Retinyl esters form lipid droplets independently of triacylglycerol and seipin

Lipid droplets store neutral lipids, primarily triacylglycerol and steryl esters. Seipin plays a role in lipid droplet biogenesis and is thought to determine the site of lipid droplet biogenesis and the size of newly formed lipid droplets. Here we show a seipin-independent pathway of lipid droplet biogenesis. In silico and in vitro experiments reveal that retinyl esters have the intrinsic propensity to sequester and nucleate in lipid bilayers. Production of retinyl esters in mammalian and yeast cells that do not normally produce retinyl esters causes the formation of lipid droplets, even in a yeast strain that produces only retinyl esters and no other neutral lipids. Seipin does not determine the size or biogenesis site of lipid droplets composed of only retinyl esters or steryl esters. These findings indicate that the role of seipin in lipid droplet biogenesis depends on the type of neutral lipid stored in forming droplets.

Equine biochemical multiple acyl-CoA dehydrogenase deficiency (MADD) as a cause of rhabdomyolysis

Two horses (a 7-year-old Groninger warmblood gelding and a six-month-old Trakehner mare) with pathologically confirmed rhabdomyolysis were diagnosed as suffering from multiple acyl-CoA dehydrogenase deficiency (MADD). This disorder has not been recognised in animals before. Clinical signs of both horses were a stiff, insecure gait, myoglobinuria, and finally recumbency. Urine, plasma, and muscle tissues were investigated. Analysis of plasma showed hyperglycemia, lactic acidemia, increased activity of muscle enzymes (ASAT, LDH, CK), and impaired kidney function (increased urea and creatinine). The most remarkable findings of organic acids in urine of both horses were increased lactic acid, ethylmalonic acid (EMA), 2-methylsuccinic acid, butyrylglycine (iso)valerylglycine, and hexanoylglycine. EMA was also increased in plasma of both animals. Furthermore, the profile of acylcarnitines in plasma from both animals showed a substantial elevation of C4-, C5-, C6-, C8-, and C5-DC-carnitine. Concentrations of acylcarnitines in urine of both animals revealed increased excretions of C2-, C3-, C4-, C5-, C6-, C5-OH-, C8-, C10:1-, C10-, and C5-DC-carnitine. In addition, concentrations of free carnitine were also increased. Quantitative biochemical measurement of enzyme activities in muscle tissue showed deficiencies of short-chain acyl-CoA dehydrogenase (SCAD), medium-chain acyl-CoA dehydrogenase (MCAD), and isovaleryl-CoA dehydrogenase (IVD) also indicating MADD. Histology revealed extensive rhabdomyolysis with microvesicular lipidosis predominantly in type 1 muscle fibers and mitochondrial damage. However, the ETF and ETF-QO activities were within normal limits indicating the metabolic disorder to be acquired rather than inherited. To our knowledge, these are the first cases of biochemical MADD reported in equine medicine.

Partial purification and characterization of ferritin from the liver and intestinal mucosa of chickens, turtledoves and mynahs

Ferritin is the iron-storage protein responsible for sequestering excess iron, to be stored in a safe way in the liver or to be shed with the intestinal epithelial cells. The properties of ferritin in iron-overload-susceptible birds have not been elucidated. Furthermore, there is only scarce information on mucosal ferritin, with no information at all in avian species. Here we have studied the liver and proximal intestine ferritins of iron-overload-susceptible (Indian hill mynahs, common mynahs) and non-susceptible (turtledoves, chicken) bird species. A brief purification process preceded native polyacrylamide gel electrophoresis and staining the gels for protein and iron. Protein amounts and iron-binding characteristics of ferritin were measured and ferritin saturation levels were calculated. Although ferritin protein amounts did not differ significantly, liver and mucosal ferritins of sensitive bird species incorporated much more iron, leading to high saturation levels. Significantly higher ferritin iron content and saturation were observed in the liver of both mynah species and in the intestinal ferritin of Indian hill mynahs when compared with the non-susceptible species. Ferritin appears not to play a major role in the regulation of iron absorption, implicating other phases in iron transport to be more important in the onset and process of iron overload in birds.

The nitric oxide donor sodium nitroprusside inhibits mineralization in ATDC5 cells

The aim of this study was to test whether the nitric oxide (NO) donor sodium nitroprusside (SNP) has an effect on mineralization in ATDC5 cells. Mineralization in ATDC5 cell culture was induced by addition of beta-glycerophosphate or inorganic phosphate, visualized by staining precipitated calcium with an alizarin red stain, and quantified using atomic absorption spectrometry. SNP was shown to inhibit the mineralization of ADTC5 cells. This inhibition was not affected by inhibitors of guanylyl cyclase nor mimicked by a cyclic guanosine monophosphate (cGMP) analog. Furthermore, SNP did not inhibit phosphate uptake or inhibit apoptosis in ATDC5 cells. These findings indicate that SNP can specifically inhibit matrix mineralization via a cGMP-independent pathway and that the effect is not mediated by inhibition of phosphate transport or apoptosis. These results suggest a preventive role of NO in premature or pathological mineralization.

Luteinizing hormone inhibits Fas-induced apoptosis in ovarian surface epithelial cell lines

Gonadotrophins including LH have been suggested to play an important role in the etiology of epithelial ovarian cancers. The goal of the present study was to obtain more insight in the mechanism of gonadotrophin action on ovarian surface epithelium (OSE) cells. As the Fas system is known to be a major player in the regulation of the process of apoptosis in the ovary, we investigated whether LH interfered with Fas-induced apoptosis in the human OSE cancer cell lines HEY and Caov-3. Activation of Fas receptor by an agonistic anti-Fas receptor antibody induced apoptosis, as was evaluated by caspase-3 activation, poly(ADP-ribose) polymerase fragmentation, phosphatidylserine externalization and morphological changes characteristic of apoptosis. Co-treatment with LH reduced the number of apoptotic cells following activation of Fas in a transient manner, while LH by itself did not affect apoptosis or cell proliferation. The anti-apoptotic effect of LH could be mimicked by the membrane-permeable cAMP analog 8-(4-chlorophenylthio) cAMP (8-CPT-cAMP), and blocked by H89, a specific inhibitor of protein kinase A (PKA). In conclusion, these findings suggest that LH protects HEY cells against Fas-induced apoptosis through a signaling cascade involving PKA. Although it is plausible that in vivo LH might also enhance OSE tumor growth through inhibition of apoptosis, further research is necessary to confirm this hypothesis.

