhibits nitrate secretion. 8. The data suggest that nitrate interacts with one of the two chloride sites of the chloride transporter.

Cl- secretion by cultured shark rectal gland cells. I. Transepithelial transport

To facilitate analysis of the regulation of epithelial Cl- transport by hormones, neurotransmitters, and autocrine mediators, we have developed a primary monolayer culture system for shark rectal gland (SRG) epithelial cells. Cultures exhibit vigorous transcellular Cl- secretion which can be measured using short-circuit current or 36Cl flux methods. Transport is markedly reduced by bumetanide or barium, inhibitors of Na(+)-K(+)-2Cl- cotransport and K+ channels, respectively. This indicates that Cl- secretion by SRG monolayers occurs by a mechanism similar to that described in numerous native Cl- secretory epithelia. Forskolin, 10 microM 2-chloroadenosine, or vasoactive intestinal peptide, potent secretagogues in the isolated perfused SRG, stimulate Cl- secretion by SRG cultures. Submicromolar concentrations of 2-chloroadenosine, which inhibit Cl- secretion in the native SRG, reduce forskolin-stimulated short-circuit current in SRG cultures. Somatostatin, another inhibitor of Cl- secretion by the native SRG, reduces forskolin-stimulated adenosine 3',5'-cyclic monophosphate accumulation in SRG cultures. These results demonstrate that SRG cultures are fully responsive to mediators which activate or inhibit secretion by the native epithelium.

Chloride secretagogues stimulate inositol phosphate formation in shark rectal gland tubules cultured in suspension

Neuroendocrine activation of transepithelial chloride secretion by shark rectal gland cells is associated with increases in cellular cAMP, cGMP, and free calcium concentrations. We report here on the effects of several chloride secretagogues on inositol phosphate formation in cultured rectal gland tubules. Vasoactive intestinal peptide (VIP), atriopeptin (AP), and ionomycin increase the total inositol phosphate levels of cultured tubules, as measured by ion exchange chromatography. Forskolin, a potent chloride secretagogue, has no effect on inositol phosphate formation. The uptake of 3H-myo-inositol into phospholipids is very slow, preventing the detection of increased levels of inositol trisphosphate. However, significant increases in inositol monophosphate (IP1) and inositol biphosphate (IP2) were measured. The time course of VIP- and AP-stimulated IP1 and IP2 formation is similar to the effects of these agents on the short-circuit current responses of rectal gland monolayer cultures. In addition, aluminum fluoride, an artificial activator of guanine nucleotide-binding proteins, stimulates IP1 and IP2 formation. We conclude that rectal gland cells contain VIP and AP receptors coupled to the activation of phospholipase C. Coupling may be mediated by G-proteins. Receptor-stimulated increases in inositol phospholipid metabolism is one mechanism leading to increased intracellular free calcium concentrations, an important regulatory event in the activation of transepithelial chloride secretion by shark rectal gland epithelial cells.

Electrical parameters of the toad skin: effects of forskolin

Forskolin stimulated short-circuit current (SCC) and transepitelial electrical conductance (G) in the isolated skin of the toad Bufo arenarum in a concentration-dependent manner, between 1.0 x 10(-6) and 2.4 x 10(-5) M. At the latter concentration, glandular secretion appeared to be stimulated also. The increase in G was considerably greater in skins bathed in Ringer solution than in solutions containing no chloride. The increased SCC was abolished by amiloride, a specific blocker of sodium transport in amphibian membranes, irrespective of the anion present in the solution bathing the skin. G was also decreased by amiloride to control values in skins bathed in solutions without chloride, but remained elevated in the presence of Cl-. The increase in SCC following exposure to forskolin, 4.4 x 10(-6) M, was not altered when furosemide, a specific blocker of chloride transport, was present in the Ringer solution bathing the dermal side of the skin. The response to forskolin, 2.4 x 10(-5) M, however, was significantly decreased by dermal furosemide; the inhibitor was ineffective in the absence of chloride. The data indicate that forskolin acts on at least two sites: stratum granulosum cells (the main pathway for sodium transport, and an alternate site, responsible for the increase in permeability to chloride. In addition, at high concentration of the agent, glandular secretion is also stimulated. The data suggest that the adenylate cyclase-cyclic AMP system is involved in the regulation of the permeability of the toad skin to sodium and chloride, probably by separate cell types.

