Arginine induced acute pancreatitis alters the actin cytoskeleton and increases heat shock protein expression in rat pancreatic acinar cells

Arginine induced acute pancreatitis was evaluated as a novel and distinct form of experimental pancreatitis with particular attention to the actin cytoskeleton and expression of heat shock or stress proteins. Arginine induced a dose related necrotising pancreatitis in rats, as shown by histological evaluation, and an increase in serum amylase. Severe pancreatitis induced by 4.5 g/kg arginine was accompanied by dramatic changes in the actin cytoskeleton, as visualised with rhodamine phallodin. Intermediate filaments were also disrupted, as visualised by cytokeratin 8/18 immunocytochemistry. Arginine pancreatitis was accompanied by a stress response with a large increase in the small heat shock protein HSP27, as well as HSP70, peaking at 24 hours and localised to acinar cells. There was a lower increase in HSP60 and HSP90 and no effect on GRP78. HSP27 was also shifted to phosphorylated forms during pancreatitis. A lower dose of arginine (3.0 g/kg) induced less pancreatitis but a larger increase in HSP70 and HSP27 expression and phosphorylation of HSP27. Thus HSP expression can be overwhelmed by severe damage. The present work in conjunction with earlier work on caerulein induced pancreatitis indicates that changes in the actin cytoskeleton are an early component in experimental pancreatitis.

Molecular cloning and expression of an inwardly rectifying K(+) channel from bovine corneal endothelial cells

PURPOSE

To determine the presence of a putative inwardly rectifying K(+) channel in bovine corneal endothelial (BCE) cells and to characterize its molecular and electrophysiological properties.

METHODS

An RT-PCR strategy was used to clone an IRK1 channel sequence from BCE mRNA. Northern blot analysis was used to confirm expression of this sequence in cultured BCE cells. Two-electrode voltage-clamp and whole-cell patch-clamp recordings were used to characterize the cloned channel expressed in Xenopus oocytes and the native channels in cultured BCE cells, respectively.

RESULTS

A full-length (1284 bp) coding sequence that shares 99.7% nucleotide sequence and 100% amino acid sequence identity to bovine lens IRK1 (Kir2.1) was cloned. The authors designate this sequence BCE IRK1 or BCIRK1. Northern blot analysis indicated that BCIRK1 mRNA is expressed in cultured BCE cells with two major transcripts of 7.5 and 5.5 kb. BCIRK1 cDNA was subcloned into the vector, pcDNA3.1(-), and cRNA transcribed from the BCIRK1 cDNA clone was injected into Xenopus oocytes. Two-electrode voltage-clamp recordings from injected oocytes revealed inwardly rectifying K(+) currents that were blocked by external Ba(2+) and Cs(+) in a concentration- and voltage-dependent manner. Whole-cell patch-clamp recordings from dissociated cultured BCE cells revealed strongly inwardly rectifying K(+) currents with similar properties.

CONCLUSIONS

Corneal endothelial cells express IRK1 (Kir2.1) inwardly rectifying K(+) channels. Consistent with the properties of IRK1 channels, BCIRK1 is likely involved in regulating membrane potential and possibly other cellular functions in corneal endothelial cells.

Secretagogues cause ubiquitination and down-regulation of inositol 1, 4,5-trisphosphate receptors in rat pancreatic acinar cells

BACKGROUND & AIMS

The action of several exocrine pancreas secretagogues depends on the second messenger inositol 1,4, 5-trisphosphate (IP3), which, via endoplasmic reticulum-located IP3 receptors, mobilizes intracellular Ca2+ stores. Signaling pathways like this one are regulated at multiple loci. To determine whether IP3 receptors are one of these loci, we measured IP3 receptor concentration, distribution, and modification in secretagogue-stimulated rat pancreatic acinar cells.

METHODS

Isolated rat pancreatic acinar cells were exposed to cholecystokinin and other secretagogues, or rats were injected intraperitoneally with cerulein. Then samples of cells or pancreata were probed for IP3 receptor content and distribution as well as for ubiquitin association with IP3 receptors.

RESULTS

Secretagogues rapidly down-regulated acinar cell IP3 receptors both in vitro and in vivo. They also elicited receptor redistribution and caused receptors to become ubiquitinated, indicating that the ubiquitin/proteasome proteolytic pathway contributes to the down-regulation. Surprisingly, however, proteasome inhibitors did not block IP3 receptor down-regulation, and phospholipase Cbeta1 and protein kinase C also were down-regulated. Thus, secretagogues simultaneously activate an additional proteolytic pathway.

CONCLUSIONS

Secretagogues rapidly down-regulate IP3 receptors and other proteins involved in intracellular signaling by a mechanism that involves, but is not limited to, the ubiquitin/proteasome pathway. Loss of these proteins may account for the disruption of Ca2+ mobilization that occurs in models of acute pancreatitis, and may contribute to cell adaptation under physiological conditions.

Salt restriction induces pseudohypoaldosteronism type 1 in mice expressing low levels of the beta-subunit of the amiloride-sensitive epithelial sodium channel

The amiloride-sensitive epithelial sodium channel (ENaC) is a heteromultimer of three homologous subunits (alpha-, beta-, and gamma-subunits). To study the role of the beta-subunit in vivo, we analyzed mice in which the betaENaC gene locus was disrupted. These mice showed low levels of betaENaC mRNA expression in kidney (approximately 1%), lung (approximately 1%), and colon (approximately 4%). In homozygous mutant betaENaC mice, no betaENaC protein could be detected with immunofluorescent staining. At birth, there was a small delay in lung-liquid clearance that paralleled diminished amiloride-sensitive Na+ absorption in tracheal explants. With normal salt intake, these mice showed a normal growth rate. However, in vivo, adult betaENaC m/m mice exhibited a significantly reduced ENaC activity in colon and elevated plasma aldosterone levels, suggesting hypovolemia and pseudohypoaldosteronism type 1. This phenotype was clinically silent, as betaENaC m/m mice showed no weight loss, normal plasma Na+ and K+ concentrations, normal blood pressure, and a compensated metabolic acidosis. On low-salt diets, betaENaC-mutant mice developed clinical symptoms of an acute pseudohypoaldosteronism type 1 (weight loss, hyperkalemia, and decreased blood pressure), indicating that betaENaC is required for Na+ conservation during salt deprivation.

Immunolocalization of CRHSP28 in exocrine digestive glands and gastrointestinal tissues of the rat

The 28-kDa (on SDS-PAGE) Ca2+-regulated heat stable protein (CRHSP28) was recently purified as novel phosphoprotein in exocrine pancreas, since it undergoes an immediate increase in serine phosphorylation when acini are stimulated with Ca2+-mobilizing agonists. Examination of CRHSP28 protein expression in rat revealed that most was highly expressed in pancreas and other morphologically related exocrine tissues, including the parotid, lacrimal, and submandibular glands. Immunofluorescence staining in pancreas indicated that CRHSP28 was specifically concentrated in zymogen granule-rich areas in the apical cytoplasm of acinar cells. Lack of colocalization with pancreatic lipase in dual immunofluorescence studies confirmed localization of CRHSP28 to the area immediately surrounding the granules. Western analysis of pancreatic zymogen granule membrane proteins indicated CRHSP28 was not associated with the granules following their purification. A similar pattern of apical cytoplasmic secretory granule staining was noted in lacrimal and submandibular glands. CRHSP28 protein was also expressed at relatively high levels in mucosal epithelial cells of the stomach and small intestine. CRHSP28 was found in the supranuclear apical cytoplasm of cells lining the small intestinal crypts, including Paneth cells, and was abundant in the cytoplasm of goblet cells. In the stomach, strong CRHSP28 staining was seen in mucus-secreting cells in the upper portion of the gastric glands and in the apical, granule-rich cytoplasm of chief cells located in the lower portions of the glands. Dual labeling with anti-H+-K+-ATPase demonstrated a comparatively lower expression of CRHSP28 in parietal cells. Collectively, the high relative expression of CRHSP28 in various secretory cell types within the digestive system, together with its intracellular localization surrounding the acinar cell secretory granules, strongly supports a role for CRHSP28 in Ca2+-mediated exocrine secretion.

