Effect of thioridazine on gap junction intercellular communication in connexin 43-expressing cells

Propagation of electrical activity between myocytes in the heart requires gap junction channels, which contribute to coordinated conduction of the heartbeat. Some antipsychotic drugs, such as thioridazine and its active metabolite, mesoridazine, have known cardiac conduction side-effects, which have resulted in fatal or nearly fatal clinical consequences in patients. The physiological mechanisms responsible for these cardiac side-effects are unknown. We tested the effect of thioridazine and mesoridazine on gap junction-mediated intercellular communication between cells that express the major cardiac gap junction subtype connexin 43. Micromolar concentrations of thioridazine and mesoridazine inhibited gap junction-mediated intercellular communication between WB-F344 epithelial cells in a dose-dependent manner, as measured by fluorescent dye transfer. Kinetic analyses demonstrated that inhibition by 10 micromol/L thioridazine occurred within 5 min, achieved its maximal effect within 1 h, and was maintained for at least 24 h. Inhibition was reversible within 1 h upon removal of the drug. Western blot analysis of connexin 43 in a membrane-enriched fraction of WB-F344 cells treated with thioridazine revealed decreased amounts of unphosphorylated connexin 43, and appearance of a phosphorylated connexin 43 band that co-migrated with a "hyperphosphorylated" connexin 43 band present in TPA-inhibited cells. When tested for its effects on cardiomyocytes isolated from neonatal rats, thioridazine decreased fluorescent dye transfer between colonies of beating myocytes. Microinjection of individual cells with fluorescent dye also showed inhibition of dye transfer in thioridazine-treated cells compared to vehicle-treated cells. In addition, thioridazine, like TPA, inhibited rhythmic beating of myocytes within 15 min of application. In light of the fact that the thioridazine and mesoridazine concentrations used in these experiments are in the range of those used clinically in patients, our results suggest that inhibition of gap junction intercellular communication may be one factor contributing to the cardiac side-effects observed in some patients taking these medications.

Prevention of organochlorine-induced inhibition of gap junctional communication by chaetoglobosin K in astrocytes

Innumerable toxic substances present in the environment inhibit gap junctions, intercellular membrane channels that play fundamental roles in coordinated function of cells and tissues. Included are persistent organochlorine compounds, which pose health risks to humans and animals owing to their widespread use, bioaccumulation, and ability to inhibit gap junction channel-mediated intercellular communication in liver, lung, skin, heart, and brain cells. In this study, the organochlorine xenobiotics dieldrin and endosulfan, at micromolar concentrations, were found to inhibit gap junction-mediated intercellular communication and induce hypophosphorylation of connexin 43 in cultured rat astrocytes, the predominant cell type in the brain coupled through gap junctions. This inhibition of gap junctional communication was substantially reduced by preincubation with chaetoglobosin K (ChK), a bioactive natural produce previously shown to have ras tumor suppressor activity. Chaetoglobosin K also prevented dieldrin and endosulfan-induced hypophosphorylation of connexin 43 and prevented dieldrin-induced connexin 43 plaque dissolution in both astrocytes and cultured liver epithelial cells. The results suggest that stabilization of the native, phosphorylated form of connexin 43 by ChK may contribute to its ability to prevent organochlorine-induced inhibition of gap junction-mediated communication and dissolution of gap junction plaques within the plasma membrane.

Stimulation of cell proliferation and inhibition of gap junctional intercellular communication by linoleic acid