Aggregation of Cryptococcus neoformans by surfactant protein D is inhibited by its capsular component glucuronoxylomannan

Cryptococcus neoformans is an opportunistic pathogen invading the immunocompromised host. Infection starts with the inhalation of acapsular or sparsely encapsulated cells, after which capsule synthesis is initiated. The capsule is the main virulence factor of this yeast-like fungus. Pulmonary surfactant protein D (SP-D) is an important component of the local innate defense system. In the present study, interactions of SP-D with intact C. neoformans cells and their isolated capsular components were investigated. Although encapsulated cryptococci were bound, SP-D showed the highest affinity for acapsular C. neoformans. Only acapsular cryptococci were aggregated by SP-D. Furthermore, the cryptococcal capsular components glucuronoxylomannan (GXM) and mannoprotein 1 (MP1) were bound with relatively high affinity, in contrast to GalXM and MP2. Binding as well as aggregation of acapsular C. neoformans by SP-D could be inhibited by GXM in concentrations that are likely to be present in the lung after infection, suggesting that not only the capsule hampers SP-D function within the innate defense system of the lung but also the secreted capsular component GXM.

The juxtamembrane lysine and arginine residues of surfactant protein C precursor influence palmitoylation via effects on trafficking

Surfactant protein (SP)-C propeptide (proSP-C) becomes palmitoylated on cysteines 5 and 6 before mature SP-C is formed by several proteolytic steps. To study the structural requirements for the palmitoylation of proSP-C, his-tagged human proSP-C (his-proSP-C) and his-proSP-C mutants were expressed in Chinese hamster ovary cells and analyzed by metabolic labeling with [(3)H]palmitate and immunocytochemistry. Substitution of cysteines 5 and 6 by serines showed that these were the only two cysteine residues palmitoylated in his-proSP-C. Substitution of the juxtamembrane basic residues lysine and arginine by uncharged glutamines led to a large decrease in palmitoylation level of proSP-C. The addition of brefeldin A nearly abolished this decrease for the lysine and double mutant; the palmitoylation of the arginine mutant increased also, but not to wild-type (WT) levels. Fluorescence immunocytochemistry showed that WT proSP-C was localized in punctate vesicles throughout the cell, whereas the mutant lacking the juxtamembrane positive charges was found more perinuclear, probably in the endoplasmic reticulum (ER). This indicates that the two basic juxtamembrane residues influence palmitoylation of proSP-C by preventing the transport of proSP-C out of the ER, implying that proSP-C becomes palmitoylated normally in a compartment distal to the ER.

Lung surfactant proteins A and D in innate immune defense

The lung surfactant proteins (SP) A and D are large multimeric proteins and belong to a family of collagenous C-type lectins designated collectins. Both SP-A and SP-D are believed to play a role in the innate immunity of the lung. SP-A and SP-D bind to a broad spectrum of pathogens, including bacteria, viruses, fungi and yeasts but also lipopolysaccharides and allergens. Furthermore, SP-A and SP-D enhance the clearing of various pathogens by neutrophils and macrophages in vitro. Recent in vivo studies on SP-A deficient mice also support a role of SP-A in host defense.

Differential role of cyclic GMP-dependent protein kinase II in ion transport in murine small intestine and colon

BACKGROUND & AIMS

The aim of this study was to determine the role of guanosine 3',5'-cyclic monophosphate (cGMP)-dependent protein kinase (cGK) type II in intestinal fluid homeostasis under basal conditions and following exposure to cGMP-linked secretagogues, e.g., Escherichia coli heat-stable enterotoxin (STa) and guanylin.

METHODS

Fluid and ion transport was determined in different segments of the intestine of wild-type and cGK II-deficient mice by ligated loop assays in vivo, and by short-circuit current and isotope flux measurements in vitro.

RESULTS

Small intestinal fluid absorption in vivo was enhanced in cGK II-deficient mice under basal conditions and in the presence of STa. Furthermore, STa, guanylin, and 8-pCPT-cGMP stimulation of electrogenic anion secretion and inhibition of Na(+) absorption in vitro were markedly reduced in the small intestine from cGK II -/- mice but not in proximal colon. The type III phosphodiesterase inhibitor amrinone mimicked STa action in cGK II -/- mice, and also stimulated ion secretion in humans.

CONCLUSIONS

This study shows that the cGMP/cGK II pathway regulates fluid homeostasis in the small intestine under basal conditions and mediates STa effects by both increasing anion secretion and inhibiting Na(+) absorption. It also demonstrates the presence of a cGK II-independent pathway for STa/cGMP-provoked secretion predominantly in the colon, which possibly involves a cGMP-inhibitable phosphodiesterase and/or activation of the cAMP-dependent protein kinase pathway.

Nitric oxide and cGMP regulate gene expression in neuronal and glial cells by activating type II cGMP-dependent protein kinase

Nitric oxide (NO) and cGMP have been implicated in many neuronal functions, including regulation of gene expression, but little is known about the downstream targets of NO/cGMP in the nervous system. We found that type II cGMP-dependent protein kinase (G-kinase), which is widely expressed in the brain, mediated NO- and cGMP-induced activation of the fos promoter in cells of neuronal and glial origin; the enzyme was ineffective in regulating gene expression in fibroblast-like cells. The effect of G-kinase II on gene expression did not require calcium uptake but was synergistically enhanced by calcium. G-kinase II was membrane associated and did not translocate to the nucleus; however, a soluble G-kinase II mutant translocated to the nucleus and regulated gene expression in fibroblast-like cells. Soluble G-kinase I also regulates fos promoter activity, but membrane targeting of G-kinase I prevented the enzyme from translocating to the nucleus and regulating transcription in multiple cell types, including glioma cells; this suggests that cell type-specific factor(s) that mediate the transcriptional effects of extranuclear G-kinase II are not regulated by G-kinase I. Our results suggest that G-kinase I and II control gene expression by different mechanisms and that NO effects on neuronal plasticity may involve G-kinase II regulation of gene expression.-Gudi, T., Hong, G. K.-P., Vaandrager, A. B., Lohmann, S. M., Pilz, R. B. Nitric oxide and cGMP regulate gene expression in neuronal and glial cells by activating type II cGMP-dependent protein kinase.

