Targeted dual inhibition of c-Met/VEGFR2 signalling by foretinib improves antitumour effects of nanoparticle paclitaxel in gastric cancer models

Elevated expression of multiple growth factors and receptors including c‐Met and VEGFR has been reported in gastric adenocarcinoma (GAC) and thus provides a potentially useful therapeutic target. The therapeutic efficacy of foretinib, a c‐Met/VEGFR2 inhibitor, was determined in combination with nanoparticle paclitaxel (NPT) in GAC. Animal studies were conducted in NOD/SCID mice in subcutaneous and peritoneal dissemination xenografts. The mechanism of action was assessed by Immunohistochemical and Immunoblot analyses. In c‐Met overexpressing MKN‐45 cell‐derived xenografts, NPT and foretinib demonstrated inhibition in tumour growth, while NPT plus foretinib showed additive effects. In c‐Met low‐expressing SNU‐1 or patient‐derived xenografts, the foretinib effect was smaller, while NPT had a similar effect compared with MKN‐45, as NPT plus foretinib still exhibited an additive response. Median mice survival was markedly improved by NPT (83%), foretinib (100%) and NPT plus foretinib (230%) in peritoneal dissemination xenografts. Subcutaneous tumour analyses exhibited that foretinib increased cancer cell death and decreased cancer cell proliferation and tumour vasculature. NPT and foretinib suppressed the proliferation of GAC cells in vitro and had additive effects in combination. Further, foretinib caused a dramatic decrease in phosphorylated forms of c‐Met, ERK, AKT and p38. Foretinib led to a decrease in Bcl‐2, and an increase in p27, Bax, Bim, cleaved PARP‐1 and cleaved caspase‐3. Thus, these findings highlight the antitumour impact of simultaneous suppression of c‐Met and VEGFR2 signalling in GAC and its potential to enhance nanoparticle paclitaxel response. This therapeutic approach might lead to a clinically beneficial combination to increase GAC patients’ survival.

Alternative polyadenylation drives oncogenic gene expression in pancreatic ductal adenocarcinoma

Alternative polyadenylation (APA) is a gene regulatory process that dictates mRNA 3'-UTR length, resulting in changes in mRNA stability and localization. APA is frequently disrupted in cancer and promotes tumorigenesis through altered expression of oncogenes and tumor suppressors. Pan-cancer analyses have revealed common APA events across the tumor landscape; however, little is known about tumor type-specific alterations that may uncover novel events and vulnerabilities. Here, we integrate RNA-sequencing data from the Genotype-Tissue Expression (GTEx) project and The Cancer Genome Atlas (TCGA) to comprehensively analyze APA events in 148 pancreatic ductal adenocarcinomas (PDACs). We report widespread, recurrent, and functionally relevant 3'-UTR alterations associated with gene expression changes of known and newly identified PDAC growth-promoting genes and experimentally validate the effects of these APA events on protein expression. We find enrichment for APA events in genes associated with known PDAC pathways, loss of tumor-suppressive miRNA binding sites, and increased heterogeneity in 3'-UTR forms of metabolic genes. Survival analyses reveal a subset of 3'-UTR alterations that independently characterize a poor prognostic cohort among PDAC patients. Finally, we identify and validate the casein kinase CSNK1A1 (also known as CK1alpha or CK1a) as an APA-regulated therapeutic target in PDAC. Knockdown or pharmacological inhibition of CSNK1A1 attenuates PDAC cell proliferation and clonogenic growth. Our single-cancer analysis reveals APA as an underappreciated driver of protumorigenic gene expression in PDAC via the loss of miRNA regulation.

Microbial biomass and activity distribution in an anoxic, hypersaline basin

The Orca Basin is a hypersaline depression in the northern Gulf of Mexico with anoxic conditions observed in the lower 200 m of the water column. Measurements of adenosine 5'-triphosphate, heterotrophic potential, and uridine uptake made above and across the interface into the anoxic zone revealed the presence of an active microbial population approximately 100 m above the interface. Biomass and activity decreased at and just below the interface but increased near the bottom, consistent with similar observations made in the Cariaco Trench. The maximum adenosine 5'-triphosphate concentration above the interface of 5.9 ng/liter (2,173 m) is about eight times greater than the value found in oxygenated waters of corresponding depth in the absence of an anoxic zone. The maximum adenosine 5'-triphosphate concentration in the anoxic zone is approximately 15 times greater than that found in oxygenated water of similar depth, suggesting anoxia will support the development of a larger bacterial population. Our findings suggest that autotrophic bacteria may be the dominant physiological group in the region just above the interface.