The effect of linoleic acid (LA) on gap-junction permeability, connexin 43 mRNA level, protein level, and phosphorylation, and the numbers of gap-junctional membrane plaques were studied in the rat liver epithelial cell line WB-F344 to determine whether changes in these parameters correlated with the enhanced cell growth and the inhibition of gap-junction function. When cultured in a medium with low serum (1%), these cells exhibited a slower growth rate than in the high serum medium (7%). Addition of linoleic acid (0.01-3 mg/ml) to the low serum medium increased the growth rate and inhibited gap junctional intercellular communication (GJIC) in a dose-dependent manner. In a comparison of short-term and long-term treatments with LA, GJIC in short-term treated (1 h) WB cells was inhibited at 3 mg/ml LA but readily recovered by washing and removing LA from cells, whereas GJIC in long-term treated (6 days) WB cells did not recover by washing and removing LA from WB cells. Western blot analysis of connexin 43 showed that a short-term incubation with linoleic acid increased the relative amount of unphosphorylated connexin 43 protein, but a long-term incubation with linoleic acid decreased the amount of unphosphorylated connexin 43 protein and increased the relative amount of hyperphosphorylated connexin 43 protein. Connexin 43 and p53 mRNA levels decreased in a time- and dose-dependent manner in linoleic acid-treated cells. These results suggest that growth stimulation and gap junctional intercellular communication inhibition of rat liver epithelial cells by linoleic acid may be mediated in part through modulation of p53 expression and function.

Localization and function of the connexin 43 gap-junction protein in normal and various oncogene-expressing rat liver epithelial cells

Clones of rat liver epithelial cells genotypically altered by mutation or by a variety of oncogenes were analyzed by microinjection-dye transfer, immunofluorescence confocal microscopy, and western blotting to determine at what level and to what degree these transformations disrupted gap-junctional intercellular communication (GJIC) mediated by connexin 43 (Cx43). Compared with normal rat liver epithelial cells, cells neoplastically transformed by src, neu, ras, and myc/ras all displayed reduced degrees of GJIC, reduced levels of membrane-associated Cx43 plaques, and hypophosphorylation of Cx43. Confocal analysis further demonstrated that the Cx43 protein was localized, at least in part, to the nucleus rather than to the plasma membrane in the src- and neu-transformed cells, but not in the ras- and myc/ras-transformed cells. Nuclei isolated from WB-neu cells showed substantially higher levels of Cx43 on western blotting than did nuclei from WB-neo control cells, supporting the idea that the nuclear-localized immunopositive material detected by confocal microscopy was Cx43 protein. In a GJIC-deficient mutant rat liver epithelial cell line containing normal numbers of plasma membrane-localized Cx43 plaques that appeared to be reduced in size, the Cx43 protein was also found to be hypophosphorylated. Cells overexpressing myc, on the other hand, displayed a normal degree of GJIC, increased levels of plasma membrane-localized Cx43 plaques, and hyperphosphorylation of the Cx43 protein. Cells expressing raf, previously shown to be GJIC competent, showed Cx43 immunostaining patterns similar to those in normal cells, whereas a cell line established from a tumor induced by injection of these raf-expressing cells into a mouse showed a marked reduction in GJIC and plasma membrane-associated Cx43 immunostaining. These data suggest that altered localization of the gap-junction protein Cx43, mediated in part by changes in the phosphorylation of this protein, contributes to the disruption of GJIC in neoplastically transformed rat liver epithelial cells.

Upregulation of gap junctional intercellular communication in immortalized gonadotropin-releasing hormone neurons by stimulation of the cyclic AMP pathway

Increased gap junctional intercellular communication induced by agents that stimulate the adenylyl cyclase/cAMP pathway was observed in the GnRH-secreting neuronal cell line, GT1-7, and possible underlying mechanisms were examined. A 24-hour treatment of GT1-7 neurons with 100 microM dibutyryl cAMP + 100 microM IBMX or with 2 microM forskolin increased by greater than 2-fold the percentage of cells that were dye coupled, using the noninvasive dye coupling assay, fluorescent recovery after photobleaching (FRAP). Longer treatment times (48 h) and higher concentrations of dibutyryl cAMP (500 microM) did not further increase the percentage of dye-coupled cells, while there was no increase in dye coupling observed between untreated cells and cells treated for 2 h or less. The increase in dye coupling induced by dibutyryl cAMP/IBMX was inhibited by octanol or dieldrin, agents known to block gap junction-mediated intercellular coupling in other cell types. Western blot analysis of total protein or membrane protein-enriched extracts revealed no apparent difference in the cellular levels of connexin 26, a connexin subtype previously shown to be expressed by GT1-7 cells, between untreated cells and cells treated for 24 h with dibutyryl cAMP/IBMX or forskolin. In addition, expression of connexin 32 or 43 protein before or after treatment was not detected. On the other hand, a dramatic increase in both the number of neurites and neurites that immunostained positive for connexin 26 was observed in dibutyryl cAMP/IBMX-treated cells. We hypothesize that the observed increase in dye coupling between GT1-7 neurons following stimulation of the adenylyl cyclase/cAMP pathway results from an augmentation of cell-cell contacts due to an increased number of neurites containing gap junctional plaques, possibly through an effect on cellular differentiation.

