Roles of Ca2+ and Na+ in the inward current and action potentials of guinea pig ureteral myocytes

Physiological roles of Ca2+ vs. Na+ in membrane currents and action potentials of ureteral myocytes were investigated on freshly dissociated guinea pig ureteral myocytes with the patch-clamp method. The myocytes are spindle shaped, with cell volume of 2,473 microm3, surface area of 2,014 microm2, capacitance of 48.2 pF, resting potential of -47.9 mV, and membrane conductance of 840 pS. The membrane current consists of a slow inward Ca2+ current (ICa) conducted by L-type Ca2+ channels and an actively fluctuating Ca2+-activated K+ current [IK(Ca)] conducted by Ca2+-activated maxi-K+ channels. ICa dominates the membrane current by being long lasting and more active at less depolarized potentials than IK(Ca) and by regulating IK(Ca). Ca2+-free media, Co2+, and nifedipine reduce or block ICa, whereas high extracellular Ca2+ concentration and BAY K 8644 enhance it. Action potential amplitudes and plateaus are regulated correspondingly. Related changes are also seen in IK(Ca) In contrast, no fast inward current attributable to Na+ was found. Replacing extracellular Na+ with tris(hydroxymethyl)aminomethane had no apparent effects on the inward or outward current or on the action potentials.

Properties of inward calcium current in guinea pig ureteral myocytes

Ureteral myocytes of guinea pigs have L-type Ca2+ channels (I(Ca)). In 3 mM Ca2+, maximum I(Ca) was 3.38 microA/cm2. Voltage at which conductance is 50% of maximum (V0.5) of I(Ca) was -1.0 mV in 3 mM Ca2+ and +22 mV in 30 mM Ca2+, with slope factors of 8 mV. V0.5 of steady-state inactivation of I(Ca) was -16.2 and +1.1 mV in 3 and 30 mM Ca2+, respectively, with similar slope factors of about -6 mV. A window current reaching 20-25% of the maximum I(Ca) was active between -20 and 0 mV. I(Ca) inactivated very slowly, with time constants of 217.6 and 2,455.9 ms with no voltage dependency. When Ba2+ was used as the charge carrier, the amplitude and inactivation kinetics of the Ba2+ current were similar to those for I(Ca). These results indicate that the ureteral myocyte has little Ca2+-mediated Ca2+ channel inactivation, a feature significantly associated with the slow I(Ca) inactivation. The slow inactivation and the window current are essential for the sustained membrane depolarization during the plateau of ureteral action potentials.

Scheduling nursing personnel on a microcomputer

Suggests that with the shortage of nursing personnel, hospital administrators have to pay more attention to the needs of nurses to retain and recruit them. Also asserts that improving nurses' schedules is one of the most economic ways for the hospital administration to create a better working environment for nurses. Develops an algorithm for scheduling nursing personnel. Contrary to the current hospital approach, which schedules nurses on a person-by-person basis, the proposed algorithm constructs schedules on a day-by-day basis. The algorithm has inherent flexibility in handling a variety of possible constraints and goals, similar to other non-cyclical approaches. But, unlike most other non-cyclical approaches, it can also generate a quality schedule in a short time on a microcomputer. The algorithm was coded in C language and run on a microcomputer. The developed software is currently implemented at a leading hospital in Taiwan. The response to the initial implementation is quite promising.

Plasma-induced grafted polymerization of acrylic acid and subsequent grafting of collagen onto polymer film as biomaterials