Atrial natriuretic peptide-stimulated Ca2+ reabsorption in rabbit kidney requires membrane-targeted, cGMP-dependent protein kinase type II

Atrial natriuretic peptide (ANP) and nitric oxide (NO) are key regulators of ion and water transport in the kidney. Here, we report that these cGMP-elevating hormones stimulate Ca2+ reabsorption via a novel mechanism specifically involving type II cGMP-dependent protein kinase (cGK II). ANP and the NO donor, sodium nitroprusside (SNP), markedly increased Ca2+ uptake in freshly immunodissected rabbit connecting tubules (CNT) and cortical collecting ducts (CCD). Although readily increasing cGMP, ANP and SNP did not affect Ca2+ and Na+ reabsorption in primary cultures of these segments. Immunoblot analysis demonstrated that cGK II, and not cGK I, was present in freshly isolated CNT and CCD but underwent a complete down-regulation during the primary cell culture. However, upon adenoviral reexpression of cGK II in primary cultures, ANP, SNP, and 8-Br-cGMP readily increased Ca2+ reabsorption. In contrast, no cGMP-dependent effect on electrogenic Na+ transport was observed. The membrane localization of cGK II proved to be crucial for its action, because a nonmyristoylated cGK II mutant that was shown to be localized in the cytosol failed to mediate ANP-stimulated Ca2+ transport. The Ca2+-regulatory function of cGK II appeared isotype-specific because no cGMP-mediated increase in Ca2+ transport was observed after expression of the cytosolic cGK Ibeta or a membrane-bound cGK II/Ibeta chimer. These results demonstrate that ANP- and NO-stimulated Ca2+ reabsorption requires membrane-targeted cGK II.

Membrane targeting of cGMP-dependent protein kinase is required for cystic fibrosis transmembrane conductance regulator Cl- channel activation

A recently cloned isoform of cGMP-dependent protein kinase (cGK), designated type II, was implicated as the mediator of cGMP-provoked intestinal Cl- secretion based on its localization in the apical membrane of enterocytes and on its capacity to activate cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels. In contrast, the soluble type I cGK was unable to activate CFTR in intact cells, although both cGK I and cGK II could phosphorylate CFTR in vitro. To investigate the molecular basis for the cGK II isotype specificity of CFTR channel gating, we expressed cGK II or cGK I mutants possessing different membrane binding properties by using adenoviral vectors in a CFTR-transfected intestinal cell line, and we examined the ability of cGMP to phosphorylate and activate the Cl- channel. Mutation of the cGK II N-terminal myristoylation site (Gly2 --> Ala) reduced cGK II membrane binding and severely impaired cGK II activation of CFTR. Conversely, a chimeric protein, in which the N-terminal membrane-anchoring domain of cGK II was fused to the N terminus of cGK Ibeta, acquired the ability to associate with the membrane and activate the CFTR Cl- channel. The potency order of cGK constructs for activation of CFTR (cGK II > membrane-bound cGK I chimer >> nonmyristoylated cGK II > cGK Ibeta) correlated with the extent of 32P incorporation into CFTR observed in parallel measurements. These results strongly support the concept that membrane targeting of cGK is a major determinant of CFTR Cl- channel activation in intact cells.

Distinct and specific functions of cGMP-dependent protein kinases

cGMP-dependent protein kinases I and II conduct signals from widespread signaling systems. Whereas the type I kinase mediates numerous effects of natriuretic peptides and nitric oxide in cardiovascular cells, the type II kinase transduces signals from the Escherichia coli heat-stable enterotoxin, STa, and from the endogenous intestinal peptide, guanylin, stimulating Cl- conductance of the cystic fibrosis transmembrane conductance regulator (CFTR). Although the two kinases may be interchangeable for several functions, CFTR regulation specifically requires the type II kinase.

Endogenous type II cGMP-dependent protein kinase exists as a dimer in membranes and can Be functionally distinguished from the type I isoforms

In mammalian tissues two types of cGMP-dependent protein kinase (cGK) have been identified. In contrast to the dimeric cGK I, cGK II purified from pig intestine was shown previously to behave as a monomer. However, recombinant rat cGK II was found to have hydrodynamic parameters indicative of a homodimer. Chemical cross-linking studies showed that pig cGK II in intestinal membranes has a dimeric structure as well. However, after purification, cGK II was found to be partly proteolyzed into C-terminal monomeric fragments. Phosphorylation studies in rat intestinal brush borders revealed that the potency of cGMP analogs to stimulate or inhibit native cGK II in vitro (i.e. 8-(4-chlorophenylthio)-cGMP > cGMP > beta-phenyl-1,N2-etheno-8-bromo-cGMP > beta-phenyl-1,N2-etheno-cGMP and Rp-8-(4-chlorophenylthio)-cGMPs > Rp-beta-phenyl-1, N2-etheno-8-bromo-cGMPs, respectively) correlated well with their potency to stimulate or inhibit cGK II-mediated Cl- secretion across intestinal epithelium but differed strikingly from their potency to affect cGK I activity. These data show that the N terminus of cGK II is involved in dimerization and that endogenous cGK II displays a distinct activation/inhibition profile with respect to cGMP analogs, which permits a pharmacological dissection between cGK II- and cGK I-mediated physiological processes.

cGMP stimulation of cystic fibrosis transmembrane conductance regulator Cl- channels co-expressed with cGMP-dependent protein kinase type II but not type Ibeta

In order to investigate the involvement of cGMP-dependent protein kinase (cGK) type II in cGMP-provoked intestinal Cl- secretion, cGMP-dependent activation and phosphorylation of cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels was analyzed after expression of cGK II or cGK Ibeta in intact cells. An intestinal cell line which stably expresses CFTR (IEC-CF7) but contains no detectable endogenous cGK II was infected with a recombinant adenoviral vector containing the cGK II coding region (Ad-cGK II) resulting in co-expression of active cGK II. In these cells, CFTR was activated by membrane-permeant analogs of cGMP or by the cGMP-elevating hormone atrial natriuretic peptide as measured by 125I- efflux assays and whole-cell patch clamp analysis. In contrast, infection with recombinant adenoviruses expressing cGK Ibeta or luciferase did not convey cGMP sensitivity to CFTR in IEC-CF7 cells. Concordant with the activation of CFTR by only cGK II, infection with Ad-cGK II but not Ad-cGK Ibeta enabled cGMP analogs to increase CFTR phosphorylation in intact cells. These and other data provide evidence that endogenous cGK II is a key mediator of cGMP-provoked activation of CFTR in cells where both proteins are co-localized, e. g. intestinal epithelial cells. Furthermore, they demonstrate that neither the soluble cGK Ibeta nor cAMP-dependent protein kinase are able to substitute for cGK II in this cGMP-regulated function.

Guanosine 3',5'-cyclic monophosphate-dependent protein kinase II mediates heat-stable enterotoxin-provoked chloride secretion in rat intestine

BACKGROUND & AIMS

Escherichia coli heat-stable enterotoxins (STa) provoke electrogenic Cl- secretion in the intestine through a guanosine 3',5'-cyclic monophosphate (cGMP)-dependent signal transduction pathway. The cGMP receptor involved in the activation of the Cl- channel is not known with certainty but may comprise either adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase (cAK) or cGMP-dependent protein kinase (cGK) type II. The aim of this study was to discriminate between these possibilities using specific kinase inhibitors.

METHODS

Intestinal electrogenic Cl- secretion was determined by measuring short-circuit current (Isc) in a Ussing chamber.