Localization and function of the connexin 43 gap-junction protein in normal and various oncogene-expressing rat liver epithelial cells

Clones of rat liver epithelial cells genotypically altered by mutation or by a variety of oncogenes were analyzed by microinjection-dye transfer, immunofluorescence confocal microscopy, and western blotting to determine at what level and to what degree these transformations disrupted gap-junctional intercellular communication (GJIC) mediated by connexin 43 (Cx43). Compared with normal rat liver epithelial cells, cells neoplastically transformed by src, neu, ras, and myc/ras all displayed reduced degrees of GJIC, reduced levels of membrane-associated Cx43 plaques, and hypophosphorylation of Cx43. Confocal analysis further demonstrated that the Cx43 protein was localized, at least in part, to the nucleus rather than to the plasma membrane in the src- and neu-transformed cells, but not in the ras- and myc/ras-transformed cells. Nuclei isolated from WB-neu cells showed substantially higher levels of Cx43 on western blotting than did nuclei from WB-neo control cells, supporting the idea that the nuclear-localized immunopositive material detected by confocal microscopy was Cx43 protein. In a GJIC-deficient mutant rat liver epithelial cell line containing normal numbers of plasma membrane-localized Cx43 plaques that appeared to be reduced in size, the Cx43 protein was also found to be hypophosphorylated. Cells overexpressing myc, on the other hand, displayed a normal degree of GJIC, increased levels of plasma membrane-localized Cx43 plaques, and hyperphosphorylation of the Cx43 protein. Cells expressing raf, previously shown to be GJIC competent, showed Cx43 immunostaining patterns similar to those in normal cells, whereas a cell line established from a tumor induced by injection of these raf-expressing cells into a mouse showed a marked reduction in GJIC and plasma membrane-associated Cx43 immunostaining. These data suggest that altered localization of the gap-junction protein Cx43, mediated in part by changes in the phosphorylation of this protein, contributes to the disruption of GJIC in neoplastically transformed rat liver epithelial cells.

Re-expression of the intermediate filament nestin in reactive astrocytes

The intermediate filament nestin is highly expressed in multipotential stem cells of the developing central nervous system (CNS). During neuro- and gliogenesis, nestin is replaced by cell type-specific intermediate filaments, e.g. neurofilaments and glial fibrillary acidic protein (GFAP). In this study, we demonstrate that nestin expression is re-induced in reactive astrocytes in the lesioned adult brain. Following ischaemic and mechanical lesioning, a strong and sustained expression of nestin was noted in GFAP-positive cells surrounding the lesion site. Lesion experiments in transgenic mice carrying the lacZ gene under control of regulatory sequences from the nestin gene suggested that the upregulation of nestin in reactive astrocytes is mediated via the same sequences that control nestin expression during CNS development. These observations and recent data on the co-expression of glial and neuronal marker antigens in reactive astrocytes point to a close relationship between proliferating astrocytes and neuroepithelial precursor cells.

Inhibition of gap junctional intercellular communication and malignant transformation of rat liver epithelial cells by neu oncogene