Polyacrylic acid (pAA) was introduced onto Ar-plasma treatment silicone rubber (SR) membrane surfaces by plasma-induced grafted polymerization. Collagen (type III) was also linked with the carboxylic group of pAA grafted onto the SR surface via a carbodiimine agent to obtain a secondary structure of SR. The SR surface properties were characterized by ATR-FTIR, ESCA, contact angle, and SEM. The biocompatibility of the SR surface was evaluated by a culture of cornea epithelial (CE) cells. Subsequently, 75-450 micrograms cm-2 of pAA were obtained on the SR surfaces under different reactive conditions; 3-12 micrograms cm-2 of collagen were linked on modified surfaces of SR. Moreover, ATR-FTIR and ESCA were utilized to confirm the proceedings of these reactions. The hydrophility of the modified SR was measured by a contact angle meter. The values of contact angle for SR grafted with pAA were approximately 45-50 degrees; a 50-55 degrees contact angle on pAA-g-SR to be further linked with collagen was subsequently obtained. Moreover, the influence of surface properties toward migration, growth and attachment of CE cells on the modified surfaces was also examined. Here, untreated SR was used as a control. Experimental results indicated that the number of CE cells attached onto the controlled SR was negligible. The attachment of cells onto pAA-grafted surfaces was clearly observed and peusopoda occurred; however, cell growth was depressed. This depression may have been caused by the acid environment of the pAA-grafted membrane. Nevertheless, both cell attachment and growth onto collagen-linked surfaces were significant. In addition, the morphology of the cells attached onto this surface was considered normal for primary cells. Collagen introduced on the SR surface was not denatured, i.e the natural properties of collagen were maintained. The results obtained in this study will hopefully lead to the successful development of modified SR for clinical applications.

Localization and interaction of the cis-acting elements for abscisic acid, VIVIPAROUS1, and light activation of the C1 gene of maize

The C1 regulatory gene of the maize anthocyanin pathway is regulated by a combination of developmental and environmental signals that include the Viviparous1 (Vp1) gene, abscisic acid (ABA), and light. Using protoplast electroporation and particle bombardment assays, we have defined c/s-acting elements that are necessary and sufficient for the activation of C1 by ABA, VP1, and light, respectively. The sequence from positions -142 to -132 (CGTCCATGCAT) is essential for VP1 activation, whereas a larger overlapping element from -147 to -132 (CGTGTCGTCCATGCAT) is necessary and sufficient for activation by ABA. A separate light (blue and red)-responsive c/s element is located between positions -116 and -59. Light interacts synergistically with the ABA and VP1 responses in transient expression assays, suggesting that combinatorial interaction between modules plays a role in integrating these signals in the developing seed.

Reduced growth capacity of hepatocytes from c-myc and c-myc/TGF-alpha transgenic mice in primary culture

We have previously shown that coexpression of c-myc and TGF-alpha in the liver results in accelerated replicative senescence and promotes tumor development in young adult transgenic mice. Here we describe the characteristics of hepatocyte proliferation in primary cultures established from 10-week-old control, c-myc and c-myc/TGF-alpha transgenic mice. A variety of cellular and functional changes occurred in the transgenic livers at this age including enhanced polypoidization and impairment of hepatic functions. Control mouse hepatocytes demonstrated a high level of DNA synthesis in serum-free medium with a maximum at day three in culture at which time 70% of the cells were in S phase. In contrast, DNA synthesis peaked one day later and was reduced by 50% in the cultured c-myc and c-myc/TGF-alpha hepatocytes. Also, higher frequency of apoptosis was observed in the transgenic hepatocytes. However, in hepatocytes isolated from c-myc/TGF-alpha mice, which show early appearance of preneoplastic lesions in vivo, the DNA synthesis continued for 6 days in culture in contrast to a sharp decrease in the labeling index of control and c-myc hepatocytes after 3-4 days in culture. The results suggest that proliferative features of the transgenic hepatocytes in vitro reflect the general properties of these cells in vivo and thus may provide a model for studies on senescence and transformation of hepatocytes.