A1 adenosine receptors inhibit chloride transport in the shark rectal gland. Dissociation of inhibition and cyclic AMP

In the in vitro perfused rectal gland of the dogfish shark (Squalus acanthias), the adenosine analogue 2-chloroadenosine (2Clado) completely and reversibly inhibited forskolin-stimulated chloride secretion with an IC50 of 5 nM. Other A1 receptor agonists including cyclohexyladenosine (CHA), N-ethylcarboxamideadenosine (NECA) and R-phenylisopropyl-adenosine (R-PIA) also completely inhibited forskolin stimulated chloride secretion. The "S" stereoisomer of PIA (S-PIA) was a less potent inhibitor of forskolin stimulated chloride secretion, consistent with the affinity profile of PIA stereoisomers for an A1 receptor. The adenosine receptor antagonists 8-phenyltheophylline and 8-cyclopentyltheophylline completely blocked the effect of 2Clado to inhibit forskolin-stimulated chloride secretion. When chloride secretion and tissue cyclic (c)AMP content were determined simultaneously in perfused glands, 2Clado completely inhibited secretion but only inhibited forskolin stimulated cAMP accumulation by 34-40%, indicating that the mechanism of inhibition of secretion by 2Clado is at least partially cAMP independent. Consistent with these results, A1 receptor agonists only modestly inhibited (9-15%) forskolin stimulated adenylate cyclase activity and 2Clado markedly inhibited chloride secretion stimulated by a permeant cAMP analogue, 8-chlorophenylthio cAMP (8CPT cAMP). These findings provide the first evidence for a high affinity A1 adenosine receptor that inhibits hormone stimulated ion transport in a model epithelia. A major portion of this inhibition occurs by a mechanism that is independent of the cAMP messenger system.

Resolution of apical from basolateral membrane of shark rectal gland

Membrane fractions were isolated from the rectal gland of Squalus acanthias using differential centrifugation and a sucrose gradient run in the presence of 1 M KBr. Using the basolateral membrane marker Na+-K+-ATPase, we obtained a sixfold purification with the most highly purified fraction from the gradient (sp act = 336 +/- 37 mumol X mg protein-1 X h-1). Electrogenic Br- transport was used as a marker activity of the apical membrane, which enabled the identification and purification of a membrane fraction that is highly resolved from the basolateral membrane. The most active fraction was purified approximately 50-fold compared with the crude homogenate. In this fraction, the specific activity of electrogenic anion transport was 296 +/- 87 nmol X mg protein-1 X min-1, whereas the ATPase was only 17.6 +/- 5.7 mumol X mg protein-1 X h-1, representing about a 4-5% contamination of the apical fraction with the basolateral membrane.

Inositol lipid turnover and adenosine 3,5 cyclic monophosphate in the salt-secreting rectal gland of the dogfish (Scyliorhinus canicula)

The rectal gland of the dogfish is rich in inositol lipids. Total phospholipids from the gland contained 9.1 mol% of phosphatidylinositol (PtdIns), 1.0 mol% of phosphatidylinositol 4-phosphate (PtdIns4P) and 0.9 mol% of phosphatidylinositol 4,5-biphosphate (PtdIns4,5P2). [32P]Orthophosphate was readily incorporated into PtdIns, phosphatidic acid (PtdA) and especially into PtdIns4P and PtdIns4,5P2 in salt gland slices incubated in elasmobranch Ringer with glucose and no other additions over a 2 hr period. The calcium ionophore A23187 stimulated incorporation into PtdIns and PtdA, but not into PtdIns4P or PtdIns4,5P2. Oxygen uptake by rectal gland slices was maximally stimulated by 0.08mM forskolin, 2.5mM 8-chlorophenylthio cyclic AMP, 2.0mM dibutyryl cyclic AMP and 0.25mM theophylline. Stimulated oxygen uptake was inhibited by 0.1mM ouabain in all cases. Incorporation of [32P]orthophosphate into PtdIns, PtdA, PtdIns4P and PtdIns4,5P2 was inhibited by 0.08mM forskolin and 2.0mM dibutyryl cyclic AMP over a 2 hr period. The results are discussed in relation to the control of salt secretion by the rectal gland.