A retrovirus containing a neu oncogene was introduced into a Fischer F344 rat liver epithelial cell line (WB-F344) to study the effect of the expression of neu oncoprotein on gap junctional intercellular communication (GJIC), the ability to form colonies in soft agar and the ability to form tumors in rat liver by these cells. After viral infection, five different neu-transduced epithelial clones were randomly selected for further analysis. Southern blot analysis of HindIII-digested genomic DNA hybridized with a neu-specific probe indicated that the neu oncogene carried by the retrovirus was integrated into different chromosomal locations in the five different neu-transduced WB cell lines. Using the fluorescence recovery after photobleaching (FRAP) assay, we found that GJIC was significantly reduced in neu-transduced WB clones, compared with control virus-infected and parental WB cells. Western blot analysis of connexin 43 in the neu-transduced cell lines showed altered phosphorylation patterns compared with the normal WB-rat liver cell line. Confocal image analysis of the neu-transduced cells showed that the connexin 43 protein, as detected by fluorescent immunostaining, was localized in the cell nucleus. The neu-transduced WB cell lines also acquired the ability to grow in soft agar. Furthermore, cells from three of the five neu-transduced cell lines, when injected into the liver of Fischer F344 rats through the portal vein, were highly tumorigenic (multiple focal hepatic tumors developed within 2 weeks). Cells derived from the tumor were shown to be G-418 resistant, demonstrating that the tumor was derived from the injected WB-neu cells. The results of this study demonstrate that the expression of the neu oncogene is able to block GJIC and to induce tumorigenicity in the rat liver WB-F344 cell line.

Microtubule-associated protein 2 as an early indicator of ischemia-induced neurodegeneration in the gerbil forebrain

Microtubule-associated protein 2 (MAP-2) was studied in the gerbil hippocampus and striatum after transient ischemia. Western immunoblot analysis shows that there is a significant decrease of MAP-2 in the dorsolateral sector of the striatum and a slight decrease of MAP-2 in the CA1 region of the hippocampus 6-12 h after ischemia in the gerbil forebrain. The immunohistochemical staining pattern of MAP-2 in these two regions also shows a loss of immunostaining of MAP-2. In particular, a beaded MAP-2 immunostaining pattern at the apical dendritic region of the CA1 neurons of the hippocampus was found within 12 h after ischemia compared with the smooth dendritic immunostaining of MAP-2 in normal CA1 neurons. In vitro assays of MAP-2 degradation suggest that dendritic loss of immunoreactivity after ischemia seen on western blots may be due to calpain I degradation of MAP-2. Loss of MAP-2 in both the striatum and hippocampus was found to occur earlier than spectrin degradation by western blot analysis. These results suggest that loss of MAP-2 may participate in the initial phase of neuronal dysfunction and that dendritic breakdown may be a first sign of neurodegeneration.

Changes in gap-junction permeability, phosphorylation, and number mediated by phorbol ester and non-phorbol-ester tumor promoters in rat liver epithelial cells

The effects of three tumor promoters on gap-junction permeability; connexin 43 and 26 mRNA levels, protein levels, and phosphorylation; and the numbers of gap-junctional membrane plaques were studied in the rat liver epithelial cell line WB-F344 to determine whether changes in these parameters correlated with the inhibition of gap-junction function. 12-O-tetradecanoylphorbol-13-acetate (TPA; 10 ng/mL), dieldrin (10 micrograms/mL), and heptachlor epoxide (10 micrograms/mL) inhibited gap-junctional intercellular communication (GJIC) assayed by fluorescent dye transfer by 80-90% after a 5-min exposure and by more than 90% within 1 h. Decreases in steady-state connexin 43 mRNA levels were detected by northern blot analysis within 1 h and paralleled changes in steady-state beta-actin mRNA, but these changes did not occur rapidly enough to account for the rapid loss of gap-junction function. A substantial loss in the number of connexin 43 immunostained gap-junctional membrane plaques was detected after a 15-min exposure to all three promoters, but little change had occurred at 5 min. Western blot analyses using connexin 43-specific antibodies showed changes in the degree of connexin 43 phosphorylation for all three tumor promoters. TPA induced the appearance of a fourth connexin 43-immunoreactive band (P3) and a concomitant decrease in the relative intensity of the unphosphorylated (P0) band within 5 min of treatment. P3, in addition to bands P1 and P2, disappeared after treatment with alkaline phosphatase. In contrast, dieldrin and heptachlor expoxide induced loss of P2 with a concomitant increase in the relative staining intensity of P0 within 1 h of exposure, but no changes were seen after 5 min. Connexin 43 phosphorylation levels recovered in parallel with the recovery of GJIC for all three tumor promoters. Connexin 26 mRNA levels showed little change after a 1-h exposure to three promoters, but reductions in connexin 26 immunofluorescent staining were observed. These results suggest that (i) TPA-induced hyperphosphorylation of connexin 43 occurred fast enough to account for inhibition of GJIC, (ii) dieldrin and heptachlor expoxide modulated connexin phosphorylation in a manner different from TPA by promoting hypophosphorylation of connexin 43, (iii) redistribution of plasma membrane gap-junctional plaques after treatment with phorbol ester and non-phorbol-ester tumor promoters occurred subsequent to changes in gap-junction permeability, and (iv) changes in connexin mRNA levels could not account for the losses in fluorescent dye coupling induced by these promoters.