Metabolic activities of the intestinal microflora of a deep-sea invertebrate

The intestinal microflora of deep-sea amphipods, in enrichment culture employing starch, urea, and N-acetyl-d-glucosamine and when examined under simulated in situ conditions, exhibited growth rates and substrate conversion approximately equal to, or greater than, atmospheric controls during short-term incubation. These observations are significant since these microorganisms may play an important role in biodegradation in the deep sea.

KCNQ channels mediate IKs, a slow K+ current regulating excitability in the rat node of Ranvier

Mutations that reduce the function of KCNQ2 channels cause neuronal hyperexcitability, manifested as epileptic seizures and myokymia. These channels are present in nodes of Ranvier in rat brain and nerve and have been proposed to mediate the slow nodal potassium current I(Ks). We have used immunocytochemistry, electrophysiology and pharmacology to test this hypothesis and to determine the contribution of KCNQ channels to nerve excitability in the rat. When myelinated nerve fibres of the sciatic nerve were examined by immunofluorescence microscopy using antibodies against KCNQ2 and KCNQ3, all nodes showed strong immunoreactivity for KCNQ2. The nodes of about half the small and intermediate sized fibres showed labelling for both KCNQ2 and KCNQ3, but nodes of large fibres were labelled by KCNQ2 antibodies only. In voltage-clamp experiments using large myelinated fibres, the selective KCNQ channel blockers XE991 (IC50 = 2.2 microm) and linopirdine (IC50 = 5.5 microm) completely inhibited I(Ks), as did TEA (IC50 = 0.22 mm). The KCNQ channel opener retigabine (10 microm) shifted the activation curve to more negative membrane potentials by -24 mV, thereby increasing I(Ks). In isotonic KCl 50% of I(Ks) was activated at -62 mV. The activation curve shifted to more positive potentials as [K+]o was reduced, so that the pharmacological and biophysical properties of I(Ks) were consistent with those of heterologously expressed homomeric KCNQ2 channels. The ability of XE991 to selectively block I(Ks) was further exploited to study I(Ks) function in vivo. In anaesthetized rats, the excitability of tail motor axons was indicated by the stimulus current required to elicit a 40% of maximal compound muscle action potential. XE991 (2.5 mg kg(-1) i.p.) eliminated all nerve excitability functions previously attributed to I(Ks): accommodation to 100 ms subthreshold depolarizing currents, the post-depolarization undershoot in excitability, and the late subexcitability after a single impulse or short trains of impulses. Due to reduced spike-frequency adaptation after XE991 treatment, 100 ms suprathreshold current injections generated long trains of action potentials. We conclude that the nodal I(Ks) current is mediated by KCNQ channels, which in large fibres of rat sciatic nerve appear to be KCNQ2 homomers.

HERG K(+) currents in human prolactin-secreting adenoma cells

To investigate the presence and possible function of ether-à-go-go-related gene (erg) K(+) channels in human lactotroph cells (HERG channels), primary cultures were prepared from human prolactinoma tissue. In almost all primary cultures, HERG currents could be recorded in identified prolactin cells using an external high-K(+) solution. The antiarrhythmic agent E-4031, a specific blocker of erg channels, served to isolate HERG currents as the drug-sensitive currents. In cells of two tumours tested, thyrotropin-releasing hormone significantly reduced the amplitude of the HERG currents. The potential dependence of HERG current availability and the deactivation kinetics differed significantly even between prolactin cells derived from one adenoma. For comparison, corresponding values were obtained for heterologously expressed rat erg1, erg2 and erg3 channels. The expression of the three HERG channel subunits was investigated in nine human adenomas using RT-PCR. Transcripts for HERG1 were present in all adenomas and although transcripts for HERG2 and HERG3 were also detected, their expression level was more variable. The results demonstrate the functional expression of HERG channels in human prolactin-secreting tumours and are compatible with a physiological role for these channels in the control of prolactin secretion, as has been shown in normal rat lactotroph cells.