RESULTS

The general protein kinase inhibitors staurosporine and H-8 inhibited rat cGK II activity in vitro with 50% inhibitory concentration values of 4 nmol/L and 3 mumol/L, respectively, which are lower than those reported for cAK. Both staurosporine and H-8, when added to rat proximal colon at concentrations that did not affect the Isc response to 8-bromo-cAMPS, inhibited the STa- and 8-bromo-cGMP-provoked Isc response for more than 80%. Furthermore, the relative specific cGK inhibitor Rp isomer of 8-(chlorophenylthio)-cGMP, but not the cAK inhibitor RP isomer of (Rp) 8-bromo-cAMPS, inhibited the Isc response to submaximal levels of STa in rat proximal colon.

CONCLUSIONS

These data provide further evidence for an important role of cGK II in STa-mediated Cl- secretion in native rat intestinal epithelium.

Signalling by cGMP-dependent protein kinases

The second messenger cGMP is a major intracellular mediator of the vaso-active agents nitric oxide and natriuretic peptides. The principal targets of cGMP are (i) phosphodiesterases, resulting in interference with the cAMP-signalling pathway, (ii) cGMP-gated cation channels, and (iii) cGMP-dependent protein kinases (cGKs). Only two mammalian isotypes of cGK have been described so far: type I cGK, consisting of an alpha and a beta isoform, presumably splice variants of a single gene, and identified as the most prominent cGK isotype in the cardio-vascular system; and type II cGK, expressed mainly in the intestine, the kidney and the brain. High levels of cGK I are found in vascular smooth muscle cells, endothelial cells and platelets. In these cells, cGK I is thought to counteract the increase in contraction provoked by Ca-mobilizing agonists, to reduce endothelial permeability and to inhibit platelet aggregation, respectively. Relatively low levels of cGK I are found in cardiomyocytes. In this cell type, cGK is implicated in the negative inotropic effect of cGMP, presumably through modulation of Ca channels and by diminishing the Ca-sensitivity of contractile proteins.

N-terminal myristoylation is required for membrane localization of cGMP-dependent protein kinase type II

The apical membrane of intestinal epithelial cells harbors a unique isozyme of cGMP-dependent protein kinase (cGK type II) which acts as a key regulator of ion transport systems, including the cystic fibrosis transmembrane conductance regulator (CFTR)-chloride channel. To explore the mechanism of cGK II membrane-anchoring, recombinant cGK II was expressed stably in HEK 293 cells or transiently in COS-1 cells. In both cell lines, cGK II was found predominantly in the particulate fraction. Immunoprecipitation of solubilized cGK II did not reveal any other tightly associated proteins, suggesting a membrane binding motif within cGK II itself. The primary structure of cGK II is devoid of hydrophobic transmembrane domains; cGK II does, however, contain a penultimate glycine, a potential acceptor for a myristoyl moiety. Metabolic labeling showed that cGK II was indeed able to incorporate [3H]myristate. Moreover, incubation of cGK II-expressing 293 cells with the myristoylation inhibitor 2-hydroxymyristic acid (1 mM) significantly increased the proportion of cGK II in the cytosol from 10 +/- 5 to 35 +/- 4%. Furthermore, a nonmyristoylated cGK II Gly2 --> Ala mutant was localized predominantly in the cytosol after transient expression in COS-1 cells. The absence of the myristoyl group did not affect the specific enzyme activity or the Ka for cGMP and only slightly enhanced the thermal stability of cGK II. These results indicate that N-terminal myristoylation fulfills a crucial role in directing cGK II to the membrane.

Production and localization of cGMP and PGE2 in nitroprusside-stimulated rat colonic ion transport

Nitrovasodilators, such as sodium nitroprusside (SNP), release nitric oxide (NO) and stimulate intestinal electrolyte transport. However, the second messengers involved in this process are unknown. NO stimulates soluble guanylate cyclase activity in other tissues, but stimulation of this enzyme has not previously been described for intestine. We report a 20-fold increase in guanosine 3',5'-cyclic monophosphate (cGMP) production by radioimmunoassay in colonic mucosal strips stimulated with SNP. SNP also caused a significant increase in prostaglandin (PG) E2 release but did not stimulate release of the prostanoids thromboxane B2 or 6-keto-PGF1alpha. Stimulation of isolated colonic crypts and the remaining subepithelial mucosa demonstrated that the latter was the major source of the increases in cGMP and PGE2. Immunostaining of colonic mucosa revealed minimal basal cGMP immunoreactivity but large increases in abundance, localizing to the subepithelium, after SNP treatment. Under basal conditions, there was diffuse immunostaining for constitutive NO synthase in both the epithelial and subepithelial compartments, which was corroborated with NADPH diaphorase staining. In conclusion, SNP was an NO donor stimulates production of cGMP and PGE2 from the subepithelium. NO may be an important mediator of colonic secretion and other processes predominantly via its direct effects on cells of the lamina propria.

Isotype-specific activation of cystic fibrosis transmembrane conductance regulator-chloride channels by cGMP-dependent protein kinase II

Type II cGMP-dependent protein kinase (cGKII) isolated from pig intestinal brush borders and type I alpha cGK (cGKI) purified from bovine lung were compared for their ability to activate the cystic fibrosis transmembrane conductance regulator (CFTR)-Cl- channel in excised, inside-out membrane patches from NIH-3T3 fibroblasts and from a rat intestinal cell line (IEC-CF7) stably expressing recombinant CFTR. In both cell models, in the presence of cGMP and ATP, cGKII was found to mimic the effect of the catalytic subunit of cAMP-dependent protein kinase (cAK) on opening CFTR-Cl-channels, albeit with different kinetics (2-3-min lag time, reduced rate of activation). By contrast, cGKI or a monomeric cGKI catalytic fragment was incapable of opening CFTR-Cl- channels and also failed to potentiate cGKII activation of the channels. The cAK activation but not the cGKII activation was blocked by a cAK inhibitor peptide. The slow activation by cGKII could not be ascribed to counteracting protein phosphatases, since neither calyculin A, a potent inhibitor of phosphatase 1 and 2A, nor ATP gamma S (adenosine 5'-O-(thiotriphosphate)), producing stable thiophosphorylation, was able to enhance the activation kinetics. Channels preactivated by cGKII closed instantaneously upon removal of ATP and kinase but reopened in the presence of ATP alone. Paradoxically, immunoprecipitated CFTR or CF-2, a cloned R domain fragment of CFTR (amino acids 645-835) could be phosphorylated to a similar extent with only minor kinetic differences by both isotypes of cGK. Phosphopeptide maps of CF-2 and CFTR, however, revealed very subtle differences in site-specificity between the cGK isoforms. These results indicate that cGKII, in contrast to cGKI alpha, is a potential activator of chloride transport in CFTR-expressing cell types.