Immunohistochemical demonstration of neuron-specific enolase and microtubule-associated protein 2 in reactive astrocytes after injury in the adult forebrain

Transformation of normal resting astrocytes to reactive astrocytes in the adult brain after injury has been well documented. Using double immunofluorescent labeling methods, we report that astrocytes in both the ischemically damaged and the retrogradely/anterogradely degenerating forebrain nuclei express not only the glial cell markers glial fibrillary acidic protein and vimentin, but also the neuronal markers neuron-specific enolase and microtubule-associated protein 2. Since these neuronal markers are expressed in glial precursor cells, these results suggest that one of the characteristic responses of astrocytes in the adult brain after injury may be re-expression of fetal trait(s) of early differentiating glial cells/neurons.

Immortalized hypothalamic luteinizing hormone-releasing hormone neurons express a connexin 26-like protein and display functional gap junction coupling assayed by fluorescence recovery after photobleaching

Expression of gap junction proteins was studied in the LHRH neuronal cell line, GT1-7, as a first step in defining the signalling mechanisms responsible for the pulsatile secretion of LHRH. GT1-7 cells were found to express a connexin 26-like protein that comigrated with mouse liver connexin 26 and that reacted with connexin 26-specific antibodies on Western blots. Immunofluorescent staining revealed punctate staining in a fraction of the cells, often present at points of apparent contact with neighboring cell bodies or processes. Fluorescence recovery after photobleaching analysis of 5,6-carboxyfluorescein loaded GT1-7 cells showed dye coupling among 20-30% of cells that made contact with other cells, suggesting the presence of functional gap junctions in this cell line.

Identification of a G-protein in depolarizing rod bipolar cells

Synaptic transmission from photoreceptors to depolarizing bipolar cells is mediated by the APB glutamate receptor. This receptor apparently is coupled to a G-protein which activates cGMP-phosphodiesterase to modulate cGMP levels and thus a cGMP-gated cation channel. We attempted to localize this system immunocytochemically using antibodies to various components of the rod phototransduction cascade, including Gt (transducin), phosphodiesterase, the cGMP-gated channel, and arrestin. All of these antibodies reacted strongly with rods, but none reacted with bipolar cells. Antibodies to a different G-protein, G(o), reacted strongly with rod bipolar cells of three mammalian species (which are depolarizing and APB-sensitive). Also stained were subpopulations of cone bipolar cells but not the major depolarizing type in cat (b1). G(o) antibody also stained certain salamander bipolar cells. Thus, across a wide range of species, G(o) is present in retinal bipolar cells, and at least some of these are depolarizing and APB-sensitive.

Neuroprotective activity of dimer of 16,16'-dimethyl-15-dehydroprostaglandin B1 (di-Calciphor) in cerebral ischemia

Post-ischemic treatment of di-Calciphor (16,16'-dimethyl-15- dehydroprostaglandin B1) significantly improves animal survival and prevents ischemia-induced neurodegeneration of vulnerable forebrain regions assessed with histochemical and biochemical techniques in gerbils. Neuronal degeneration seen by Cresyl violet staining and silver impregnation in the CA1 sector of the hippocampus and the dorso-lateral sector of the striatum was significantly reduced in animals treated with di-Calciphor. In addition, the early onset of selective degradation of calpain I substrates spectrin and microtubule-associated protein (MAP2) in these same vulnerable regions was prevented. The lack of adverse side effects may facilitate the potential therapeutic use of this drug in preventing neuronal damage caused by stroke.