11-Oxo-tetrodotoxin and a specifically labelled 3H-tetrodotoxin

Tetrodotoxin was oxidized to a hydrated aldehyde, 11-oxo-tetrodotoxin, which shares the specificity of tetrodotoxin for the Na+ channel of the isolated voltage-clamped frog skeletal muscle fiber, but is four to five times more potent. It binds to the solubilized Na+ channel of eel electroplax with a similarly higher potency, because of an equilibrium dissociation constant about 0.25, and a dissociation rate constant 2.4 times slower than those for tetrodotoxin. 11-Oxo-tetrodotoxin can be reduced to regenerate a tetrodotoxin, which is chemically and biologically indistinguishable from the original tetrodotoxin. By reducing with tritiated sodium borohydride, a 3H marker can be inserted regiospecifically to yield 11-[3H]-tetrodotoxin. Because it has a defined specific activity of > 2.5 Ci/mmole, and a 3H marker which does not exchange with solvent proton, 11-[3H]-tetrodotoxin is an ideal tracer for tetrodotoxin. It may enable studies of problems which require higher signals and/or better stability of the marker than those obtainable from currently available tracer Na(+)-channel ligands.

Artificial cornea: surface modification of silicone rubber membrane by graft polymerization of pHEMA via glow discharge

A method for producing various surfaces of silicone rubber membrane (SR) was developed in this study by grafting various amounts of poly(2-hydroxy ethyl methacrylate) (pHEMA) onto SR by plasma-induced grafted polymerization (PIP) as a homobifunctional membrane. The elemental composition and different carbon bindings on the surface of SR were examined by electron spectroscopy for chemical analysis with the amount of O1s/C1s being approximately 0.7 at 1 min, 60 W, 200 mTorr of Ar-plasma treatment. The peroxide group introduced on SR was measured via 1,1-diphenyl-2-picrylhydrazyl (DPPH) and the amount of 6.85 x 10(-8) mol cm-2 reached optimum value at 1 min of Ar-plasma treatment. After Ar-plasma treated SR, the peroxide group (33D peak) was introduced on the surface of SR by negative spectra of secondary ion mass spectroscopy analysis, whereas ester groups (72D peak) were observed for pHEMA-grafted SR. For the in vitro test, the influence of various surfaces of SR on attachment and growth of rabbit corneal epithelial cells (CEC) was studied by cell culture assay. These results indicated that 56-150 micrograms cm-2 of pHEMA grafted onto SR were suitable values for attachment and growth of CEC. On the contrary, the large grafted amounts (500-1650 micrograms cm-2) of pHEMA on SR were insufficient for attachment and growth of CEC. For the in vivo test, the migration of CEC from host cornea to implant was investigated by slit lamp microscopy. The experimental results indicated that SRs grafted with pHEMA were completely covered with CEC 3 weeks after implantation of the membranes into the host cornea. These results provide a valuable reference for developing an artificial cornea.

Two types of normal human breast epithelial cells derived from reduction mammoplasty: phenotypic characterization and response to SV40 transfection

A culture method to grow two morphologically distinguishable normal human breast epithelial cell types derived from reduction mammoplasty has been developed. Type I cells were characterized by a more variable cell shape, smooth cell colony boundaries, the expression of epithelial membrane antigen (EMA) and keratin 18 and the non-expression of keratin 14 and alpha 6 integrin. In addition, the Type I cells were growth stimulated by fetal bovine serum (FBS) and were deficient in gap junctional intercellular communication (GJIC). In contrast, Type II cells were characterized by a uniform cell shape, expression of keratin 14 and alpha 6 integrin and the non-expression of EMA and keratin 18. In addition, Type II cells were growth inhibited by FBS and were proficient in GJIC. Type I cells can be induced by cholera toxin to change their morphology to a Type II cell morphology. Hence, Type I cells antigenically resemble luminal epithelial cells, while the Type II cells more closely resemble basal epithelial cells. Type I and Type II cells were transfected with SV40 DNA. Clones with extended lifespans were obtained from both Type I and Type II cells by SV40 transfection. Some (2/9) of the SV40-transfected Type I cell clones became immortal (> 100 cumulative population doubling level), whereas none (0/8) of the SV40-transfected Type II cell clones became immortal. The SV-40-transfected Type I and Type II cell-derived extended life clones and immortal cell lines phenotypically resembled their parental cells with respect to EMA, keratin 14 and keratin 18 expression and GJIC. Each (9/9) of the SV40 transfected Type I cell clones grew in soft agar; none (0/8) of the SV40-transfected Type II cell clones were capable of growing in soft agar. These results provide evidence that normal human breast epithelial cells, derived from reduction mammoplasty, can be separated into two morphologically and antigenically different cell types and that these two different cell types significantly differ in their response to an oncogenic (SV40) stimulus.