Expression of cGMP-dependent protein kinase I and phosphorylation of its substrate, vasodilator-stimulated phosphoprotein, in human endothelial cells of different origin

Previous studies demonstrated that the thrombin-induced permeability of endothelial cell monolayers is reduced by the elevation of cGMP. In the present study, the presence of cGMP-dependent protein kinase (cGMP-PK) immunoreactivity and activity in various types of human endothelial cells (ECs) and the role of cGMP-PK in the reduction of thrombin-induced endothelial permeability was investigated. cGMP-PK type I was demonstrated in freshly isolated ECs from human aorta and iliac artery as well as in cultured ECs from human aorta, iliac vein, and foreskin microvessels. Addition of the selective cGMP-PK activator 8-(4-chlorophenylthio)-cGMP (8-pCPT-cGMP) to these ECs caused phosphorylation of the vasodilator-stimulated phosphoprotein (VASP), an established cGMP-PK substrate, which is localized at cell-cell contact sites of confluent ECs. cGMP-PK type I expression decreased during serial passage of ECs, which correlated with a diminished ability of 8-pCPT-cGMP to induce VASP phosphorylation. Preincubation of aorta and microvascular EC monolayers with 8-pCPT-cGMP caused a 50% reduction of the thrombin-stimulated permeability, as determined by measuring the peroxidase passage through EC monolayers on porous filters. Furthermore, the thrombin-induced rise in cytoplasmic [Ca2+]i was strongly attenuated by the cGMP-PK activator in fura 2-loaded aorta ECs. In contrast, cGMP-PK could not be demonstrated in freshly isolated and cultured human umbilical vein ECs. Incubation of umbilical vein ECs with 8-pCPT-cGMP did not cause VASP phosphorylation and had no effect on the thrombin-induced increases in cytoplasmic Ca2+ and endothelial permeability.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous expression of type II cGMP-dependent protein kinase mRNA and protein in rat intestine. Implications for cystic fibrosis transmembrane conductance regulator

Certain pathogenic bacteria produce a family of heat stable enterotoxins (STa) which activate intestinal guanylyl cyclases, increase cGMP, and elicit life-threatening secretory diarrhea. The intracellular effector of cGMP actions has not been clarified. Recently we cloned the cDNA for a rat intestinal type II cGMP dependent protein kinase (cGK II) which is highly enriched in intestinal mucosa. Here we show that cGK II mRNA and protein are restricted to the intestinal segments from the duodenum to the proximal colon, with the highest amounts of cGK II protein in duodenum and jejunum. cGK II mRNA and protein decreased along the villus to crypt axis in the small intestine, whereas substantial amounts of both were found in the crypts of cecum. In intestinal epithelia, cGK II was specifically localized in the apical membrane, a major site of ion transport regulation. In contrast to cGK II, cGK I was localized in smooth muscle cells of the villus lamina propria. Short circuit current (ISC), a measure of Cl- secretion, was increased to a similar extent by STa and by 8-Br-cGMP, a selective activator of cGK, except in distal colon and in monolayers of T84 human colon carcinoma cells in which cGK II was not detected. In human and mouse intestine, the cyclic nucleotide-regulated Cl- conductance can be exclusively accounted for by the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel. Viewed collectively, the data suggest that cGK II is the mediator of STa and cGMP effects on Cl- transport in intestinal-epithelia.

Heat-stable enterotoxin receptor/guanylyl cyclase C is an oligomer consisting of functionally distinct subunits, which are non-covalently linked in the intestine

Guanylyl cyclase (GC) C is a heat-stable enterotoxin (STa) receptor with a monomeric M(r) of approximately 140,000. We calculated from its hydrodynamic parameters that an active GC-C complex has a M(r) of 393,000, suggesting that GC-C is a trimer under native conditions. Both trimeric and dimeric GC-C complexes were detected by 125I-STa binding and SDS-polyacrylamide gel electrophoresis under non-reducing conditions. The GC activity and STa binding from intestinal brush border membranes comigrated in gel filtration and velocity sedimentation with recombinant GC-C. However, 125I-STa cross-linking demonstrated that STa receptors with molecular masses of 52 and 74 kDa are non-covalently attached to GC in the intestine. Radiation inactivation revealed different functional sizes for basal GC activity, STa-stimulated GC activity, and STa binding (59, 210-240, and 32-52 kDa, respectively). At low radiation doses, basal GC activity was stimulated, suggesting that GC-C is inhibited by a relatively large, probably internal structure. These results suggest that STa may activate GC-C by promoting monomer-monomer interaction (internal "dimerization") within a homotrimeric GC-C complex, and that GC-C is proteolytically modified in the brush border membrane but retains its function.

Heat-stable enterotoxin activation of immunopurified guanylyl cyclase C. Modulation by adenine nucleotides

We studied the activation and inactivation of recombinant guanylyl cyclase (GC) C stably expressed in human kidney 293 cells. Activation of GC-C by heat-stable enterotoxin (STa), Cd2+, hemin, or the detergent Triton X-100 was followed by a rapid inactivation of the enzyme. Adenine nucleotides were found to prevent the inactivation process in native membranes, as well as following enzyme solubilization and immunopurification. Inactivation of GC-C was not associated with dephosphorylation. However, the phosphorylation of GC-C was promoted by phorbol esters, known activators of protein kinase C. The activation of purified GC-C by STa required the presence of a nonspecific factor as exemplified by bovine serum albumin. When immunopurified GC-C, stabilized by ATP and bovine serum albumin, was analyzed by SDS-polyacrylamide gel electrophoresis under nonreducing conditions, proteins with almost twice the molecular mass (220 and 245 kDa) of the monomer were observed. The mobility of these high M(r) GC-C forms was reduced by STa, suggesting that STa induces a conformation change in a preexisting GC-C dimer. These results indicate that ATP interacts directly with GC-C, stabilizing its active conformation and that the activation of GC-C may occur in the absence of other specific regulatory factors.