A role for the p38 mitogen-activated protein kinase/Hsp 27 pathway in cholecystokinin-induced changes in the actin cytoskeleton in rat pancreatic acini

Cholecystokinin (CCK) and other pancreatic secretagogues have recently been shown to activate signaling kinase cascades in pancreatic acinar cells, leading to the activation of extracellular signal-regulated kinases and Jun N-terminal kinases. We now show the presence of a third kinase cascade activating p38 mitogen-activated protein (MAP) kinase in isolated rat pancreatic acini. CCK and osmotic stress induced by sorbitol activated p38 MAP kinase within minutes; their effects were dose-dependent, with maximal activation of 2.8- and 4.4-fold, respectively. The effects of carbachol and bombesin on p38 MAP kinase activity were similar to those of CCK, whereas phorbol ester, epidermal growth factor, and vasoactive intestinal polypeptide stimulated p38 MAP kinase by 2-fold or less. Both CCK and sorbitol also increased the tyrosyl phosphorylation of p38 MAP kinase. Using the specific inhibitor of p38 MAP kinase, SB 203580, we found that p38 MAP kinase activity was required for MAP kinase-activated protein kinase-2 activation in pancreatic acini. SB 203580 reduced the level of basal phosphorylation and blocked the increased phosphorylation of Hsp 27 after stimulation with either CCK or sorbitol. CCK treatment induced an initial rapid decrease in total F-actin content of acini, followed by an increase after 40 min. Preincubation with SB 203580 significantly inhibited these changes in F-actin content. Staining of the actin cytoskeleton with rhodamine-conjugated phalloidin and analysis by confocal fluorescence microscopy showed disruption of the actin cytoskeleton after 10 and 40 min of CCK stimulation. Pretreatment with SB 203580 reduced these changes. These findings demonstrate that the activation of p38 MAP kinase is involved not only in response to stress, but also in physiological signaling by gastrointestinal hormones such as CCK, where activation of Gq-coupled receptors stimulates a cascade in which p38 MAP kinase activates MAP kinase-activated protein kinase-2, resulting in Hsp 27 phosphorylation. Activation of p38 MAP kinase, most likely through phosphorylation of Hsp 27, plays a role in the organization of the actin cytoskeleton in pancreatic acini.

Purification and characterization of a novel physiological substrate for calcineurin in mammalian cells

Although the calcium/calmodulin-regulated protein phosphatase calcineurin has been shown to play a role in a number of intracellular processes, relatively few of the downstream phosphoproteins that are dephosphorylated by this enzyme in cells have been described. Calcineurin was previously shown to play a role in amylase secretion by rat pancreatic acinar cells and to specifically dephosphorylate a 24-kDa cytosolic protein. The present study describes the purification and characterization of this novel phosphoprotein, termed CRHSP-24 (calcium-regulated heat-stable protein with a molecular mass of 24 kDa). Microgram quantities of CRHSP-24 were purified from a large-scale rat pancreas preparation in a procedure involving heat and acid precipitation, anion-exchange chromatography, preparative electrophoresis, electroelution, and two-dimensional electrophoresis. Internal amino acid sequence was obtained from two peptides following trypsin digestion and high pressure liquid chromatography. Both sequences matched with 100% identity nucleotide sequences of expressed sequence tags from human placenta and rat PC-12 cells. Two CRHSP-24 transcripts of 0.7 and 2. 9 kilobases were detected in multiple rat tissues by Northern analysis, whereas a single 24-kDa protein was observed by Western blotting. The CRHSP-24 protein is 147 amino acids in length, is composed of nearly 14% proline, and is phosphorylated entirely on serine residues. Western analysis and 32P metabolic labeling of acini revealed CRHSP-24 to be maximally phosphorylated in control cells and to undergo a rapid sustained dephosphorylation on at least 3 serine residues in response to calcium-mobilizing stimuli. Dephosphorylation of CRHSP-24 was completely inhibited by pretreatment of acini with cyclosporin A or FK506. Furthermore, the inhibitory effects of FK506 were blocked by excess rapamycin. The ubiquitous expression of CRHSP-24 in rat tissues suggests that this novel calcineurin substrate plays a common role in calcium-mediated signal transduction.

Overexpression of Rab3D enhances regulated amylase secretion from pancreatic acini of transgenic mice

Rab3D, a member of the ras-related GTP-binding protein Rab family, is localized to secretory granules of various exocrine tissues such as acinar cells of the pancreas, chief cells of the stomach, and parotid and lacrimal secretory cells. To elucidate the function of Rab3D in exocytosis, we have generated transgenic mice that over-express Rab3D specifically in pancreatic acinar cells. Hemagglutinin-tagged Rab3D was localized to zymogen granules by immunohistochemistry, and was shown to be present on zymogen granule membranes by Western blotting; both results are similar to previous studies of endogenous Rab3D. Secretion measurements in isolated acinar preparations showed that overexpression of Rab3D enhanced amylase release. Amylase secretion from intact acini of transgenic mice 5 min after 10 pM cholecystokinin octapeptide (CCK) stimulation was enhanced by 160% of control. In streptolysin-O-permeabilized acini of transgenic mice, amylase secretion induced by 100 microM GTP-gamma-S was enhanced by 150%, and 10 microM Ca2+-stimulated amylase secretion was augmented by 206% of that of the control. To further elucidate Rab3D involvement in stimulus-secretion coupling, we examined the effect of CCK on the rate of GTP binding to Rab3D. Stimulation of permeabilized acini with 10 pM CCK increased the incorporation of radiolabeled GTP into HA-tagged Rab3D. These results indicate that overexpression of Rab3D enhances secretagogue-stimulated amylase secretion through both calcium and GTP pathways. We conclude that Rab3D protein on zymogen granules plays a stimulatory role in regulated amylase secretion from pancreatic acini.

Chemokine gene expression in rat pancreatic acinar cells is an early event associated with acute pancreatitis

BACKGROUND & AIMS

The molecular mechanisms underlying pancreatitis are largely unknown. The goal of this study was to identify an early genetic event that correlated with pancreatitis.

METHODS

Differential display of messenger RNAs (mRNAs) was conducted on normal pancreas vs. those of animals with secretagogue-induced pancreatitis. Northern blots from normal animals and animals with experimental acute pancreatitis were probed with cloned complementary DNAs for chemokines. Pancreatitis was induced with cerulein and by retrograde injection of bile salts. Immunocytochemistry was used to identify the source of chemokine expression. Pyrrolidine dithiocarbamate was tested for effects on chemokine expression and pancreatitis.

RESULTS

A differentially amplified band was consistently observed early after cerulein hyperstimulation. This band was identified as a portion of the mob-1 gene, an alpha-chemokine. Northern analysis indicated that mRNAs for mob-1 and another chemokine, mcp-1, were induced after cerulein hyperstimulation in vivo. mob-1 mRNA was also induced by retrograde injection of bile salts and by cerulein in acinar cells in vitro. mob-1 protein was localized to exocrine cells in pancreata of diseased animals. Pyrrolidine dithiocarbamate inhibited both chemokine gene expression and early inflammatory characteristics of pancreatitis.

CONCLUSIONS

Chemokines are induced in acinar cells by treatments that induce pancreatitis and may play an important role in the early stages of the disease.

Heterotrimeric G-protein Gq/11 localized on pancreatic zymogen granules is involved in calcium-regulated amylase secretion

The heterotrimeric G-protein Gq/11 was identified on pancreatic acinar zymogen granules and its function in calcium-regulated exocytosis was examined. Western blotting showed alphaq/11, but not alphas or alphao, to be localized to the zymogen granule membrane along with G-protein beta-subunit; all three alpha subunits were present in a plasma membrane fraction and the alphaq/11 signal was 30-fold more enriched in the plasma membrane as compared with granule membrane. Neither CCK receptors nor alpha subunits of the sodium pump, both plasma membrane markers were present on granule membranes. Immunohistochemistry of pancreatic lobules showed that alphaq/11 localized to the zymogen granule-rich apical region of acinar cells together with a much stronger signal at the basolateral plasma membrane. When the substance-P-related peptide GPAnt-2a, an antagonist of Gq/11, was introduced into streptolysin-O permeabilized acini to bypass the plasma membrane, the amylase release induced by 10 microM free calcium was potentiated in a concentration-dependent manner. By contrast, another substance-P-related peptide, GPAnt-1, an antagonist of Go and Gi, showed no effect on calcium-induced amylase release from permeabilized acini. GPAnt-2a peptide also exerted an inhibitory effect on the total GTPase activity of the purified zymogen granules and a larger inhibitory effect on the GTPase activity of the Gq/11 protein immunopurified from zymogen granules. GPAnt-1, however, did not inhibit GTPase activity of either zymogen granules or immunopurified Gq/11. These results suggest that GPAnt-2a peptide augmented calcium-induced amylase release from permeabilized acini by inhibiting GTPase activity of the Gq/11 protein on zymogen granules. We conclude that Gq/11 protein on zymogen granules plays a tonic inhibitory role in calcium-regulated amylase secretion from pancreatic acini.