A fluorescent lipid analogue can be used to monitor secretory activity and for isolation of mammalian secretion mutants

The use of reporter proteins to study the regulation of secretion has often been complicated by posttranslational processing events that influence the secretion of certain proteins, but are not part of the cellular mechanisms that specifically regulate secretion. This has been a particular limitation for the isolation of mammalian secretion mutants, which has typically been a slow process. To provide a reporter of secretory activity independent of protein processing events, cells were labeled with the fluorescent lipid analogue C5-DMB-ceramide (ceramide coupled to the fluorophore boron dipyrromethene difluoride) and its secretion was followed by fluorescence microscopy and fluorescence-activated cell sorting. Brefeldin A, which severely inhibits secretion in Chinese hamster ovary cells, blocked secretion of C5-DMB-ceramide. At high temperature, export of C5-DMB-ceramide was inhibited in HRP-1 cells, which have a conditional defect in secretion. Using C5-DMB-ceramide as a reporter of secretory activity, several different pulse-chase protocols were designed that selected mutant Chinese hamster ovary cells that were resistant to the drug brefeldin A and others that were defective in the transport of glycoproteins to the cell surface. Mutant cells of either type were identified in a mutagenized population at a frequency of 10(-6). Thus, the fluorescent lipid C5-DMB-ceramide can be used as a specific marker of secretory activity, providing an efficient, general approach for isolating mammalian cells with defects in the secretory pathway.

Effects of lidocaine on rat myometrial sodium channels and implications for the management of preterm labor

OBJECTIVE

We examined the effects of lidocaine on ionic channels of single uterine myocytes and on the duration of pregnancy.

STUDY DESIGN

The electrophysiologic effects of lidocaine on the sodium, calcium, and potassium channels of freshly dissociated myocytes from rat uteri in late pregnancy were studied with the tight-seal patch-clamp method. Dose-response relations on the sodium and calcium currents and effects on the availability relation for sodium current were examined. Also studied were the effects of long-term administration of lidocaine on the duration of pregnancy.

RESULTS

Lidocaine blocks Na+ channels selectively, with an ED50 (concentration of lidocaine capable of blocking half of the Na+ current) of 0.11 mmol/L. It shifts the availability relations for sodium ion to more negative voltages. When administered from day 20 until delivery, lidocaine prolonged term by 7%.

CONCLUSION

Lidocaine can selectively block myometrial Na+ channels, thereby reducing excitability. It can prolong term in the pregnant rat. These observations suggest that lidocaine might be useful in the clinical management of preterm labor.

A method for linking VL and VH region genes that allows bulk transfer between vectors for use in generating polyclonal IgG libraries

Libraries of Ab fragments have been produced by others from light and heavy chain cDNAs derived from populations of B lymphocytes and expressed in bacteria. However, such libraries have not been transferred to eukaryotic expression vectors to generate polyclonal libraries of intact glycosylated Abs that can mediate effector functions. We present a method for transferring pairs of linked VL-VH region genes between circular prokaryotic and eukaryotic vectors. The key feature of the transfer is that the VL and VH region genes are linked head to head (<-->) in opposite transcriptional orientations. To illustrate this method, a pair of VL and VH region cDNAs derived from an existing hybridoma cell line were linked head to head by PCR, transferred as a unit between vectors, and expressed as an IgG Ab with Ag binding activity. Although we tested the transfer of a single VL-VH region gene pair, this system is expected to allow the bulk transfer of physically linked VL-VH region gene combinations between different circular vectors and the expression of the same library as either Ab fragments or intact Abs.