Expression of gamma-aminobutyric acid immunoreactivity in reactive astrocytes after ischemia-induced injury in the adult forebrain

Transient ischemia induces an increase in glial fibrillary acidic protein (GFAP) immunoreactivity which can be detected in specific forebrain regions of the adult gerbil as early as day 2, becomes prominent by day 4-7 and persists for at least 3 months. These forebrain areas include layers 2/3 of the somatosensory and auditory cortices, the CA1 and CA4 sectors of the hippocampus, the dorsolateral region of the striatum, and the dorsolateral subregion of the medial septal nucleus. In addition, astrocytes in the ischemically lesioned areas stain with gamma aminobutyric acid (GABA) antiserum. These GABA-immunoreactive astrocytes are not found in non-damaged areas. The time-course of expression of GABA immunoreactivity is similar to that of GFAP immunoreactivity. Using a double immunofluorescent staining method, reactive astrocytes which express GABA immunoreactivity were also found to immunostain with either GFAP or vimentin. On the other hand, astrocytes were not found to be immunoreactive with antibodies to glutamic acid decarboxylase or glutamate. Our present finding demonstrates, in an in vivo model, an aberrant expression of GABA immunoreactivity by astrocytes which is not observed in non-ischemic adult animals.

Tubulin in bovine retinal rod outer segments

Bovine rod outer segment (ROS) preparations contain a major 58 kDa protein doublet that was identified by immunoblot as tubulin. Quantification by gel densitometry showed that the total amount of tubulin was 5- to 10-fold higher than that attributable to the rod axoneme, suggesting additional role(s) for tubulin in photoreceptor cells. Approximately 20% of this nonaxonemal tubulin (15% of total tubulin) is tightly associated with outer segment membranes. This fraction remains membrane-associated after extensive low- or high-salt washing, requiring detergents or protein denaturants for release from ROS membranes. Unlike ROS soluble tubulin it associates tightly with liposomes upon detergent solubilization and reconstitution. The ROS membrane-associated tubulin is highly enriched in isolated ROS plasma membrane fractions compared to the total outer segment membrane pool and can be localized to the plasma membrane but not to disks by immunofluorescent staining, suggesting a possible role in the structure or electrophysiology of the rod outer segment plasma membrane.

Tissue-dependent association of muscarinic acetylcholine receptors with guanine nucleotide-binding regulatory proteins

The muscarinic acetylcholine receptors in heart and cerebellum form a stable association with guanine nucleotide-binding regulatory proteins (G proteins) in the presence of receptor agonists. This has been confirmed by purification of the muscarinic receptor-G protein complexes using an immunoprecipitation protocol. The isolated complexes were subjected to Western blotting to identify the G protein subunits present in the complexes. At saturating concentrations of carbachol, the muscarinic receptors in atrial membranes co-purified exclusively with Go, whereas in cerebellar and ventricular membranes an association with both Gi and Go was demonstrated. Further characterization of the G protein subunits allowed identification of the species of Gi alpha subunits present in the complexes of muscarinic receptor and G protein; in ventricle Gi alpha 2 was the only subtype present, whereas in cerebellum both Gi alpha 1 and Gi alpha 2 were present. These results demonstrate that a single muscarinic receptor subtype, depending on the tissue studied, is capable of interacting with more than one G protein subtype. The concentrations of agonist required to promote receptor-G protein association in atrial and ventricular membranes correlated with the high affinity component of receptor occupancy by agonist, as measured in equilibrium binding assays. Furthermore, incubation of cardiac membranes with saturating concentrations of pilocarpine or McN A343 resulted in reduced amounts of receptor-G protein complexes, compared with carbachol. Overall, our results suggest that the specificity of cellular effects of muscarinic agonists may relate, in part, to the selective interaction of receptor with G proteins.

Atropine dissociates complexes of muscarinic acetylcholine receptor and guanine nucleotide-binding protein in heart membranes

Complexes of muscarinic acetylcholine receptor and guanine nucleotide-binding protein (G protein) are formed in the presence of the agonist carbachol. The complexes remain stable after removal of agonist, and survive subsequent solubilization from cardiac membranes and purification. Dissociation of the receptor from the G protein occurs when the antagonist atropine is added following removal of agonist. This is the first direct demonstration of destabilization of receptor-G protein complexes by the binding of an antagonist.