Mechanism of NaCl secretion in rectal gland tubules of spiny dogfish (Squalus acanthias). III. Effects of stimulation of secretion by cyclic AMP

Segments of rectal gland tubules (RGT) the spiny dogfish (Squalus acanthias) were perfused in vitro to study the cellular mechanism by which NaCl secretion is stimulated. Transepithelial PD (PDte), transepithelial resistance (Rte), the PD across the basolateral membrane (PDbl), the fractional resistance of the lumen membrane (FR1), and the cellular activities for Cl-, Na+, and K+ (alpha cell x) were measured. In series 1 the effects of stimulation (S) (dbcAMP 10(-4, adenosine 10(-4), and forskolin 10(-6) mol x 1(-1) on these parameters were recorded and compared to nonstimulated state (NS). PDte increased from -1.9 +/- 0.2 mV to -11.0 +/- 0.9 mV (n = 51). PDbl depolarized from -86 +/- 1 to -74 +/- 1.4 mV (n =52), Rte fell from 29 +/- 2.8 to 21 +/- 2 omega cm2 (n = 23), and FR1 fell from 0.96 +/- 0.005 to 0.79 +/- 0.04 (n = 9). alpha cell K+ was constant (123 +/- 13 versus 128 +/- 17 mmol x 1(-1) (n = 6), but alpha cell cl- fell significantly from 48 +/- 4 to 41 +/- 3 mmol x 1(-1) (n = 7). alpha cell Na+ increased from 11 +/- 2.1 to 29.5 +/- 6.6 mmol x 1(-1) (n = 4). In series 2 the conductivity properties were examined by rapid K+, and Cl- concentration steps on the basolateral and luminal cell side respectively in NS and S states. In NS-segments reduction of bath K+ led to a hyperpolarization of PDbl with a mean slope of 28 +/- 1.3 mV/decade (n = 9) (as compared to 19 mV/decade for S-state).(ABSTRACT TRUNCATED AT 250 WORDS)

An endogenous peptide stimulates secretory activity in the elasmobranch rectal gland

Extraction and partial purification of peptide material from the intestine of the elasmobranch Scyliorhinus canicula yielded a fraction that shows potent stimulatory activity in the rectal gland. The extracted material appears to contain an endogenous peptide (or peptides) that represents the natural hormone responsible for the control of rectal gland secretion in vivo.

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.

Ouabain binding in rectal gland of Squalus acanthias

In an attempt to examine the mechanisms of activation of (Na, K)-ATPase when epithelial transport is stimulated, the binding of ouabain to rectal gland tissue was measured before and after stimulation with dibutyryl cAMP and theophylline. Stimulation significantly altered the characteristics of ouabain binding to slices of Squalus acanthias rectal gland and to isolated rectal gland cells, accelerating the rate of binding and increasing the amount of ouabain bound at equilibrium when low concentrations of ouabain (10(-9) to 10(-7) M) were present in the medium. Scatchard plots of ouabain binding were nonlinear, suggesting at least two classes of binding sites, one of higher and one of lower affinity. Stimulation with cAMP and theophylline appeared to increase the affinity of the high-affinity site. Ouabain binding was increased by cAMP and theophylline even in the presence of furosemide (10(-4) M) or bumetanide (10(-5) M), and when Li+ was substituted for Na+, or NO3- for Cl- -maneuvers known to inhibit rectal gland secretion. The changes in ouabain binding induced by cAMP and theophylline do not appear, therefore, to be secondary to secretory activity but may reflect a change in the configuration, environment or location of existing enzyme so as to enhance its activity. Stimulation of ouabain binding cannot be demonstrated in whole homogenates of rectal gland, indicating that intact cells are necessary for the cyclic AMP-induced increase in ouabain binding to become manifest.