A method for linking VL and VH region genes that allows bulk transfer between vectors for use in generating polyclonal IgG libraries

Libraries of Ab fragments have been produced by others from light and heavy chain cDNAs derived from populations of B lymphocytes and expressed in bacteria. However, such libraries have not been transferred to eukaryotic expression vectors to generate polyclonal libraries of intact glycosylated Abs that can mediate effector functions. We present a method for transferring pairs of linked VL-VH region genes between circular prokaryotic and eukaryotic vectors. The key feature of the transfer is that the VL and VH region genes are linked head to head (&lt;--&gt;) in opposite transcriptional orientations. To illustrate this method, a pair of VL and VH region cDNAs derived from an existing hybridoma cell line were linked head to head by PCR, transferred as a unit between vectors, and expressed as an IgG Ab with Ag binding activity. Although we tested the transfer of a single VL-VH region gene pair, this system is expected to allow the bulk transfer of physically linked VL-VH region gene combinations between different circular vectors and the expression of the same library as either Ab fragments or intact Abs.

Release of TEA blockade of maxi-K+ channels by isoproterenol

The outward K+ current induced by step depolarization of freshly dispersed myocytes of guinea-pig taenia coli decreased about 80% upon treatment with 3 mM tetraethylammonium chloride (TEA). Isoproterenol (ISO, 2-5 microM) restored it to a large extent. This restoration did not occur in the presence of propranolol (2 microM). In single-channel recordings from cell-attached patches, the activity of maxi-K+ channel is dominant. When 3 mM TEA is incorporated in the pipette solution, the dominant channel-openings observed had much smaller unitary conductance. On the addition of ISO (2 microM) to the bath solution, but not to the pipette solution, K(+)-channel openings with unitary conductance similar to that without TEA treatment appeared. Cyclic AMP incorporated into the cytoplasm through the pipette was ineffective. These results indicate that ISO release TEA decrease of maxi-K+ channel conductance through some intracellular second messenger system other than adenylyl cyclase-protein kinase A system.

Enhancement of beta-adrenergic receptor activation of maxi-K+ channels by GM1 ganglioside

In freshly dispersed guinea-pig taenia coli myocytes, beta-adrenergic receptor agonist isoproterenol (ISO, 2-5 microM) increased the open probability (po) of the maxi-K+ channels through the G-protein, G. Subunit B of cholera toxin (0.1 nM) suppressed the ISO-induced increase of po of maxi-Ks+ channels but did not affect that induced by forskolin. Brief (20 min) treatment of the myocytes with GM1 ganglioside (GM1, 0.1-1.0 microM) enhanced the effectiveness of ISO-induced increase of po. This effect was blocked by 0.5% trypsin, which is known to prevent the incorporation of exogeneous GM1 into the membrane. The effect of GM1 was not shared by GM2 and GM3 gangliosides. These results suggest that the membrane-bound endogeneous GM1 may participate in the regulation of cellular response to beta-adrenergic agents.

Chimeric antibodies with anti-dextran-derived complementarity-determining regions and anti-p-azophenylarsonate-derived framework regions

The framework regions of antibodies fold into a conserved beta-sheet structure that acts as scaffolding for the antigen-contacting complementarity-determining regions (CDR). To test the structural equivalence of the frameworks between two antibodies with widely different combining sites, we created chimeric H and L chains by grafting the CDR of an alpha(1--&gt;6)dextran specific antibody onto the framework of a p-azophenylarsonate (Ars) specific antibody through oligonucleotide-directed mutagenesis of the anti-Ars variable region genes. Antibodies consisting of various chain combinations of the chimeric, anti-dextran, and anti-Ars derived H and L chains were generated in transfectomas and tested for binding to dextran and Ars. Of the newly created chimeric and/or hybrid antibodies, an antibody with the chimeric H chain and the anti-dextran L chain bound to dextran with the same association constant as the parental anti-dextran antibody, and like the anti-dextran antibody was shown by immunochemical mapping to have a site complementary to six glucose residues. None of the other new variable region combinations, including the all-chimeric combination, showed binding to either dextran or Ars. These results indicate that the H chain but not the L chain anti-dextran and anti-Ars frameworks are functionally equivalent. Attempts to confer dextran binding on the H and L chain chimeric antibody, by mutagenizing selected framework residues, were unsuccessful. This study demonstrates the important role of the frameworks in the precise alignment of the CDR for Ag binding.