Pharmacological and biochemical characterization of complexes of muscarinic acetylcholine receptor and guanine nucleotide-binding protein

Complexes of agonist-bound muscarinic acetylcholine receptor (mAChR) and guanine nucleotide-binding protein (G protein) were solubilized and isolated from rat heart. Heart membranes were incubated with mAChR agonists or antagonists, solubilized using digitonin and cholate, and subjected to chromatography over wheat germ agglutinin-Affi-Gel. Eluted fractions were precipitated using a cardiac-selective anti-mAChR antibody (Luetje, C. W., Brumwell, C., Norman, M. G., Peterson, G. L., Schimerlik, M. I., and Nathanson, N. M. (1987) Biochemistry 26, 6892-6898). Using samples obtained from membranes initially incubated with carbachol (10 nM, 100 nM, or 1 mM), G alpha immunoreactivity was detected on Western blots probed using antibodies with specificity for G alpha subunits. The G alpha immunoreactivity was not detected when atropine alone (10 nM or 1 microM) or when excess atropine (1 microM) plus carbachol (100 nM) was used during the membrane preincubation. G beta immunoreactivity, when detectable on Western blots, was present in substoichiometric amounts relative to that of G alpha. The G alpha immunoreactivity was not present if GTP was included during incubation of membranes with agonist and following membrane solubilization. Further results indicate that although agonist binding to receptors is rapidly reversed by GTP or GDP (t1/2 less than 10 min), the mAChR-G protein complex is reversed more slowly or not at all. It was also shown that at high agonist concentrations, the cardiac mAChR interacts with both Go and Gi-like proteins. Together, these results demonstrate the utility of an immunoaffinity approach to the purification and biochemical study of receptor-G protein interactions.

Incorporation of serine into Paramecium ethanolamine phospholipid and phosphonolipid head groups

Ethanolamine phospholipid head groups in Paramecium were synthesized directly from ethanolamine. As in other cell types, radioactivity from ethanolamine failed to incorporate significantly into head groups of ethanolamine phosphonolipids, indicating that the phosphonolipids are not derived from their phospholipid analogues. Unlike other systems previously examined, radioactivity from serine is incorporated into both ethanolamine phospholipid and phosphonolipid head groups of glycerolipids and sphingolipids in this ciliate. These observations suggest that synthesis of ethanolamine phosphonolipids involves synthesis de novo of free phosphonoserine, which is then incorporated into lipids, and then lipid-bound phosphonoserine intermediates (glycerolipids or sphingolipids) undergo decarboxylation, forming lipidbound phosphonoethanolamine compounds.

Characterizations of six ethanolamine sphingophospholipids from Paramecium cells and cilia

Six ethanolamine sphingophospholipids from axenically cultured Paramecium tetraurelia were isolated from cells and purified ciliary fractions, and were characterized. The sphingolipids comprised 10.7% of whole cell and 32.5% of ciliary ethanolamine phospholipid fractions purified by ion exchange column chromatography. The individual sphingolipids were characterized by thin-layer chromatographic analyses of parent compounds and the polar head group and long chain base moieties, gas-liquid chromatography, and mass spectrometry of amide-linked fatty acids and long chain bases, and nuclear magnetic resonance of the compounds. Colorimetric assays of differential hydrolysis products and 31P nuclear magnetic resonance were used to determine the nature of phosphorus linkages. The sphingolipids were identified as N-acyl-trans-4-hydroxy-sphinganine-1- phosphonoethanolamine , N-acyl-trans-4-hydroxy-sphinganine-1-phosphoethanolamine, N-acyl-sphingenine-1- phosphonoethanolamine , N-acyl-sphingenine-1-phosphoethanolamine, N-acyl-sphinganine-1- phosphonoethanolamine and N-acyl-sphinganine-1-phosphoethanolamine. All six had greater than 90% saturated fatty acids. These sphingolipids were quantified by radioisotope methods and plate densitometry of thin-layer chromatograms. Changes in the relative amounts of each species were detected in cells grown in different culture media as well as in cells at different culture ages.