Epithelial localization of a reptilian Na+/H+ exchanger homologous to NHE-1

Basolateral membranes of turtle (Pseudemys scripta) colon epithelial cells exhibit robust Na+/H+ exchange activity that can be activated by cell shrinkage and is blocked by amiloride [M. A. Post and D. C. Dawson. Am. J. Physiol. 262 Cell Physiol. 31):C1089-C1094, 1992]. The colonic epithelium actively absorbs Na+ and secretes K+ and HCO3-, but the role of basolateral Na+/H+ exchange, if any, in transepithelial transport is unknown. The current studies were undertaken to identify the gene product(s) responsible for the observed basolateral Na+/H+ exchange activity and to determine the cellular localization of the reptilian Na+/H+ exchange protein. We cloned and sequenced partial-length cDNAs that are likely to encode a reptilian homologue of the mammalian NHE-1 Na+/H+ exchanger isoform. The partial-length cDNAs were > 80% identical to mammalian NHE-1 homologues at the nucleotide level and recognized a transcript (approximately 5.8-6.0 kb) in RNA isolated from turtle colon, small intestine, stomach, kidney, urinary bladder, heart, and liver. In situ hybridization showed that mRNA encoding the reptile homologue of NHE-1 was expressed predominantly in the epithelial cells of these tissues. Immunofluorescent localization of the reptilian Na+/H+ exchanger protein using an antibody raised against a human NHE-1 fusion protein confirmed that protein expression paralleled abundant mRNA expression in epithelial cells of turtle stomach and colon, as well as in some nephron segments, and showed that the reptile NHE-1 homologue was localized exclusively to the basolateral membranes of these cells. The relatively high level of NHE-1 expression in epithelial cells, particularly those of the colon and stomach, suggests that NHE-1 function is important for the maintenance or regulation of ion transport processes that occur in these cell types.

Immunolocalization of the ubiquitin-protein ligase Nedd4 in tissues expressing the epithelial Na+ channel (ENaC)

The epithelial Na+ channel (ENaC) was previously shown to be expressed in several Na(+)- and fluid-absorbing epithelia, particularly those of the kidney, colon, and lung. We have recently identified the ubiquitin-protein ligase Nedd4 as an interacting protein with ENaC and demonstrated that Nedd4 binds by its WW domains to the proline-rich PY motifs of ENaC. These PY motifs were recently shown to be deleted/mutated in patients afflicted with Liddle's syndrome, a hereditary form of systemic renal hypertension. Such mutations cause elevated channel activity by an increase in channel number/stability at the plasma membrane and by increased channel opening. We then proposed that Nedd4, by regulating channel stability/ degradation, may be a suppressor of ENaC. To test whether Nedd4 is localized to those tissues/regions that express ENaC, we performed immunocytochemical analysis of rat Nedd4 (rNedd4) distribution in rat kidney, colon, and lung tissues. Our results show that, in the kidney, rNedd4 is primarily localized to the cortical collecting tubules and outer and inner medullary collecting ducts. These tubular segments were previously shown to express ENaC. The epithelium lining medullary calyxes was also intensely stained, and microvillar borders of proximal convoluted tubules expressed variable amounts of rNedd4. In the lung, rNedd4 was mainly expressed in the epithelia lining the airways, in the submucosal glands and ducts, and in the distal respiratory epithelium. These sites resemble the pattern of ENaC expression. In contrast, in the distal colon, rNedd4 was strongly expressed in the epithelia lining the crypts but not in the ENaC-expressing surface epithelium. Low-salt diet (to elevate serum aldosterone levels) had no effect on rNedd4 distribution in the kidney or colon. Thus Nedd4 is coexpressed and likely colocalizes with ENaC in specific regions within the kidney and lung but not in the colon. We speculate this difference in colocalization may reflect differences in the regulation of channel stability in those tissues.

Delivery of newly synthesized Na(+)-K(+)-ATPase to the plasma membrane of A6 epithelia

Na(+)-K(+)-ATPase is localized to the basolateral cell surface of most epithelial cells. Conflicting results regarding the intracellular trafficking of Na(+)-K(+)-ATPase in Madin-Darby canine kidney cells have been reported, with delivery to both apical and basolateral membranes or exclusively to the basolateral cell surface. We examined the delivery and steady-state distribution of Na(+)-K(+)-ATPase in the amphibian epithelial cell line A6 using an antibody raised against Na(+)-K(+)-ATPase alpha-subunit and sulfo-N-hydroxysuccinimidobiotin to tag cell surface proteins. The steady-state distribution of the Na(+)-K(+)-ATPase was basolateral, as confirmed by immunocytochemistry. Delivery of newly synthesized Na(+)-K(+)-ATPase to the cell surface was examined using [35S]methionine and [35S]cysteine in a pulse-chase protocol. After a 20-min pulse, the alpha-subunit and core glycosylated beta-subunit were present at both apical and basolateral cell surfaces. The alpha-subunit and core glycosylated beta-subunit delivered to the apical cell surface were degraded within 2 h. Mature alpha/beta-heterodimer was found almost exclusively at the basolateral surface after a 1- to 24-h chase. These data suggest that immature Na(+)-K(+)-ATPase alpha-subunit and core glycosylated beta-subunits are not retained in the endoplasmic reticulum of A6 cells and apparently lack sorting signals. Mature Na(+)-K(+)-ATPase is targeted to the basolateral surface, suggesting that basolateral targeting of the protein is conformation dependent.

Evidence that zymogen granules are not a physiologically relevant calcium pool. Defining the distribution of inositol 1,4,5-trisphosphate receptors in pancreatic acinar cells

A key event leading to exocytosis of pancreatic acinar cell zymogen granules is the inositol 1,4,5-trisphosphate (InsP3)-mediated release of Ca2+ from intracellular stores. Studies using digital imaging microscopy and laser-scanning confocal microscopy have indicated that the initial release of Ca2+ is localized to the apical region of the acinar cell, an area of the cell dominated by secretory granules. Moreover, a recent study has shown that InsP3 is capable of releasing Ca2+ from a preparation enriched in secretory granules (Gerasimenko, O., Gerasimenko, J., Belan, P., and Petersen, O. H., (1996) Cell 84, 473-480). In the present study, we have investigated the possibility that zymogen granules express InsP3 receptors and are thus Ca2+ release sites. Immunofluorescence staining, obtained with antisera specific to types I, II, or III InsP3 receptors and analyzed by confocal fluorescence microscopy revealed that all InsP3 receptor types were present in acinar cells. The type II receptor localized exclusively to an area close to or at the luminal plasma membrane. While types I and III InsP3 receptors displayed a similar luminal distribution, these receptors were also present at low levels in nuclei. The localization of InsP3 receptor was in marked contrast to the distribution of amylase, a zymogen granule content protein. In a zymogen granule fraction prepared in an identical manner to the aforementioned report demonstrating InsP3-induced Ca2+ release, immunoblotting demonstrated the presence of types I, II, and III InsP3 receptors. Ca2+ release from this preparation in response to InsP3, but not thapsigargin, could also be demonstrated. In contrast, when the zymogen granules were further purified on a Percoll gradient, InsP3 receptors were undetectable, and InsP3 failed to release Ca2+. Transmission electron microscopy performed on both preparations showed that the Percoll-purified granule preparation consisted of essentially pure zymogen granules, whereas the granules prepared without this step were enriched in granules but also contained significant contamination by mitochondria, endoplasmic reticulum, and nuclei. It is concluded that zymogen granules do not express InsP3 receptors and thus are not a site of Ca2+ release relevant to the secretory process in the pancreatic acinar cell.

Biosynthesis and cell surface delivery of the NHE1 isoform of Na+/H+ exchanger in A6 cells

The Na+/H+ exchanger isoform NHE1 is localized to the basolateral membrane of renal and intestinal epithelia. We examined the plasma membrane distribution, biosynthesis, and cell surface delivery of NHE1 in A6 epithelia. NHE1 was localized to the basolateral membrane. Studies of NHE1 biosynthesis with a pulse-chase protocol demonstrated that a core glycosylated, endoglycosidase H-sensitive, 90-kDa NHE1 was present 0-5 h into the chase period and that mature 110-kDa NHE1 was present 1-24 h into the chase period. Studies of plasma membrane delivery of newly synthesized NHE1 demonstrated that the 90-kDa NHE1 was detected at both apical and basolateral membranes 2-5 h into the chase period. The 110-kDa NHE1 was observed at the basolateral membrane 5-24 h into the chase period. These results suggest that NHE1 is expressed primarily at the basolateral membrane of A6 cells, that core glycosylated NHE1 is delivered to the plasma membrane in a nonpolarized manner, and that nature 110-kDa NHE1 is delivered to the basolateral membrane.

Rab3D localizes to zymogen granules in rat pancreatic acini and other exocrine glands

Rab3 proteins are members of the family of Ras-like monomeric GTP-binding proteins that have been implicated in secretion in neuronal cells. Although an isoform of Rab3 has been assumed to exist in pancreatic acini, its identity has not yet been established. We now report that Rab3D is present in rat pancreatic acini and is localized to the zymogen granule membrane. Reverse transcription-polymerase chain reaction (PCR) was used with primers based on mouse Rab3D to amplify Rab3D from rat pancreas. The PCR product without primer sites consisted of 580 base pairs and was 94% identical to the mouse Rab3D cDNA sequence previously cloned from adipocytes. Western blotting with a polyclonal antiserum raised against Rab3D-specific carboxyterminal amino acids identified Rab3D in rat pancreatic acini and revealed its concentration on zymogen granule membranes. Immunocytochemistry of pancreatic lobules showed that Rab3D localized to the apical region in a pattern similar to amylase. Confocal fluorescence microscopy of lobules double immunolabeled with antibodies to Rab3D and the granule membrane marker protein glycoprotein-2 (GP-2) revealed a similar localization of these proteins to zymogen granules. Immunocytochemistry also revealed the presence of Rab3D in chief and enterochromaffin-like cells in the stomach, acinar cells in lacrimal and parotid gland, and Paneth cells in the intestine. These results show that Rab3D is expressed in rat pancreatic acini and other exocrine secretory cells. Its location implies it may be involved in regulated exocytosis.