Chimeric antibodies with anti-dextran-derived complementarity-determining regions and anti-p-azophenylarsonate-derived framework regions

The framework regions of antibodies fold into a conserved beta-sheet structure that acts as scaffolding for the antigen-contacting complementarity-determining regions (CDR). To test the structural equivalence of the frameworks between two antibodies with widely different combining sites, we created chimeric H and L chains by grafting the CDR of an alpha(1-->6)dextran specific antibody onto the framework of a p-azophenylarsonate (Ars) specific antibody through oligonucleotide-directed mutagenesis of the anti-Ars variable region genes. Antibodies consisting of various chain combinations of the chimeric, anti-dextran, and anti-Ars derived H and L chains were generated in transfectomas and tested for binding to dextran and Ars. Of the newly created chimeric and/or hybrid antibodies, an antibody with the chimeric H chain and the anti-dextran L chain bound to dextran with the same association constant as the parental anti-dextran antibody, and like the anti-dextran antibody was shown by immunochemical mapping to have a site complementary to six glucose residues. None of the other new variable region combinations, including the all-chimeric combination, showed binding to either dextran or Ars. These results indicate that the H chain but not the L chain anti-dextran and anti-Ars frameworks are functionally equivalent. Attempts to confer dextran binding on the H and L chain chimeric antibody, by mutagenizing selected framework residues, were unsuccessful. This study demonstrates the important role of the frameworks in the precise alignment of the CDR for Ag binding.

The transduction system in the isoproterenol activation of the Ca(2+)-activated K+ channel in guinea pig taenia coli myocytes

In freshly dispersed guinea pig taenia coli myocytes the activity of the large conductance Ca(2+)-activated K+ channel (maxi-K+ channel) predominates. The open probability (Po) of this channel is increased by micromolar concentrations of the beta-adrenergic agonist isoproterenol (ISO). Low concentrations of cholera toxin (CTX, 1 pM) and guanosine 5'-O-2-thiodiphosphate (GDP beta S, 0.5 mM) suppress the ISO-induced increase of Po. Higher concentrations of CTX (e.g., 0.5 nM) as well as forskolin and dibutyryl cAMP increase the Po. 1,9-Dideoxyforskolin, the forskolin analogue, which lacks the adenylate cyclase-stimulating effect, does not. A specific protein kinase A inhibitor (Wiptide), applied intracellularly via diffusion from the patch electrode, suppresses the ISO-induced increase of whole-cell outward K+ current during step depolarization. In contrast, intracellularly applied protein kinase C (19-36), a specific protein kinase C inhibitor, has no effect on the whole-cell current. TMB-8, an inhibitor of intracellular calcium mobilization, does not affect either the whole-cell outward K+ current during step depolarization or the Po. These observations show that ISO increases the Po of the maxi-K+ channels in the guinea pig taenia coli myocytes through the G protein-adenylate cyclase-protein kinase A system.

On the apparent absence of maxi-K+ channel in rat aortic myocyte

In cultured rat aortic myocytes, no high-conductance Ca(2+)-and voltage-dependent K+ channels (maxi-K+ channels) such as those found in the human and rabbit were observed. However, we have found that in freshly dissociated rat aortic myocytes, the activities of high-conductance Ca(2+)-and voltage-dependent channels were present and predominant. In cell-attached patches measured with pipettes filled with normal saline solution, their single channel conductances were 139.6 +/- 2.5 picosiemens (mean +/- SE). Under the 140-mM symmetrical K+ condition (both bath and pipette solutions contained 140 mM KCl), that measurement became 207.2 +/- 3.6 picosiemens and increased by about 50% with detaching of the membrane patch. The relative conductances of the channel to K+, NH4+, and Rb+ were 1:0.61:0.48. The conductance of these channels can readily be reduced by more than 90% by extracellularly applied 0.5 mM tetraethylammonium chloride. These characteristics show that they were maxi-K+ channels.