Carbonic anhydrase localization in the elasmobranch rectal gland

Carbonic anhydrase (CA) activity was histochemically localized in the elasmobranch rectal gland at the light and electron microscopic levels. Reaction product in the secretory tubules was localized coincident with that reported for sodium-potassium activated adenosine triphosphatase (Na-K-ATPase): along the highly amplified basolateral plasma membranes of the epithelial cells. Reaction product was also localized along the plasma membrane of adjacent central canal epithelial cells. The results suggest that CA plays a role in modulating the environment of the intercellular space which in the secretory tubule is believed to be the paracellular pathway for sodium. The results also draw attention to the possible role of the central canal epithelium in modification of the secreted fluid.

Amphotericin B and the elasmobranch rectal gland: implications for the relationship between oxygen consumption and ion transport

In slices of the dogfish rectal gland, amphotericin B produced increases in oxygen consumption and ouabain binding similar to those produced by cyclic AMP, except that the increases induced by the latter were inhibited by furosemide whereas those resulting from the addition of amphotericin B were unaffected by the diuretic. Unlike cyclic AMP, however, amphotericin B failed to stimulate secretion by the isolated perfused gland. These results support the suggestion that the increases in ouabain binding and oxygen consumption produced by cyclic AMP result simply from an increased sodium entry into the cells, and as such can be mimicked by amphotericin B, whereas secretion rate itself depends on some additional, cyclic AMP-induced process. The implications of this for determinations of the relationship between rates of oxygen consumption and ion transport are discussed.

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

In epithelia that secrete sodium chloride, high-conductance tight junctions between cells have been proposed as the primary pathway for transepithelial sodium flux. We examined the properties of tight junctions in the perfused rectal gland of the dogfish shark during basal secretion and following adenosine 3',5'-cyclic monophosphate stimulation of sodium chloride secretion. Freeze-fracture electron microscopy revealed extensive interdigitation of adjacent cells with an associated amplification in the length of tight junctions per area of luminal surface, averaging 102 +/- 4.7 m/cm2 in outer regions of 80 +/- 6.7 in inner regions of the gland. Marked heterogeneity of junctional structure was present with junctional elements varying from single strands to three duplex elements and junctional depth varying from 15 to 60 nm. In glands perfused with lanthanum chloride, ionic lanthanum filled the intercellular space up to but not through the tight junctions. Characteristics of tight junctions were not different during basal and maximally stimulated sodium chloride secretion. These studies define tight junctions in the rectal gland as an anatomical barrier capable of restricting the passage of relatively small molecules such as urea while providing a greatly amplified junctional area for the passive diffusion of sodium and water.

Effects of orthovanadate on salt and water effluxes from the gills of seawater eels, Anguilla anguilla

Orthovanadate (5 . 10(-7) M) perfused through isolated gills at a constant rate increased the perfusion pressure by 40% but inhibited the effluxes of Na+ and Cl- by 40%. Water efflux was unaltered. Ouabain (10(-4) M) and rotenone (10(-4) M) influenced salt and water effluxes in the same way but did not alter perfusion pressures. Orthovanadate (10(-5) M) perfused at constant rate increased the pressure nearly 2.5-fold; under these conditions effluxes of Na+, Cl- and H2O were all increased approximately 2.5-fold.

Freeze-fracture and morphometric analysis of occluding junctions in rectal glands of elasmobranch fish

The structure of occluding junctions in secretory and ductal epithelium of salt-secreting rectal glands from two species of elasmobranch fish, the spiny dogfish Squalus acanthias and the stingray Dasyatis sabina, was examined by thin-section and freeze-fracture electron microscopy. In both species, occluding junctions between secretory cells are shallow in their apical to basal extent and are characterized by closely juxtaposed parallel strands. Average strand number in the dogfish was 3.5 +/- 0.2. with a mean depth of 56 +/- 5 nm; in the stingray a mean of 2.0 +/- 0.2 strands encompassed an average depth of 18 +/- 3 nm. In contrast, the linear extent of these junctions was remarkably large due to the intermeshing of the narrow apices of the secretory cells to form the tubular lumen. Morphometric analysis gave values of 66. 8 +/- 2.5 and 74.9 +/- 4.6 m/cm2 for the length of junction per unit of luminal surface area in the dogfish and stingray, respectively. This junctional morphology is similar to that generally described for "leaky" epithelia. In comparison, the stratified ductal epithelium which carries the NaCl-rich secretion to the intestine is characterized by extensive occluding junctions which extend 0.6-0.8 mum in depth and consist of a mean of 12 strands arranged in an anastomosing network, an architectural pattern typical of "tight" epithelia. The length density of these junctions in the dogfish rectal gland was 7.6 +/- 0.1 m/cm2. The junctional architecture of the rectal gland secretory epithelium (few strands, large junctional length densities) is similar to that described for several other hypertonic secretory epithelia [20, 34] and is compatible with the recent model for salt secretion in rectal glands [39] and in other C1- secretory epithelia which posits a conductive paracellular pathway for trans-epithelial Na+ secretion from intercellular space to the lumen to form the NaCl-rich secretory product.