Muscarinic receptor sequestration in SH-SY5Y neuroblastoma cells is inhibited when clathrin distribution is perturbed

The possibility that clathrin plays a role in the agonist-mediated sequestration of muscarinic cholinergic receptors in human SH-SY5Y neuroblastoma cells has been investigated by the application of experimental paradigms previously established to perturb clathrin distribution and receptor cycling events. Preincubation of SH-SY5Y cells under hypertonic conditions resulted in a pronounced inhibition of agonist-induced muscarinic receptor sequestration (70-80% at 550 mOsm), which was reversed when cells were returned to isotonic medium. Depletion of intracellular K+ or acidification of the cytosol also resulted in > 80% inhibition of muscarinic receptor sequestration. Under conditions of hypertonicity, depletion of intracellular K+, or acidification of cytosol, muscarinic receptor-stimulated phosphoinositide hydrolysis and Ca2+ signaling events were either unaffected or markedly less inhibited than receptor sequestration. That these same experimental conditions did perturb clathrin distribution was verified by immunofluorescence studies. Hypertonicity and depletion of intracellular K+ resulted in a pronounced accumulation of clathrin in the perinuclear region, whereas acidification of the cytosol resulted in the appearance of microaggregates of clathrin throughout the cytoplasm and at the plasma membrane. The results are consistent with the possibility that muscarinic receptors in SH-SY5Y cells are endocytosed via a clathrin-dependent mechanism.

NaK-ATPase pump sites in cultured bovine corneal endothelium of varying cell density at confluence

PURPOSE

The driving force for ion and water flow necessary for efficient deturgesence of the corneal stroma resides in the ouabain-sensitive sodium (Na) pump of corneal endothelial cells. Using a cell culture model of corneal endothelial cell hypertrophy, the authors examined the expression of Na pumps at the cell surface to see how this central element of the endothelial pump changed as corneal endothelial cell density decreased to a level associated with corneal decompensation in vivo.

METHODS

3H-ouabain binding to NaK-ATPase at saturating conditions was used to quantitate the number of Na pump sites on cultured bovine corneal endothelial cells as the confluent density decreased from approximately 2750 cells/mm2 to approximately 275 cells/mm2.

RESULTS

The mean number of Na pump sites per cell at confluence (1.92 +/- 0.07 x 10(6)) did not change as the cell density decreased 2.7-fold from 2763 cells/mm2 to 1000 cells/mm2. However, pump site expression doubled to approximately 4 x 10(6) sites/cell as the cell density decreased from 1000 cells/mm2 to 275 cells/mm2. Despite the incremental increase in Na pump site expression that occurred as the cells hypertrophied below a density of 1000/mm2 to achieve confluence, this increase was insufficient to prevent a decrease in Na pump site density of the intact monolayer, expressed as pump sites/mm2.

CONCLUSION

The confluent cell density of cultured bovine corneal endothelial cells can be varied from that found in the normal native cornea to that associated with corneal decompensation. In confluent cultures with cell densities ranging from 2750 cells/mm2 to 1000 cells/mm2, the number of pump sites per cell remains relatively unchanged. Below cell densities of 1000 cells/mm2, the number of pump sites per cell progressively increases. The increased Na pump site abundance in markedly hypertrophied endothelial cells cannot adequately compensate for the progressive reduction in the number of transporting cells per unit area within the intact monolayer. Even when considered with the decrease in the size of the paracellular ion conductive pathway that is a consequence of progressive endothelial hypertrophy, the overall pumping capacity of the intact endothelial monolayer declines.

Cholecystokinin stimulates the down-regulation of CTP:phosphocholine cytidylyltransferase in pancreatic acinar cells

Stimulation of rat pancreatic acinar cells with cholecystokinin (CCK) is known to result in a significant inhibition of CTP:phosphocholine cytidylyltransferase (CT), a rate-limiting enzyme in phosphatidylcholine biosynthesis. Immunoprecipitation of CT from 32P-labeled acinar cells revealed that CCK treatment also caused a marked reduction in CT phosphate levels. The effects of CCK were maximal over 60 min and dependent on concentration, exhibiting an EC50 of 800 pM. Other calcium mobilizing secretagogues such as carbamylcholine (100 microM) and bombesin (10 nM) also reduced CT phosphate levels to 20 and 39% of control, respectively. Treatment of cells with thapsigargin and/or 12-O-tetradecanoyl-phorbol-13-acetate established that a combination of increased intracellular Ca2+ and protein kinase C activation was necessary to decrease phosphorylated CT content. Conversely, secretin (10 nM) or 8-(4-chlorophenylthio)-cAMP (100 microM) added alone had no effects. Use of the compound JMV-180 indicated CCK was acting through the low affinity state of the CCKA receptor to reduce CT phosphate levels. Further, the decrease in phosphorylated CT caused by CCK was blocked by the phosphatase inhibitors okadaic acid (3 microM) and calyculin A (100 nM). Finally, immunoblotting from whole cell lysates revealed CT was partially degraded in response to CCK, providing a novel mechanism by which the inhibition of CT enzyme activity occurs in response to the hormone. Moreover, this degradation was also blocked by a phosphatase inhibitor. These data suggest that the dephosphorylation of either CT itself or some other regulatory molecule(s) which mediates the CCK-induced protease activation may play a central role in reducing CT enzyme levels in acinar cells.

Salt stress increases abundance and glycosylation of CFTR localized at apical surfaces of salt gland secretory cells

Osmotic stress elicits hypertonic NaCl secretion and promotes structural and biochemical differentiation in avian salt glands. In addition to cholinergic control, Cl- secretion is stimulated by vasoactive intestinal peptide (VIP), suggesting that the cystic fibrosis transmembrane conductance regulator (CFTR) may be present and that its expression may be regulated by chronic salt stress. Anion efflux, assayed by 6-methoxy-N-(3-sulfopropyl)quinolinium fluorescence changes in single cells, was stimulated by VIP or 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate. Immunoblots with a COOH-terminal peptide antibody to human CFTR revealed approximately 170- and approximately 180-kDa bands in lysates from control and salt-stressed glands, respectively. Both variants reduced to approximately 140 kDa after N-glycanase digestion and gave identical tryptic phosphopeptide maps after immunoprecipitation and phosphorylation by protein kinase A. CFTR was localized to apical membranes by immunofluorescence and, additionally, to subapical vesicles by immunoelectron microscopy. Salt stress induced an approximately twofold increase in CFTR abundance/cell protein (approximately 5-fold/cell) and intensified apical membrane immunofluorescence. For comparison, Na+ pump expression increased approximately fourfold per cell protein with little change in actin. Thus differentiation induced by salt stress is accompanied by alteration in CFTR abundance and glycosylation. Upregulation of CFTR likely contributes to increased efficiency of Cl- secretion.

Arginine vasopressin and forskolin regulate apical cell surface expression of epithelial Na+ channels in A6 cells

Both arginine vasopressin (AVP) and forskolin regulate vectorial Na+ transport across high-resistance epithelia by increasing the Na+ conductance of the apical membrane mediated by amiloride-sensitive Na+ channels. Pretreatment of A6 cells with brefeldin A partially inhibited the increase in Na+ transport in response to forskolin, suggesting recruitment of Na+ channels from an intracellular pool. The activation of Cl- secretion was not affected. Apical cell surface expression of Na+ channels was examined following activation of transepithelial Na+ transport across the epithelial cell line A6 by AVP or forskolin. Apical cell surface radioiodinated Na+ channels were immunoprecipitated to quantify the biochemical pool of Na+ channels at the apical plasma membrane and to determine whether an increment in the biochemical pool of Na+ channels expressed at the apical cell surface is a potential mechanism by which AVP and forskolin increase apical membrane Na+ conductance. The activation of Na+ transport across A6 cells by AVP was accompanied by a significant increase in the biochemical pool of Na+ channels at the apical plasma membrane within 5 min after addition of hormone, which was sustained for at least 30 min. The increase in apical cell surface expression of Na+ channels was also observed 30 min after application of forskolin. No changes in the oligomeric subunit composition of the channel were noted. Brefeldin A inhibited the forskolin-stimulated increase in apical cell surface expression of Na+ channels. These results suggest that AVP and forskolin regulate Na+ transport, in part, via rapid recruitment of Na+ channels to the cell surface, perhaps from a pool of channels in the subapical cytoplasm.