Oligonucleotide-directed mutagenesis of antibody combining sites

We review here our attempts to achieve a better understanding of the structure--function relationship of antibody combining sites, and to gain insights into the engineering of antibodies with desired specificity and affinity. We have focused on a model system--antibodies to the hapten p-azophenylarsonate (Ars) derived from A/J mice. Oligonucleotide-directed mutagenesis was used to alter the sequence of the variable region genes of such anti-Ars antibodies. Mutant antibodies were generated in hybridoma cells following transfection of the altered genes, and the effects of the primary structure changes on antibody specificity, affinity, and idiotypic expression were assessed. These studies suggest that an antibody combining site with basic specificity for an antigen could be created by introducing a set of a few amino acid residues in the complementarity determining regions, and that the affinity of such a site could be improved one substitution at a time in a sequential manner.

Difference in the effects of verapamil on Ca(2+)-activated K+ channels of smooth myocytes of aorta and taenia coli

In freshly dispersed taenia coli myocytes of guinea pig, verapamil decreased the whole-cell outward current during depolarization. In aortic myocytes it has the opposite effect. In cell-attached patches, verapamil increased the frequency of opening of the maxi-K+ channel of both taenia coli and aortic myocytes at the expense of the mean open time. In taenia coli myocytes, the greater frequency did not fully compensate for the reduced open time, and the overall open probability of the channel was reduced. In aortic myocytes, the increase in open frequency overwhelmed the effect of the decrease in mean open time. None of the effects could be reproduced with nifedipine or diltiazem in either cell type.

Actions of chiriquitoxin on frog skeletal muscle fibers and implications for the tetrodotoxin/saxitoxin receptor

Chiriquitoxin (CqTX) from the Costa Rican frog Atelopus chiriquensis differs from tetrodoxin (TTX) only in that a glycine residue replaces a methylene hydrogen of the C-11 hydroxymethyl function. On the voltage-clamped frog skeletal muscle fiber, in addition to blocking the sodium channel and unrelated to such an action, CqTX also slows the activation of the fast potassium current in approximately 40% of the muscle fiber population. At pH 7.25, CqTX is as potent as TTX in blocking the sodium channel, with an ED50 of 3.8 nM. Its ED50's at pH 6.50 and 8.25 are 6.8 and 2.3 nM, contrasted with 3.8 and 4.3 nM for TTX. These differences are attributable to changes in the chemical states in the glycine residue. The equipotency of CqTX with TTX at pH 7.25 is explainable by an intramolecular salt bridge between the amino and carboxyl groups of the glycine function, all other surface groups in TTX and CqTX being the same. From available information on these groups and those in saxitoxin (STX), the TTX/STX binding site is deduced to be in a pocket 9.5 A wide, 6 A high, and 5 A deep. The glycine residue of CqTX probably projects out of the entrance to this pocket. Such a view of the binding site could also account for the actions of STX analogues, including the C-11 sulfated gonyautoxins and the 21-sulfocarbamoyl analogues. In the gonyautoxins the sulfate groups are equivalently placed as the glycine in CqTX, whereas in the sulfocarbamoyl toxins the sulfate groups extend the carbamoyl side-chain, leading to steric hinderance to productive binding.