The neuropeptide head activator induces activation and translocation of the growth-factor-regulated Ca2+-permeable channel GRC

The neuropeptide head activator stimulates cell proliferation of neuronal precursor and neuroendocrine cells. The mitogenic signaling cascade requires Ca2+ influx for which, as we show in this paper, the growth-factor-regulated Ca2+-permeable cation channel, GRC, is responsible. GRC is a member of the transient receptor potential channel family. In uninduced cells only low amounts of GRC are present on the plasma membrane but, upon stimulation with head activator, GRC translocates from an intracellular compartment to the cell surface. Head activator functions as an inducer of GRC translocation in neuronal and neuroendocrine cells, which express GRC endogenously, and also in COS-7 cells after transfection with GRC. Head activator is no direct ligand for GRC, but its action requires the presence of a receptor coupled to a pertussis-toxin inhibitable G-protein. Heterologously expressed GRC becomes activated by head activator, which results in opening of the channel and Ca2+ influx. SK&F 96365, an inhibitor specific for TRP-like channels, blocks Ca2+ entry and, consequently, translocation of GRC is prevented. Head activator-induced GRC activation and translocation are also inhibited by wortmannin and KN-93, blockers of the phosphatidylinositol 3-kinase and of the Ca2+/calmodulin-dependent kinase, respectively, which implies a role for both kinases in head-activator signaling to GRC.

Ca2+ channels in clonal rat anterior pituitary cells (GH3/B6)

In clonal rat somatomammotroph cells (GH3/ B6) Ca2+ influx through voltage-dependent Ca2+ channels is important for regulating the Ca2+ concentration that mediates hormone secretion. To study the Ca2+ channel subtypes in GH3/B6 cells, Ca2+ channel currents were recorded with the whole-cell configuration of the patch-clamp technique using Ba2+ as the charge carrier. Forty-nine percent of the total Ba2+ current amplitude was mediated by a nifedipine-sensitive current (L-type). In addition, three other high-voltage-activated Ca2+ channel current components could be distinguished pharmacologically: 10 nM omega-agatoxin-IVA-sensitive current (22%; P-type), omega-conotoxin-MVIIC-sensitive current (18%; Q-type), and toxin-resistant current (24%). Since omega-conotoxin GVIA (2 microM) had no blocking effect, N-type Ca2+ channels are assumed not to be present in GH3/B6 cells. The T-type Ca2+ channel current was either absent or very small. Different pore-forming alpha1 subunits of Ca2+ channels were found to be expressed in GH3/B6 cells, which could be the molecular correlates of the different Ba2+ current subtypes: alpha1G of T-type, alpha1C, alpha1D and alpha1S of L-type, and alpha1A of P/Q-type current. In addition, transcripts for beta1, beta2 and beta3 subunits were detected. Blockage of L-type channels with 10 microM nifedipine or P/Q-type channels with 10 nM omega-agatoxin MVIIC + 200 nM omega-conotoxin blocked action potential firing in GH3/B6 cells and decreased basal prolactin secretion.

Modulation of rat erg1, erg2, erg3 and HERG K+ currents by thyrotropin-releasing hormone in anterior pituitary cells via the native signal cascade