Molecular basis of defective anion transport in L cells expressing recombinant forms of CFTR

Cystic fibrosis (CF) is caused by mutations in the gene encoding a chloride channel called the CF transmembrane conductance regulator (CFTR). A single mutation in this gene, deletion of three nucleotides that leads to the absence of phenylalanine 508 (i.e., delta F508), is found on 70% of all CF chromosomes. To explore the molecular mechanism(s) responsible for defective chloride transport in patients with CF, we have studied the processing, localization, and function of wild type (W.T.), delta F508 and G551D CFTR (a G-->D missense mutation at position 551) in retrovirus transduced L cells. Cell transduced with W.T. CFTR expressed a 170 kd CFTR protein that was endoglycosidase H (Endo H) resistant, localized to the plasma membrane, and generated a cAMP-mediated anion conductance (GCl) when stimulated with standard concentrations of forskolin (5 microM), cpt cAMP (400 microM) and IBMX (100 microM). The G551D CFTR was indistinguishable from W.T. CFTR with respect to post-translational processing and localization, but it did not produce a cAMP-activated GCl in response to the standard stimulation cocktail. However, raising the IBMX concentration to 4 mM produced GCl in G551D expressing cells. Cells transduced with delta F508 CFTR expressed an Endo H sensitive CFTR protein (approximately 140 kd) that was found in a cytosolic, perinuclear location. These cells did not respond to the standard cocktail, but approximately 20% of cells increased GCl when the cocktail contained 4 mM IBMX.(ABSTRACT TRUNCATED AT 250 WORDS)

Agonist-induced Ca2+ mobilization in cultured bovine and human corneal endothelial cells

We investigated the possibility that cultured corneal endothelial cells express receptors that are coupled to the phosphoinositide cycle/intracellular Ca2+ signaling pathway. Agonist-stimulated changes in intracellular calcium ([Ca2+]i) in single bovine and human corneal endothelial cells (BCEC and HCEC, respectively) derived from confluent cultures were measured by microspectrofluorimetry using the Ca(2+)-sensitive probe, fura-2. Total inositol phosphates accumulated during a 30 min incubation in the presence or absence of agonists was determined in Li+ containing medium with cells pre-labelled for 48 hrs with 10 microCi/ml 3H-myoinositol. Histamine (HA), ADP and ATP induced a rapid increase in [Ca2+]i. Subsequently, [Ca2+]i decreased to either a stable, agonist-dependent sustained elevation, or fell back to baseline to begin oscillatory fluctuations. The initial rise in [Ca2+]i was insensitive to removal of extracellular calcium (Ca2+o), whereas the stable elevations in [Ca2+]i and the [Ca2+]i oscillations required Ca2+o. In contrast, bradykinin (BK) and endothelin-1 (ET-1) elicited an initial rise in [Ca2+]i that returned to prestimulatory levels within 2 min despite the continued presence of agonist. The Ca(2+)-mobilizing agonists carbachol, phenylephrine, adenosine and substance P were all ineffective in elevating [Ca2+]i. Histamine-induced Ca2+ mobilization was inhibited by the H1-receptor antagonist triprolidine, but triprolidine had no effect on either BK or ATP stimulation of Ca2+ mobilization. In BCEC, 100 microM HA significantly increased total inositol phosphate accumulation (18.8-fold over unstimulated controls) and was 90% inhibited by 0.5 microM triprolidine. BK and ATP also significantly increased formation of inositol phosphates in BCEC.(ABSTRACT TRUNCATED AT 250 WORDS)

Submucosal glands are the predominant site of CFTR expression in the human bronchus

We have used in situ hybridization and immunocytochemistry to characterize the cellular distribution of cystic fibrosis (CF) gene expression in human bronchus. The cystic fibrosis transmembrane conductance regular (CFTR) was primarily localized to cells of submucosal glands in bronchial tissues from non-CF individuals notably in the serous component of the secretory tubules as well as a subpopulation of cells in ducts. Normal distribution of CFTR mRNA was found in CF tissues while expression of CFTR protein was genotype specific, with delta F508 homozygotes demonstrating no detectable protein and compound heterozygotes expressing decreased levels of normally distributed protein. Our data suggest mechanisms whereby defects in CFTR expression could lead to abnormal production of mucus in human lung.

Histamine H1 receptor-mediated Ca2+ signaling in cultured bovine corneal endothelial cells

The corneal endothelium pumps ions and water from the stroma to the aqueous humor, maintaining corneal transparency. This report investigates the possibility that cultured corneal endothelial cells express neurohormonal Ca2+ signaling pathways employed by other epithelia to regulate transport or other cellular functions. Agonist-stimulated changes in intracellular calcium ([Ca2+]i) in single bovine corneal endothelial cells (BCEC) derived from confluent cultures were measured by microspectrofluorimetry using the Ca(2+)-sensitive probe, fura 2. Mean resting [Ca2+]i in BCEC was 46 +/- 2 nM (n = 124). The muscarinic cholinergic agonist, carbachol, did not mobilize Ca2+, whereas histamine induced a rapid increase in [Ca2+]i to initial peak levels of 549 +/- 22 nM (n = 46) at maximally stimulating doses. The initial rise in [Ca2+]i in response to histamine was dose dependent, with a minimum effective dose of 50 nM, EC50 = 0.84 mumol/l, and a maximum effective dose of 10 mumol/l. [Ca2+]i decreased from the initial peak, but then stabilized to form an agonist-dependent sustained elevation or abruptly fell back to baseline to begin oscillatory fluctuations. The initial peak was insensitive to removal of extracellular calcium (Ca2+o), whereas subsequent elevations in [Ca2+]i or sustained [Ca2+]i oscillations required Ca2+o. The amplitude of the oscillations in [Ca2+]i increased with an increase in [histamine]. However, frequency was independent of [histamine] (mean = 0.62 spikes min-1 +/- 0.06, n = 33). Histamine-induced Ca2+ mobilization was inhibited by the H1 receptor antagonist triprolidine, but was unaffected by ranitidine (H2 antagonist) or thioperamide (H3 antagonist).(ABSTRACT TRUNCATED AT 250 WORDS)

Aldosterone does not alter apical cell-surface expression of epithelial Na+ channels in the amphibian cell line A6

The steroid hormone aldosterone regulates reabsorptive Na+ transport across specific high resistance epithelia. The increase in Na+ transport induced by aldosterone is dependent on protein synthesis and is due, in part, to an increase in Na+ conductance of the apical membrane mediated by amiloride-sensitive Na+ channels. To examine whether an increment in the biochemical pool of Na+ channels expressed at the apical cell surface is a mechanism by which aldosterone increases apical membrane Na+ conductance, apical cell-surface proteins from the epithelial cell line A6 were specifically labeled by an enzyme-catalyzed radioiodination procedure following exposure of cells to aldosterone. Labeled Na+ channels were immunoprecipitated to quantify the biochemical pool of Na+ channels at the apical cell surface. The activation of Na+ transport across A6 cells by aldosterone was not accompanied by alterations in the biochemical pool of Na+ channels at the apical plasma membrane, despite a 3.7-4.2-fold increase in transepithelial Na+ transport. Similarly, no change in the distribution of immunoreactive protein was resolved by immunofluorescence microscopy. The oligomeric subunit composition of the channel remained unaltered, with one exception. A 75,000-Da polypeptide and a broad 70,000-Da polypeptide were observed in controls. Following addition of aldosterone, the 75,000-Da polypeptide was not resolved, and the 70,000-Da polypeptide was the major polypeptide found in this molecular mass region. Aldosterone did not alter rates of Na+ channel biosynthesis. These data suggest that neither changes in rates of Na+ channel biosynthesis nor changes in its apical cell-surface expression are required for activation of transepithelial Na+ transport by aldosterone. Post-translational modification of the Na+ channel, possibly the 75,000 or 70,000-Da polypeptide, may be one of the cellular events required for Na+ channel activation by aldosterone.

Immunocytochemical demonstration of Na+,K(+)-ATPase in internodal axolemma of myelinated fibers of rat sciatic and optic nerves

We used postembedding electron microscopic immunocytochemistry with colloidal gold to determine the ultrastructural distribution of Na+,K(+)-ATPase in the sciatic and optic nerves of the rat. Using a polyclonal antiserum raised against the denatured catalytic subunit of brain Na+,K(+)-ATPase, we found immunoreactivity along the internodal axolemma of myelinated fibers in both nerves. This antiserum did not produce labeling of nodal axolemma. These results suggest that an important site of energy-dependent sodium-potassium exchange is along the internodal axolemma of myelinated fibers in the mammalian CNS and PNS and that there may be differences between the internodal and nodal forms of the enzyme.