Structural diversity of occluding junctions in the low-resistance chloride-secreting opercular epithelium of seawater-adapted killifish (Fundulus heteroclitus)

The structural features of the chloride-secreting opercular epithelium of seawater-adapted killifish (Fundulus heteroclitus) were examined by thin-section and freeze-fracture electron microscopy, with particular emphasis on the morphological appearance of occluding junctions. This epithelium is a flat sheet consisting predominantly of groups of mitochondriarich chloride cells with their apices associated to form apical crypts. These multicellular groups are interspersed in an otherwise continuous pavement cell epithelial lining. The epithelium may be mounted in Ussing-type chambers, which allow ready access to mucosal and serosal solutions and measurement of electrocal properties. The mean short-circuit current, potential difference (mucosal-side negative), and DC resistance for 19 opercular epithelia were, respectively, 120.0 +/- 18.2 microA/cm2, 12.3 +/- 1.7 mV, and 132.5 +/- 26.4 omega cm2. Short-circuit current, a direct measure of Cl- transport, was inhibited by ouabain (5 micron) when introduced on the serosal side, but not when applied to the mucosal side alone. Autoradiographic analysis of [3H]-ouabain-binding sites demonstrated that Na+,K+-ATPase was localized exclusively to basolateral membranes of chloride cells; pavement cells were unlabeled. Occluding junctions between adjacent chloride cells were remarkably shallow (20-25 nm), consisting of two parallel and juxtaposed junctional strands. Junctional interactions between pavement cells or between pavement cells and chloride cells were considerably more elaborate, extending 0.3-0.5 micron in depth and consisting of five or more interlocking junctional strands. Chloride cells at the lateral margins of crypts make simple junctional contacts with neighboring chloride cells and extensive junctions with contiguous pavement cells. Accordingly, in this heterogeneous epithelium, only junctions between Na+,K+-ATPase-rich chloride cells are shallow. Apical crypts may serve, therefore, as focal areas of high cation conductivity across the junctional route. This view is consistent with the electrical data showing that transmural resistance across the opercular eptihelium is low, and with recent studies demonstrating that transepithelial Na+ fluxes are passive. The simplicity of these junctions parallels that described recently for secretory cells of avian salt gland (Riddle and Ernst, 1979, J. Membr. Biol., 45:21-35) and elasmobranch rectal gland (Ernst et al., 1979, J. Cell Biol., 83:(2, Pt. 2):83 a[Abstr.]) and lends morphological support to the concept that paracellular ion permeation plays a central role in ouabain-sensitive transepithelial NaCl secretion.

Hemicyst formation stimulated by cyclic AMP in dog kidney cell line MDCK

Certain epithelial cell lines have morphologic, physiologic, biochemical and pharmacologic characteristics of transporting epithelia from intact organs. In this paper we show that dibutyryl cyclic AMP, 5' AMP, adenosine and cyclic AMP phosphodiesterase inhibitors stimulate hemicyst formation by the dog kidney cell line MDCK. It is suggested that this effect is explained by elevation of intracellular cyclic AMP levels by means of an exogenous non-metabolizable source of cyclic AMP, phosphodiesterase inhibition or adenyl cyclase stimulation. Since hemicyst formation is in part due to transepithelial fluid transport, these findings raise the possibility that this fraction might be modulated by cAMP in an established cell line. We believe that cultured epithelial cells may provide an exploitable model system to investigate at the cellular and subcellular levels, the mechanism by which cyclic AMP modifies water and solute movements across epithelia.