Retention of secretory proteins in an intermediate compartment and disappearance of the Golgi complex in an END4 mutant of Chinese hamster ovary cells

Mutant V.24.1, a member of the End4 complementation group of temperature-sensitive CHO cells, is defective in secretion at the restrictive temperature (Wang, R.-H., P. A. Colbaugh, C.-Y. Kao, E. A. Rutledge, and R. K. Draper. 1990. J. Biol. Chem. 265:20179-20187; Presley, J. F., R. K. Draper, and D. T. Brown. 1991. J. Virol. 65:1332-1339). We have further investigated the secretory lesion and report three main findings. First, the block in secretion is not due to aberrant folding or oligomerization of secretory proteins in the endoplasmic reticulum because the hemagglutinin of influenza virus folded and oligomerized at the same rate in mutant and parental cells at the restrictive temperature. Second, secretory proteins accumulated in a compartment intermediate between the ER and the Golgi. Several lines of evidence support this conclusion, the most direct being the colocalization by immunofluorescence microscopy of influenza virus hemagglutinin with a 58-kD protein that is known to reside in an intermediate compartment. Third, at the resolution of fluorescence microscopy, the Golgi complex in the mutant cells vanished at the restrictive temperature.

Actions of 6-epitetrodotoxin and 11-deoxytetrodotoxin on the frog skeletal muscle fiber

6-Epitetrodotoxin (6-epiTTX) and 11-deoxytetrodotoxin (11-deoxyTTX), isolated from an Okinawan newt, Cynops ensicauda, were tested for sodium-channel blocking effects on the voltage-clamped frog skeletal muscle fiber. In 6-epiTTX, the C-6 -OH is in an epimeric position; in 11-deoxyTTX, C-11 has a methyl in place of a hydroxymethyl group. At pH 7.25, the ED50s for reducing INa are: 4.1 nM (TTX), 96 nM (6-epiTTX), and 445 nM (11-deoxyTTX). In each analogue, the lowered potency can be attributed energetically to the loss of a hydrogen bond. By complementarity, in the sodium-channel receptor for TTX, there must be a hydrogen-acceptor group for the C-6 -OH, and another for the C-11 -OH. Therefore, the TTX molecule is bound to the receptor through an ion-pair (for the guanidinium), and five hydrogen bonds, one each for the -OH on C-9, C-10, C-4, and, as now identified, for C-6 and C-11. Considering the three-dimensional structure of the toxin molecules, these binding sites must be located in a fold or a crevice of the channel protein. If glutamate 387 of rat brain sodium channel II is the ion-pairing site for the guanidinium group, then the carbonyl oxygen of asparagine 388 is the hydrogen acceptor for the C-9 and C-10 -OHs.

Actions of decarbamoyloxysaxitoxin and decarbamoylneosaxitoxin on the frog skeletal muscle fiber

Two new analogues of the decarbamoyl series of paralytic shellfish toxins have been isolated through improved HPLC methods. In decarbamoyloxysaxitoxin (doSTX), the -OH function at C-13 of decarbamoylsaxitoxin (dcSTX) is changed to -CH3. In decarbamoylneosaxitoxin (dcneoSTX), the carbomyl side-chain of neosaxitoxin (neoSTX) has been removed. The new analogues were assayed on voltage-clamped frog skeletal muscle fiber for their potency in reducing the sodium current. Compared with neoSTX, the relative potencies of dcneoSTX are: 0.003 (at pH 6.50), 0.004 (pH 7.25), and 0.005 (pH 8.25). The influence of pH on the potency is the same in neoSTX and dcneoSTX. The fractional loss of potency caused by decarbamoylation is much greater in neoSTX than in STX, possibly because of an intramolecular interaction between the N-1 -OH in neoSTX and the -OH on C-13. Compared with STX, the ED50 for reducing INa by doSTX is 618 nM, making its relative potency 0.008 that of STX. Energetically, the decreased potency can be accounted for by the loss of two hydrogen bonds, one at the C-13 -OH of dcSTX, and the other at the amino group in the carbamoyl function of STX. These two groups resemble the C-6 and C-11 -OHs in tetrodotoxin, and probably bind to the same site-points. Thus, the near-identical actions of STX and TTX can be attributed to the common sharing of one ion-pair site and four hydrogen-bonding sites. If glutamate 387 of rat brain sodium channel II were the anionic site which ion-pairs with the 7, 8, 9 guanidinium of STX, then the carbonyl oxygen of asparagin 388 is the hydrogen-acceptor for the C-12 gem-diols.