Agonist-induced frequency modulation of Ca2+ oscillations in salt gland secretory cells

Oscillations in intracellular calcium concentration ([Ca2+]i) induced by the acetylcholine analogue carbachol (CCh) were characterized by microspectrofluorimetry of fura-2 in single secretory cells from the avian salt gland. The frequency of oscillations increased in graded fashion with [CCh] between 25 nM (2.7 +/- 0.6 min-1) and 250 nM (11.8 +/- 1.4 min-1), whereas the amplitude of the spikes was independent of [CCh]. An interperiod return to prestimulatory [Ca2+]i was generally seen only at very low (25 nM) CCh. Between 50 and 250 nM CCh, oscillations were associated with sustained elevated [Ca2+]i levels. The amplitude of the oscillatory spikes was found not to exceed that of initial spikes arising from prestimulatory [Ca2+]i, despite the dose-dependent [effective concentration at 50% (EC50) = 200 nM CCh] sustained rise in [Ca2+]i. At 1 microM CCh, oscillations gave way to a maximal sustained increase in [Ca2+]i. Reduction of [Ca2+]o to 1.5 microM during an oscillatory train or blockage of Ca2+ influx with Ni+ resulted in a reduction in sustained Ca2+i levels and in frequency, but not amplitude, of oscillations. A relationship between the sustained partial rise in [Ca2+]i derived from Ca2+ influx and the oscillatory frequency at a given [CCh] was further indicated by the lower frequency (P less than 0.01) of the early spikes in a train when interspike [Ca2+]i initially returned to near-basal levels. In some cells, oscillations were slow enough (less than 2 min-1) to resolve an interperiod of elevated baseline [Ca2+]i, showing that the latter can occur independent of the repetitive Ca2+ spikes. (ABSTRACT TRUNCATED AT 250 WORDS)

Characterization and cellular localization of the epithelial Na+ channel. Studies using an anti-Na+ channel antibody raised by an antiidiotypic route

Amiloride-sensitive Na+ channels are expressed at the apical membrane of high resistance, Na+-transporting epithelial. The specific interaction of amiloride with this transport protein suggested the feasibility of raising anti-Na+ channel antibodies by an antiidiotypic approach designed to generate antibodies directed against the amiloride-binding domain on the channel. Antiidiotypic monoclonal antibody RA6.3 mimicked the effect of amiloride by inhibiting Na+ transport across A6 cell monolayers when applied to the apical cell surface. Inhibition of transport required pretreatment of the apical cell surface with trypsin in the presence of amiloride in order to enhance accessibility of the antibody to the amiloride-binding site. This antibody specifically immunoprecipitated a large 750,000-700,000 Da protein from [35S]methionine-labeled A6 cell cultures, which was resolved further under reducing conditions as a set of polypeptides with apparent molecular masses of 260,000-230,000, 180,000, 140,000-110,000, and 70,000 Da. The antibody recognized the 140,000-Da subunit, known to contain the amiloride-binding domain, on immunoblots of purified A6 cell Na+ channel. Immunoprecipitation of apical or basolateral plasma membrane proteins selectively labeled with 125I demonstrated that expression of the oligomeric Na+ channel was restricted to the apical plasma membrane. Immunocytochemical localization in A6 cultures revealed apical membrane as well as cytosolic immunoreactive sites. Immunostaining was also observed at or near the basolateral plasma membrane.

Multiple calcium mobilization pathways in single avian salt gland cells

Agonist-induced changes in intracellular Ca2+ concentration ([Ca2+]i) in individual secretory cells from the avian salt gland were detailed using dual-wavelength microspectrofluorimetry of the Ca2(+)-sensitive fluorescent probe fura-2. Resting [Ca2+]i averaged 42 +/- 5 nM. Stimulation with the cholinergic agonist carbachol (1 microM) resulted in a rapid increase in [Ca2+]i to 308 +/- 26 nM, which was sustained at a nearly constant elevated level (328 +/- 31 nM) throughout agonist application. In the absence of extracellular Ca2+ or in the presence of an inorganic blocker of Ca2+ entry (Ni2+, 1 mM), only a transient increase in [Ca2+]i occurred on agonist stimulation, whereas subsequent readmission of Ca2+ or washout of Ni2+ reinitiated a sustained increase in [Ca2+]i. The initial transient response results from Ca2+ release from intracellular stores, whereas the sustained phase represents entry of extracellular Ca2+ into the cytoplasm. Repetitive stimulations in Ca2(+)-free medium alternating with Ca2(+)-containing medium were performed to examine the mechanisms involved in refilling of the agonist-sensitive intracellular pool. After depletion of the intracellular pool by stimulation in Ca2(+)-free medium, removal of the agonist and readmission of Ca2+ resulted in a rapid transient increase in [Ca2+]i that could be blocked by Ni2+, La3+, or elevated K+. Subsequent removal of extracellular Ca2+ and restimulation nonetheless showed that complete refilling of the intracellular pool had occurred in each case. These results suggest that two separate Ca2(+)-entry mechanisms, one sensitive to Ni2+, La3+, and elevated K+ and responsible for the agonist-induced increase in [Ca2+]i and one insensitive to the blockers and involved in refilling of the intracellular pool, may exist in salt gland cells. Spontaneous oscillations of [Ca2+]i that are independent of extracellular Ca2+ have also been observed in 10% of the cells. The abolition of the oscillations by depletion of the agonist-sensitive pool suggests this pool as the Ca2+ source for the oscillations.

Two K+ channel types, muscarinic agonist-activated and inwardly rectifying, in a Cl- secretory epithelium: the avian salt gland

Patches of membrane on cells isolated from the nasal salt gland of the domestic duck typically contained two types of K+ channel. One was a large-conductance ("maxi") K+ channel which was activated by intracellular calcium and/or depolarizing membrane voltages, and the other was a smaller-conductance K+ channel which exhibited at least two conductance levels and displayed pronounced inward rectification. Barium blocked both channels, but tetraethylammonium chloride and quinidine selectively blocked the larger K+ channel. The large K+ channel did not appear to open under resting conditions but could be activated by application of the muscarinic agonist, carbachol. The smaller channels were open under resting conditions but the gating was not affected by carbachol. Both of these channels reside in the basolateral membranes of the Cl- secretory cells but they appear to play different roles in the life of the cell.

Mechanism of ion transport by avian salt gland primary cell cultures

Confluent sheets formed from primary culture of avian salt gland secretory cells exhibit a short-circuit current (Isc) in response to cholinergic and beta-adrenergic stimulation [Lowy, R. J., D. C. Dawson, and S. A. Ernst. Am J. Physiol. 249 (Cell Physiol. 18): C41-C47, 1985]. To establish the ionic basis for the Isc, transmural fluxes of 22Na and 36Cl were measured. Under short-circuit conditions there was little net flux of either ion in the absence of agonists. Addition of carbachol elevated net serosal-to-mucosal Cl flux to 1.71 mu eq.h-1.cm-2, whereas a smaller increase to 0.85 mu eq.h-1.cm-2 occurred with isoproterenol. Neither agonist altered net Na flux. The stimulated Isc accounted for 70% of the net Cl flux induced by carbachol and nearly 100% of that induced by isoproterenol. Replacement of Cl by gluconate or Na by choline abolished (carbachol) or greatly reduced (isoproterenol) the Isc, which could be restored in a dose-dependent fashion by ion restitution. Active ion transport was preferentially inhibited by basal (vs. apical) addition of ouabain, furosemide, or barium. The results provide evidence that cholinergic and beta-adrenergic agonists elicit active transmural Cl secretion. They further suggest that transport is dependent on the Na+-K+-adenosine-triphosphatase, a Na-Cl cotransport process, and a basal K conductance, all features of a secondary active Cl secretory mechanism.

Na,K-ATPase: comparison of the cellular localization of alpha-subunit mRNA and polypeptide in mouse cerebellum, retina, and kidney

A clone encoding mouse brain Na,K-ATPase alpha-subunit was isolated from a mouse brain lambda gt11 cDNA library by using antisera to mouse and bovine brain alpha-subunit. A comparison of the nucleotide sequence of this clone with published sequences of rat brain alpha-subunit isoform clones showed it to be most similar to rat brain alpha 1. An RNA antisense probe prepared from the cDNA insert of the mouse clone detected a single mRNA of approximately 4.5 kb in Northern blots of mouse brain and kidney RNAs. This probe hybridized only to an alpha 1-cDNA insert from rat brain under high stringency conditions on Northern blots. The RNA antisense probe was used for in situ hybridization to sections of mouse kidney, cerebellum, and retina, and the cellular distribution of alpha-subunit mRNA (alpha-mRNA) was compared with that of alpha-subunit polypeptide (alpha-subunit) detected by immunofluorescence in similar sections. In kidney, alpha-mRNA distribution closely paralleled that of the polypeptide with abundant expression in ascending thick limbs and cortical distal tubules and weaker labeling in cortical proximal tubules. The co-distribution of alpha-mRNA and polypeptide in kidney where Na,K-ATPase localization is well established is consistent with the specificity of these probes. In the retina, prominent labeling with both probes was seen in photoreceptor inner segments, inner nuclear layer, and ganglion cell bodies. Plexiform layers and optic fibers expressed abundant alpha-subunit but little mRNA. Light labeling for both was seen in the outer nuclear layer. In cerebellum, alpha-mRNA and alpha-subunit were associated with soma of granule cells, basket cells, and stellate cells. Glomeruli and basket terminals contained abundant alpha-subunit but exhibited little reactivity with the riboprobe. In Purkinje cell bodies, in contrast, the antibody used to identify the cDNA clone did not resolve significant polypeptide in the somal plasmalemma despite abundant somal mRNA expression. Comparison of distribution of the two probes in cerebellum and retina indicates that message accumulation is primarily in cell bodies, while alpha-subunit epitopes may be co-expressed in cell bodies and/or transported to distant sites in cell-specific patterns.