Guanylyl cyclase C is an N-linked glycoprotein receptor that accounts for multiple heat-stable enterotoxin-binding proteins in the intestine

Guanylyl cyclase C (GC-C) is a newly discovered receptor found in the intestine, and possibly in other epithelia, that binds bacterial heat-stable enterotoxins (STa). The receptor has now been stably expressed in human embryonic 293 cells, which do not normally contain the receptor. Cyclic GMP concentrations are elevated 40-fold in response to 1 microM STa, and membranes obtained from the overproducing cells contain GC-C activity that can be stimulated about 9-fold by STa alone and an additional 1.4- to 2-fold by a combination of ATP and STa. The ATP effect does not appear to be due to enzyme activation, but instead to protection of GC-C against inactivation. Antibody raised against the carboxyl-terminal sequence of GC-C identified two major proteins (M(r) 140,000 and 160,000) in 293 cells expressing GC-C. Both proteins bound to wheat germ lectin-Sepharose, and N-glycosidase F treatment converted both forms to a single M(r) 120,000 protein, the size predicted from amino acid composition. The addition of high concentrations of tunicamycin to 293-GC-C cells also reduced the M(r) to 120,000, indicating that GC-C is an N-linked glycoprotein. When rat intestinal membranes or 293-GC-C cells were cross-linked with 125I-labeled STa, the major 125I-labeled protein complexes had M(r) ranging between 45,000 and 80,000. On immunoblots of rat intestinal membranes treated with a reducing agent, 3 major proteins of M(r) 65,000, 85,000, and 140,000 were specifically recognized by a GC-C antibody. However, under nonreducing conditions one major GC-C related protein appeared at a higher position (M(r) 230,000). Its mobility was reduced in the presence of STa, similar to rCG-C expressed in 293 cells. These data indicate that at least part of the lower M(r) STa-binding proteins found in intestinal extracts represent proteolytic products of GC-C.

Dual role for protein kinase C alpha as a regulator of ion secretion in the HT29cl.19A human colonic cell line

The involvement of protein kinase C (PKC) in the regulation of intestinal ion secretion was studied in polarized monolayers of the HT29cl.19A human colon carcinoma cell line. Carbachol, phorbol esters [PMA (phorbol 12-myristate 13-acetate) and PDB (phorbol 12,13-dibutyrate)] and 8-bromo cyclic AMP (8-Br-cAMP) induced Cl secretion, as measured by a rise in the short-circuit current (ISC). The electrical response to carbachol coincided with a transient translocation of PKC alpha from the soluble to the particulate fraction. The carbachol-, PDB- and 8-Br-cAMP-induced ISC responses were inhibited by pretreatment of the cells with PMA (0.5 microM) for 2 h, a time period in which PKC alpha, beta 1 and gamma levels were not changed. As shown by 86Rb+ and 125I- efflux studies, the main targets for this inhibition were basolateral K+ transporters rather than apical Cl- channels. Prolonged exposure to PMA (24 h) led to a 60% recovery of the 8-Br-cAMP response, but not of the carbachol- or PDB-provoked secretion. As shown by immunoblotting with PKC-isoenzyme-specific antisera, the recovery of the 8-Br-cAMP response coincided with the down-regulation of PKC alpha, whereas the levels of PKC beta 1 and gamma were unmodified. These results suggest that PKC alpha, but not PKC beta 1 or gamma, is involved in both acute stimulation and chronic inhibition of ion secretion in the HT29cl.19A colonic cell line.

Regulation of chloride transport in cultured normal and cystic fibrosis keratinocytes

Cultured normal (N) cystic fibrosis (CF) keratinocytes were evaluated for their Cl(-)-transport properties by patch-clamp-, Ussing chamber- and isotopic efflux-measurements. Special attention was paid to a 32 pS outwardly rectifying Cl- channel which has been reported to be activated upon activation of cAMP-dependent pathways in N, but not in CF cells. This depolarization-induced Cl- channel was found with a similar incidence in N and CF apical keratinocyte membranes. However, activation of this channel in excised patches by protein kinase (PK)-A or PK-C was not successful in either N or CF keratinocytes. Forskolin was not able to activate Cl- channels in N and CF cell-attached patches. The Ca(2+)-ionophore A23187 activated in cell-attached patches a linear 17 pS Cl- channel in both N and CF cells. This channel inactivated upon excision. No relationship between the cell-attached 17 pS and the excised 32 pS channel could be demonstrated. Returning to the measurement of Cl- transport at the macroscopic level, we found that a drastic rise in intracellular cAMP induced by forskolin did in N as well as CF cells not result in a change in the short-circuit current (Isc) or the fractional efflux rates of 36Cl- and 125I-. In contrast, addition of A23187 resulted in an increase of the Isc and in the isotopic anion efflux rates in N and CF cells. We conclude that Cl(-)-transport in cultured human keratinocytes can be activated by Ca2+, but not by cAMP-dependent pathways.

Biphasic increase of apical Cl- conductance by muscarinic stimulation of HT-29cl.19A human colon carcinoma cell line: evidence for activation of different Cl- conductances by carbachol and forskolin

The modulation of ion transport pathways in filter-grown monolayers of the Cl(-)-secreting subclone (19A) of the human colon carcinoma cell line HT-29 by muscarinic stimulation was studied by combined Ussing chamber and microimpalement experiments. Basolateral addition of 10(-4) M carbachol induced a complex poly-phasic change of the cell potential consisting of (i) a fast and short (30-sec) depolarization of 15 +/- 1 mV from a resting value of -52 +/- 1 mV and an increase of the fractional resistance of the apical membrane (first phase), (ii) a repolarization of 22 +/- 1 mV leading to a hyperpolarization of the cell (second phase), (iii) a depolarization of 11 +/- 1 mV and a decrease of the fractional resistance of the apical membrane (the third phase), (iv) and sometimes, a hyperpolarization of 6 +/- 1 mV and an increase of the fractional resistance of the apical membrane (fourth phase). The transepithelial potential increased with a peak value of 2.4 +/- 0.3 mV (basolateral side positive). The transepithelial PD started to increase (serosa positive), coinciding with the start of the second phase of the intracellular potential change, and continued to increase during the third phase. Ion replacements and electrical circuit analyses indicate that the first phase is caused by increase of the Cl- conductance in the apical and basolateral membrane, the second phase by increased K+ conductance of the basolateral membrane, and the third phase and the fourth phase by increase and decrease, respectively, of an apical Cl- conductance. The first and second phase of the carbachol effect could be elicited also by ionomycin. They were strongly reduced by EGTA. Phorbol dibutyrate (PDB) induced a sustained depolarization of the cell and a decrease of the apical fractional resistance. The results suggest that two different types of Cl- channels are involved in the carbachol response: one Ca2+ dependent and a second which may be PKC sensitive. In the presence of a supramaximal concentration of forskolin, carbachol evoked a further increase of the apical Cl- conductance. It is concluded that the short-lasting carbachol/Ca(2+)-dependent Cl- conductance is different from the forskolin-activated conductance. The increase of the Cl- conductance in the presence of forskolin by carbachol may be due to activation of different Cl- channels or to modulation of the PKA-activated Cl- channels by activated PKC.

Atriopeptins and Escherichia coli enterotoxin STa have different sites of action in mammalian intestine

Studies with Escherichia coli-induced heat-stable enterotoxin (STa), an activator of intestinal particulate guanylate cyclase, have established an independent role for cyclic guanosine monophosphate (cGMP) as an intracellular mediator of intestinal salt and water secretion. The present study addressed whether atriopeptins (APs), known activators of particulate guanylate cyclase in other tissues, function as physiological agonists for cGMP-linked Cl- secretion in intestine. APs, in contrast to STa, caused no or only minor changes in cGMP levels in freshly isolated rat intestinal villus and crypt cells and in cultured human colon carcinoma cell lines (HT29glc-, CaCo-2, and T84). Conversely, APs, but not STa, induced a large increase in intracellular cGMP levels in the undifferentiated small intestinal cell lines IEC-6, IEC-18, and INT407. Addition of AP II (atrial natriuretic peptide fragment 5-27) to stripped mucosa of rat proximal colon in Ussing chambers caused a transient increase in the transepithelial potential difference (PD), which most likely represents an increase in Cl- secretion. In contrast, a sustained increase in PD was observed in response to STa or 8Br-cGMP. The AP II-provoked increase in PD was blocked by the neurotoxin tetrodotoxin. Immunohistochemical detection of cGMP in this tissue provided evidence for a different localization pattern of cells responding with an increase in cGMP levels to STa (colonocytes and goblet cells) or AP (specific cells in the submucosa) in rat proximal colon. This indicates that APs, unlike STa, do not directly stimulate the colonic epithelial cells but possibly provoke Cl- secretion by release of a neurotransmitter in the submucosa.

Phorbol esters stimulate and inhibit Cl- secretion by different mechanisms in a colonic cell line

Phorbol 12-myristate 13-acetate (PMA) was found to increase both the short-circuit current (Isc) and the efflux of 125I- or 36Cl- in the colonic epithelial cell line HT-29.cl19A. Neither the PMA-provoked rise in Isc nor the stimulation of 125I- efflux was affected by the cyclooxygenase inhibitor indomethacin. The PMA-induced increase in Cl- efflux was not accompanied by a rise in adenosine 3',5'-cyclic monophosphate (cAMP) levels. A prolonged incubation with PMA (3 h), however, inhibited the PMA- and the cAMP-stimulated Isc by greater than 90%, whereas the cAMP-provoked 125I- and 36Cl- efflux was not inhibited. The long-term PMA treatment was found to inhibit the basal and cAMP-provoked 86Rb+ efflux by 65 +/- 9 and 86 +/- 7%, respectively. A 3-h incubation with PMA also strongly inhibited the Ca2+ ionophore A23187-induced increase in 86Rb+ efflux, whereas the A23187-stimulated 125I- efflux was only marginally inhibited. These data suggest that phorbol esters, presumably by activation of protein kinase C, can provoke Cl- secretion in HT-29.cl19A colonocytes independently of a prostaglandin- or cAMP-mediated pathway. Prolonged exposure to PMA, however, causes an inhibition of net electrogenic Cl- secretion by downregulation of the activity of K+ transporters.

Ca2+ and cAMP activate different chloride efflux pathways in HT-29.cl19A colonic epithelial cell line

The mechanism of adenosine 3',5'-cyclic monophosphate (cAMP)- and Ca(2+)-induced Cl- secretion was studied in monolayers of the colon carcinoma cell line HT-29.cl19A by combined short-circuit current (Isc) and 125I- or 36Cl- efflux measurements. Forskolin, a specific adenylate cyclase activator, was found to induce a large increase in Isc and a two- to threefold increase in 36Cl- efflux solely across the apical border. The fractional efflux of 36Cl-compared with 125I- (basal ratio 1.71 +/- 0.28) did not change significantly in the presence of forskolin (1.91 +/- 0.45). In contrast, the Ca2+ ionophore A23187 did not appreciably affect the Isc but enhanced 36Cl- and 125I- efflux at the apical and basolateral side of the monolayer. Furthermore, the fractional efflux ratio of 36Cl- to 125I- changed dramatically to a value of 0.36 +/- 0.14. Both forskolin- and A23187-induced 36Cl- or 125I- efflux were only weakly inhibited by the putative Cl- channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoicacid. Carbachol, a Ca(2+)-linked agonist, mimicked the effects of A23187 on the 36Cl- and 125I- efflux but additionally provoked a significant increase in Isc. These data show that Ca2+ and cAMP activate different Cl-efflux pathways in HT-29.cl19A cells. Most likely these pathways represent a cAMP-activated conductance in the apical membrane and a separate Ca(2+)-activated Cl- conductance expressed in both apical and basolateral membranes. Apparently cholinergic agonists induce net electrogenic Cl- secretion through an intracellular signaling pathway (e.g., protein kinase C activation) different from the one activated by Ca2+/Ca2+ ionophore alone.

Asymmetrical distribution of G-proteins among the apical and basolateral membranes of rat enterocytes

The distribution of the alpha and beta subunits of guanosine-nucleotide-binding proteins (G-proteins) among the apical and basolateral membranes of polarized rat enterocytes was investigated by ADP-ribosylation assays in vitro and immunoblotting with G-protein-subunit-specific antisera. The enterocytes were found to express alpha i2, alpha ji3, alpha s and beta subunits, whereas alpha i1 and alpha o subunits could not be detected. The alpha i2 and alpha i3 subunits were located predominantly in the basolateral membrane, in contrast with the alpha s and beta subunits, which were distributed uniformly among both membranes. Furthermore, 39 kDa and 78 kDa proteins, recognized by anti-alpha i1/2 but not anti-alpha i1 or anti-alpha i3 specific antisera, and resistant to ADP-ribosylation by pertussis toxin, were localized exclusively at the apical border. These Gi-related proteins might represent novel members of the G-protein family. Activation of apical G-proteins by GTP or its analogues failed to release the alpha s, alpha i and beta subunits or the 39 kDa and 78 kDa alpha i-like proteins from the membrane, suggesting a functional role for these proteins in the apical membrane itself. Our recent finding of a guanosine 5'-[gamma-thio]triphosphate-sensitive Cl- conductance in the apical membrane of rat enterocytes suggests that one or more of these G-proteins may act as local regulators of specific apical transport functions.