Immunocytochemical localization of (Na+ + K+)-ATPase in the rat hippocampus

The adult rat hippocampus was investigated by light microscopic immunocytochemistry for (Na+ + K+)-ATPase. In the CA1, CA2 and CA3 hippocampal regions, dense immunostaining for (Na+ + K+)-ATPase, exhibiting a punctate appearance, was demonstrated along the soma plasmalemma of hippocampal pyramidal cells in the stratum pyramidale, thus outlining these cells distinctly, and along dendrites extending into the stratum radiatum. (Na+ + K+)-ATPase immunostaining was dense in the neuropil of the strata oriens and radiatum of the rat hippocampus, but much lighter in the corpus callosum. Immunostaining at the periphery of pyramidal cell soma may be associated with the plexus formed by axon terminals of hippocampal basket cells.

(Na+ + K+)-adenosinetriphosphatase in the brain of Shiverer (Shi/Shi) mice

The myelin-deficient Shiverer (Shi/Shi) mutant mouse may be a useful model in assessing the dependence of brain (Na+ + K+)-ATPase concentration and composition on myelin membrane formation. Brain microsomal membranes from age-matched control (+/+) and Shiverer (Shi/Shi) mice were fractionated by differential centrifugation and sucrose gradient sedimentation. No reduction in (Na+ + K+)-ATPase specific activity was measured in whole homogenates, high- and low-speed fractions or gradient fractions from brains of Shi/Shi mice as compared to those of +/+ mice. In addition, sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting with antisera specific for mouse brain (Na+ + K+)-ATPase revealed no significant difference in catalytic subunit composition between fractions of +/+ and Shi/Shi brains. The similar results obtained for both +/+ and myelin-deficient Shi/Shi mice suggest that myelin contributes little to total brain (Na+ + K+)-ATPase.

Vasoactive intestinal peptide stimulates ion transport in avian salt gland

Avian salt glands are considered to be under the control of cholinergic nerve fibers. Here we report evidence that vasoactive intestinal peptide (VIP) also regulates ion transport. Nerve fibers stained immunocytochemically with anti-VIP were distributed throughout the tissue within the peritubular connective tissue and were in close proximity to the secretory tubules. VIP applied to primary cultures of the secretory cells elicited active ion transport as assayed by short-circuit current (Isc) analysis. The mucosal-to-serosal positive Isc was produced in a dose-dependent fashion [(EC50) = 3.1 X 10(-9) M], was potentiated by theophylline, and was inhibited by either ouabain or furosemide. This Isc was independent of activation by cholinergic agonists. VIP also increased ouabain-sensitive respiration 14-18% in acutely isolated cells from salt-stressed and unstressed animals. These data demonstrate for the first time that VIP is present in the avian salt gland and can act as a secretagogue by directly affecting the secretory cells. In addition, the results provide evidence for direct control of ion transport by an adenosine 3',5'-cyclic monophosphate-linked neurohormone in both adult unstressed and fully salt-stressed animals.

Beta-adrenergic stimulation of ion transport in primary cultures of avian salt glands

Adrenergic stimulation of transmural ion transport was identified and characterized in primary cultures of avian salt gland. Adrenergic activation was mediated by beta-receptors since stimulation of the short-circuit current (Isc) was blocked by propranolol but not phentolamine. The Isc's elicited by isoproterenol, epinephrine, and norepinephrine were dose dependent, with respective EC50 values of 1.5 X 10(-8) M, 5.0 X 10(-6) M, and 1.1 X 10(-5) M. The apparent Ki for propranolol inhibition after isoproterenol stimulation was 7.5 X 10(-10) M. 8-Br cyclic AMP (8-Br cAMP) and forskolin-elicited Isc's that were insensitive to propranolol, were potentiated by theophylline, and inhibited by furosemide or ouabain. Isoproterenol also induced an increase in ouabain-sensitive respiration in acutely dispersed cells from salt-stressed juvenile or unstressed adult animals, but not in fully salt-stressed adults. The data indicate that, in addition to the well-established cholinergic receptors, beta-adrenergic receptors can control ion transport in these glands. Furthermore, the results suggest for the first time that an intracellular effector pathway involving cAMP is present.

Muscarinic receptor-stimulated Ca2+ signaling and inositol lipid metabolism in avian salt gland cells

Activation of muscarinic cholinergic receptors was studied by measuring agonist-stimulated inositol lipid turnover and changes in [Ca2+]i in dissociated salt gland secretory cells. Carbachol stimulation of quin2-loaded cells results in a sustained 4-fold increase in [Ca2+]i, while incorporation of [32P]Pi into phosphatidylinositol (PI) and phosphatidate are similarly increased. [3H]Inositol phosphates, measured in the presence of Li+, increased 13-fold. The stimulated increment in [Ca2+]i required extracellular Ca2+, whereas [3H]inositol phosphate accumulation was independent of external Ca2+. Dose-response curves for carbachol-induced increments in [Ca2+]i, PI labeling, and labeled inositol phosphate release are similar, with EC50 values of 6, 4.5 and 8 microM, respectively. Dissociation constants for atropine vs. the quin2 and phospholipid responses are 0.59 +/- 0.3 nM and 0.48 +/- 0.28 nM, respectively. These cells thus provide a model system for the study of non-exocytotic secretion as a consequence of stimulated inositol lipid turnover.

Immunoreactivity and ouabain-dependent phosphorylation of (Na+ + K+)-adenosinetriphosphatase catalytic subunit doublets

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis with 6% polyacrylamide was used to resolve the 100-kDa catalytic (alpha subunit) polypeptide of (Na+ + K+)-adenosinetriphosphatase from various tissues. The catalytic subunit was identified on immunoblots with antisera against mouse brain catalytic subunit and lamb kidney holoenzyme. Immunoblots and Coomassie Blue-stained companion gels showed double species of the 100-kDa subunit in sucrose gradient fractions of mouse brain and kidney, bovine grey and white matter, purified lamb kidney and duck salt gland holoenzyme, electroplax microsomes, and NaI-extracted microsomes of goldfish and rat brain. The apparent molecular mass differences between the two species in each tissue all ranged between 5 and 8 kDa. Both forms in rat brain and lamb kidney enzyme contain common epitopes reactive with antibodies immunoaffinity-purified on either species from mouse brain. In addition, ouabain-dependent acid-stable inorganic phosphate incorporation was tested with mouse brain, lamb kidney, and electroplax enzyme. Ouabain-dependent phosphorylation was demonstrated in both species in lamb kidney and electroplax and in the larger of the two forms in mouse brain. These results suggest that double species of the phosphorylatable subunit are present generally in epithelial as well as excitable tissues and in fish and avian as well as mammalian species. Work is needed to elucidate their qualitative and quantitative characteristics in different tissues.

Differentiation of two distinct K conductances in the basolateral membrane of turtle colon

The K conductance of the basolateral membrane of turtle colon was measured in amphotericin-treated cell layers under a variety of ionic conditions. Changing the composition of the bathing solutions changed not only the magnitude but also the physical properties of the basolateral K conductance. The results are consistent with the notion that altered ionic environments can lead to changes in the relative abundance of two different populations of K channels in the basolateral membrane, which can be differentiated on the basis of pharmacological specificity, ion selectivity, and tracer kinetics. In the following article (Germann, W. J., S. A. Ernst, and D. C. Dawson, 1986, Journal of General Physiology, 88:253-274), we present evidence consistent with the hypothesis that one of these conductances was due to the same channels that give rise to the normal resting basolateral K conductance of the transporting cells, while the other was associated with experimental maneuvers that led to extreme swelling of the epithelial cells.