Phosphoinositide metabolism in intestinal brush borders: stimulation of IP3 formation by guanine nucleotides and Ca2+

The potential role of polyphosphoinositide (PPI) metabolism as a signal-transduction mechanism in apical membranes of polarized epithelial cells was evaluated by examining the formation and breakdown of PPI in rat intestinal brush-border membranes (BBM) prelabeled by intraluminal injection of [3H]inositol in vivo or by [gamma-32P]ATP in vitro. Freshly isolated BBM prelabeled with [3H]inositol contained higher amounts of [3H]phosphatidylinositol 4,5-diphosphate compared with a basolateral membrane (BLM) preparation (approximately 14 and 6.8% of total [3H]PPIs, respectively) and were enriched in inositol lipid kinases, diacylglycerol (DAG) kinase, and phosphomonoesterases degrading PPI, inositol bisphosphate, and inositol triphosphate (IP3). In the presence of nonhydrolysable GTP analogues and low Ca2+ (pCa 6-8) or at high Ca2+ alone (pCa 4) endogenous pools of PPI were rapidly depleted by an intrinsic PPI-specific phospholipase C apparently coupled to a GTP-binding protein (G protein). Surprisingly, despite the assignment of most G protein-coupled hormone receptors to the BLM, the capacity of isolated BBM to release [3H]IP3 in response to Ca2+ or GTP gamma S appeared comparable to that in a BLM preparation. Intestinal secretagogues acting through apical membrane receptors (adenosine, heat-stable Escherichia coli toxin), however, were unable to promote [3H]IP3 release in isolated BBM in the presence of GTP. PPI metabolism in BBM may be coupled to receptors for as yet unidentified secretagogues or may serve as an amplification mechanism for hormone-stimulated PPI breakdown in BLM. The local release of DAG and IP3 at the interior of the intestinal microvilli likely plays a role in the regulation of ion transport systems in microvillar membranes.

A sensitive technique for the determination of anion exchange activities in brush-border membrane vesicles. Evidence for two exchangers with different affinities for HCO3- and SITS in rat intestinal epithelium

A large percentage (up to 70%) of 36Cl- influx in brush-border membrane vesicles from rat small intestine under equilibrium exchange conditions was found to be mediated by SITS-inhibitable anion exchange. This Cl-/anion exchange could be measured 10-15-times more sensitive by determining the uptake of trace amounts of 125I- driven by a large Cl- gradient (in greater than out) generated by passing the vesicles through an anion-exchange column. Voltage clamping of the vesicle membrane with K+ and valinomycin did not effect the chloride driven 125I- uptake, showing that the 'overshooting' I- uptake was not mediated by an electrical diffusion potential, as might be generated by the Cl- gradient in the presence of a chloride channel. The Cl-/anion exchange was further characterized in brush-border membrane vesicles from both rat ileum and jejunum by studying the inhibitory action of various anions on the Cl- driven I- uptake. NO3-, Cl-, SCN- and formate at 2 mM could inhibit Cl-/I- exchange for more than 80%. The ileal brush-border membrane vesicles displayed a clear heterogeneity with respect to the inhibitory action of SO2-(4), SITS and HCO-3 on Cl-/I- exchange. Approximately 30% of the Cl-/I- exchange was insensitive to SO2-(4) and showed a relatively low sensitivity to SITS (IC50 = 1 mM) but could be inhibited for 80% by 2 mM HCO-3. Presumably this component represents Cl-/OH- or Cl-/HCO-3 exchange. The residual 70% showed a high sensitivity to SO2-(4) (IC50 = 0.5 mM) and SITS (IC50 = 2.5 microM) but was less sensitive to HCO-3. This part of the exchange activity showed inhibition characteristics very similar to the Cl-/I- exchange in the jejunal vesicles. The latter process was also inhibited for 80% by 2 mM oxalate. As discussed in this paper both exchangers may be involved in the electroneutral transport of NaCl across the apical membrane of the small intestinal villus cell.

Cyclic nucleotide-dependent protein kinase inhibition by H-8: effects on ion transport

We explored the potential role of cyclic nucleotide-dependent protein phosphorylation in regulating ion transport across flounder intestinal mucosa by studying the effects of N-[2(methylamino)-ethyl]-s-isoquinolinesulfonamide (H-8), a selective inhibitor of cyclic nucleotide-dependent protein kinase in vitro. Addition of H-8 reversed the inhibitory effects of 8-bromoguanosine 3',5'-cyclic-monophosphate (8-BrcGMP), 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP), atriopeptin III (AP III), and vasoactive intestinal peptide (VIP) on the short-circuit current (Isc) and transepithelial potential difference (PD). Flux measurements established that these changes in Isc and PD directly reflected changes in Na and Cl absorption by the intestine. H-8 was unable, however, to reverse the inhibitory effects on Isc and PD of the Ca ionophore ionomycin and of substance P at dosages exceeding those needed to reverse the effects of AP III, VIP, and the cyclic nucleotides. We conclude that 1) H-8 (100 microM or less) does not exert toxic effects, 2) exogenously added cyclic nucleotide analogues inhibit ion transport through activation of cyclic nucleotide-dependent kinases resulting in protein phosphorylation, 3) activation of these kinases is an essential intermediate step in the inhibitory action of AP III and VIP on ion transport, and 4) the Ca ionophore ionomycin and substance P appear to inhibit ion transport by a mechanism that is independent of cyclic nucleotide-dependent protein phosphorylation.

Modulation of salt permeabilities of intestinal brush-border membrane vesicles by micromolar levels of internal calcium

A possible modulation of ion permeabilities of rat intestinal brush-border membrane vesicles by Ca2+, a putative second messenger of salt secretion, was explored by three independent methods: (1) measurements of [3H]glucose accumulation driven by a Na+ gradient; (2) stopped-flow spectrophotometry of salt-induced osmotic swelling; (3) 86Rb+, 22Na+ and 36Cl- flux measurements. Cytoskeleton-deprived membrane vesicles were prepared from isolated brushborders by thiocyanate treatment. Intravescicular Ca2+ levels were varied by preincubating vesicles in Ca-EGTA buffers in the presence of the Ca2+-ionophore A23187. At Ca2+free greater than 10(-5) M, initial Na+-dependent glucose uptake in the presence of a 0.1 M NaSCN gradient (but not in its absence) was inhibited by about 50 per cent as compared to EGTA alone (ED50 approximately equal to 10(-6) M Ca2+). By contrast, initial rates of 22Na+ uptake and reswelling rates of vesicles exposed to a NaSCN gradient were increased at least 2-fold by 10(-5) M Ca2+free. Both observations are compatible with a Ca2+-induced increase of the Na+-permeability of the vesicle membrane. The modulation of ion transport was fully reversible and critically dependent on internal Ca2+, suggesting a localization of Ca2+-sensor sites at the inner surface of the microvillous membrane. As shown by radiotracer and osmotic swelling measurements, micromolar Ca2+ additionally increased the flux rate of K+, Rb+, Cl- and NO-3 but did not change the membrane permeability for small uncharged molecules, including glucose and mannitol. The effect of Ca2+ on ion permeabilities could be blocked by Ba2+ (10(-3) M) or Mg2+ (10(-2) M), but not by amiloride (10(-3) M), apamin (2 X 10(-7) M), trifluoperazine (10(-4) M) or quinine (5 X 10(-4) M). At present it is unclear whether Ca2+ activates a nonselective cation and anion channel or multiple highly selective channels in the vesicle membrane.