Resting and osmotically induced basolateral K conductances in turtle colon

Two types of K conductance can be distinguished in the basolateral membranes of polyene-treated colonic epithelial cells (see Germann, W. J., M. E. Lowy, S. A. Ernst, and D. C. Dawson, 1986, Journal of General Physiology, 88:237-251). The significance of these two types of K conductance was investigated by measuring the properties of the basolateral membrane under conditions that we presumed would lead to marked swelling of the epithelial cells. We compared the basolateral conductance under these conditions of osmotic stress with those observed under other conditions where changes in cell volume would be expected to be less dramatic. In the presence of a permeant salt (KCl) or nonelectrolyte (urea), amphotericin-treated colonic cell layers exhibited a quinidine-sensitive conductance. Light microscopy revealed that these conditions were also associated with pronounced swelling of the epithelial cells. Incubation of tissues in solutions containing the organic anion benzene sulfonate led to the activation of the quinidine-sensitive gK and was also associated with dramatic cell swelling. In contrast, tissues incubated with an impermeant salt (K-gluconate) or nonelectrolyte (sucrose) did not exhibit a quinidine-sensitive basolateral conductance in the presence of the polyene. Although such conditions were also associated with changes in cell volume, they did not lead to the extreme cell swelling detected under conditions that activated the quinidine-sensitive gK. The quinidine-sensitive basolateral conductance that was activated under conditions of osmotic stress was also highly selective for K over Rb, in contrast to the behavior of normal Na transport by the tissue, which was supported equally well by K or Rb and was relatively insensitive to quinidine. The results are consistent with the notion that the basolateral K conductance measured in the amphotericin-treated epithelium bathed by mucosal K-gluconate solutions or in the presence of sucrose was due to the same channels that are responsible for the basolateral K conductance under conditions of normal transport. Conditions of extreme osmotic stress, however, which led to pronounced swelling of the epithelial cells, were associated with the activation of a new conductance, which was highly selective for K over Rb and was blocked by quinidine or lidocaine.

Immunocytochemical localization of Na+,K+-ATPase catalytic polypeptide in mouse choroid plexus

Na+,K+-ATPase plays a central role in the mechanism of cerebrospinal fluid secretion by the choroid plexus. We have used an antiserum to the 100 KD catalytic polypeptide of the enzyme purified from mouse brain (30) to localize the catalytic unit in mouse choroid plexus at the light and electron microscopic levels. Pre-embedding immunostaining with the peroxidase-conjugated second antibody technique showed that microvillar borders facing the ventricle were intensely reactive. In contrast, basal and lateral plasma membrane surfaces were devoid of activity. Identical localization was obtained with a post-embedding procedure in which protein A-gold was used to stain immunoreactive sites on thin sections of Lowicryl-embedded tissue. For comparison, immunogold staining was shown to be restricted to basolateral membranes of kidney medullary ascending thick limbs. The apical localization of Na+,K+-ATPase in choroid plexus is in striking contrast to the almost exclusive basolateral localization seen in other ion-transporting tissues. The immunocytochemical data are completely consistent with physiological data on choroidal epithelial transport and with light microscopic autoradiographic localization of [3H]-ouabain binding sites.

Primary culture of duck salt gland. II. Neurohormonal stimulation of active transport

Primary cultures of structurally polarized sheets of avian salt gland secretory cells were mounted in Lucite chambers for transmural electrophysiological analysis. Transmural resistance values increased during the first 3 days of culture to 293 +/- 35 omega X cm2 and then decreased slowly thereafter. There was little short-circuit current (Isc) in the absence of secretagogues. Serosal addition of either carbachol or epinephrine resulted in a Isc consistent with positive charge flow from mucosa to serosa, thus demonstrating that these cell layers were capable of active ion transport in response to either cholinergic or adrenergic neurohormonal stimulation. Serosal ouabain or furosemide abolished the response to either agonist, while theophylline enhanced the response. Receptor specificity for the electrical responses was shown by selective inhibition of carbachol- and epinephrine-induced Isc by atropine and propranolol, respectively. The results demonstrate that these primary epithelial cell cultures are capable of active ion transport and are sensitive to known inhibitors of secretory transport, and suggest that intracellular coupling mechanisms for hormonal control are retained in culture. These cultures should be useful for studying mechanisms of ion secretory transport and their regulatory control.

Primary culture of duck salt gland. I. Morphology of confluent cell layers

Dissociated avian salt gland secretory cells were maintained in primary culture after plating on hydrated collagen gels. When seeded at 3 X 10(6) cells/cm2, confluent cell sheets formed within 2-3 days, whereas cultures seeded at lower densities formed a complex reticulum of cell aggregates, which remained nonconfluent even after 7 days. Scanning electron microscopy showed that the free surface of 3-day confluent cultures consisted of intermixed convex and flattened cell membranes with prominent junctional boundaries and abundant microvilli. Transmission electron microscopy indicated that these cultures were multilayers of 1-4 cells in thickness. The plasma membranes of the superficial cells were polarized into apical and basolateral regions displaying, respectively, microvilli and interdigitating lateral membrane folds. These membrane domains were separated by shallow occluding junctions, which consisted of both single strands and simple net-like arrays in freeze-fracture images. Underlying epithelial cells retained lateral membrane folds and formed desmosomal contacts with superficial and neighboring cells. These cultures, unlike the intact tissue, allow direct access to the apical and basolateral cell surfaces for electrophysiological analysis of transmural active ion transport.

Morphological and physiological studies of rat kidney cortex slices undergoing isosmotic swelling and its reversal: a possible mechanism for ouabain-resistant control of cell volume

Slices of rat kidney cortex were induced to swell by preincubation at 1 degree C in an isotonic Ringer's solution, and their capacity to reverse swelling, by net extrusion of cellular water, was studied during subsequent incubation at 25 degrees C. The recovery from swelling was prevented by the respiratory inhibitor, antimycin A. On the other hand, extrusion of water was little affected by ouabain. The extrusion of water continuing in the presence of ouabain (but not that in its absence) was significantly reduced when furosemide was added or when medium Cl- was replaced by NO-3 or I-. There was substantial variability in the morphological appearance of cells within the cortical slices. Different segments of the nephron showed different structural changes during swelling and its reversal, the proximal tubules being most markedly affected. Proximal tubular cells of swollen slices showed disorganization of brush borders and expansion of their apical surfaces, and contained vesicles in their apical cytoplasm. Upon recovery at 25 degrees C, the apical portions of these cells showed reversal of the expansion, but some apical vesicles remained. These vesicles were much more numerous after recovery in the presence of ouabain, but they were much reduced in numbers, or totally absent, when recovery took place in the presence of furosemide or absence of Cl-, with or without ouabain. The vesicles seen in the presence of ouabain alone appeared to fuse with each other and with infoldings of the basolateral plasma membrane. Rather similar results were obtained with distal tubular cells in the slices. We suggest that volume regulation in the proximal and distal tubular cells proceeds by way of two mechanisms. The first consists of extrusion of water coupled to the ouabain-sensitive transport of Na+ and K+. The other proceeds by way of an ouabain-resistant entry of water into apical cytoplasmic vesicles, following furosemide-sensitive movements of Cl- and Na+; the vesicles then expel their contents by exocytosis at the basolateral cell borders.

Purification of mouse brain (Na+ + K+)-ATPase catalytic unit, characterization of antiserum, and immunocytochemical localization in cerebellum, choroid plexus, and kidney

The denatured catalytic polypeptide of mouse brain (Na+ + K+)-adenosine triphosphatase(ATPase) was separated from microsomal membranes on polyacrylamide gels and used as an immunogen. The antiserum, characterized by immunoblots, recognizes the polypeptide corresponding to the catalytic unit in various fractions of mouse brain and cross-reacts with the catalytic unit from lamb kidney, duck salt gland, and electroplax. The same polypeptide in brain and salt gland is recognized by antiserum raised against purified lamb kidney enzyme. Light microscopy was performed with the peroxidase-conjugated second antibody method. In mouse cerebellum, immunochemical staining outlines Purkinje cell and granule cell perikarya. Intense activity is associated with regions of high synaptic content including the pericellular basket meshes and preaxonal regions of Purkinje cells and the glomeruli in the granular layer. In the molecular layer, the neuropil is diffusely reactive with distinct vertically oriented processes evident. White matter exhibits light stain deposition. Choroid plexus presents abundant reaction product only at ependymal apical surfaces, while the ependymal lining of the fourth ventricle displays little or no immunoreactivity. Specificity of the antiserum was demonstrated further in mouse kidney where staining conforms to the well-characterized localization of the enzyme along basolateral surfaces of cortical and medullary tubules. The biochemical and immunocytochemical data show the efficacy of generating antisera to brain (Na+ + K+)-ATPase using catalytic polypeptide as an immunogen.

Muscarinic stimulation of phospholipid turnover in dissociated avian salt gland cells

Addition of carbamylcholine to 32P-prelabeled dissociated avian salt gland cells resulted in increased turnover of phosphatidic acid, phosphatidylinositol, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate, which could be prevented by the inclusion of atropine. Carbamylcholine had no discernable effect on protein phosphorylation, measured either in the total preparation or in subcellular fractions. It is concluded that for the avian salt gland, no obligatory link is indicated between protein phosphorylation and either phospholipid turnover or salt secretion.