The mechanism of thyrotropin-releasing hormone (TRH)-induced ether-a-go-go-related gene (erg) K+ current modulation was investigated with the perforated-patch whole-cell technique in clonal somatomammotroph GH3/B6 cells. These cells express a small endogenous erg current known to be reduced by TRH. GH3/B6 cells were injected with cDNA coding for rat erg1, erg2, erg3 and HERG K+ channels. The corresponding erg currents were isolated with the help of the specific erg channel blockers E-4031 and dofetilide and their biophysical properties were determined. TRH (1 M) was able to significantly reduce the different erg currents. The voltage dependence of activation was shifted by 15 mV (erg1), 10 mV (erg2) and 6 mV (erg3) to more positive potentials without strongly affecting erg inactivation. TRH reduced the maximal available erg current amplitude by 12% (erg1), 13% (erg2) and 39% (erg3) and accelerated the time course of erg1 and erg2 channel deactivation, whereas erg3 deactivation kinetics were not significantly altered. The effects of TRH on HERG currents did not differ from those on its rat homologue erg1. In addition, coinjection of rat MiRP1 with HERG cDNA did not influence the TRH-induced modulation of HERG channels. Rat erg1 currents recorded in the cell-attached configuration were reduced by application of TRH to the extra-patch membrane in the majority of the experiments, confirming the involvement of a diffusible second messenger. Application of the phorbol ester phorbol 12-myristate 13-acetate (PMA; 1 M) shifted the voltage dependence of erg1 activation in the depolarizing direction, but it did not reduce the maximal current amplitude. The voltage shift could not be explained by a selective effect on protein kinase C (PKC) since the PKC inhibitor bisindolylmaleimide I did not block the effects of TRH and PMA on erg1. In addition, cholecystokinin, known to activate the phosphoinositol pathway similarly to TRH, did not significantly affect the erg1 current. Various agents interfering with different known TRH-elicited cellular responses were not able to completely mimic or inhibit the TRH effects on erg1. Tested substances included modulators of the cAMP-protein kinase A pathway, arachidonic acid, inhibitors of tyrosine kinase and mitogen-activated protein kinase, sodium nitroprusside and cytochalasin D. The results demonstrate that all three members of the erg channel subfamily are modulated by TRH in GH3/B6 cells. In agreement with previous studies on the TRH-induced modulation of the endogenous erg current in prolactin-secreting anterior pituitary cells, the TRH effects on overexpressed erg1 channels are not mediated by any of the tested signalling pathways.

Erg1, erg2 and erg3 K channel subunits are able to form heteromultimers

Clonal somato-mammotroph GH3/B6 cells and lactotroph MMQ cells express two (ergl, erg2) of the three cloned rat ether-à-go-go-related gene (erg) K channel subunits. To study whether the erg subunits form heteromultimers, dominant-negative mutants of erg and erg2 were constructed by point mutation (erg1G630S, erg2G480S). After co-expression of these mutants with the wild-type erg1, erg2, or erg3 in Chinese hamster ovary (CHO) cells no erg currents could be detected. In contrast, in co-expression experiments with members of the other ether-à-go-go (EAG) subfamilies (eagl, elkl) the mutant erg1G630S had no effect. These results strongly suggest that erg channel subunits are able to form heteromultimers within the erg channel subfamily. Suppression of the endogenous E-4031-sensitive currents in GH3/B6 and MMQ cells by erg1G630S confirms that they are mediated by erg channels despite the differences in gating kinetics in these cells. Reduction of the erg current in GH3/B6 cells by erg2G480S indicates that erg heteromultimers can also be formed in these cells.

Expression of mRNA for voltage-dependent and inward-rectifying K channels in GH3/B6 cells and rat pituitary

The expression of mRNA for voltage-dependent (Kv) and inward-rectifying K channels (Kir) was studied in clonal rat somato-mammotroph cells (GH3/B6 cells) and rat pituitary using reverse transcription-polymerase chain reaction (RT-PCR). In GH3/B6 cells transcripts for 16 different Kv channel alpha-subunits (seven Shaker-related: Kv1.2, Kv1.4, Kv1.5, Kv2.1, Kv3.2, Kv4.1, Kv5.1; six EAG: eag1, erg1, erg2, elk1-elk3; three KCNQ: KCNQ1-KCNQ3) and for five different Kir channel alpha-subunits (Kir1.1, Kir2.3, Kir3.2, Kir3.3, Kir6.2) were found. In addition, transcripts for a short isoform of Kvbeta2 and transcripts for Kvbeta3 subunits were present. In rat pituitary transcripts for 21 different Kv channel alpha-subunits (11 Shaker-related: Kv1.3, Kv1.4, Kv1.6, Kv2.1, Kv2.2, Kv3.2, Kv3.4, Kv4.1, Kv4.2, Kv4.3, Kv6.1; seven EAG: eag1, erg1-erg3, elk1-elk3; three KCNQ: KCNQ1-KCNQ3) and nine Kir channel alpha-subunits (Kir1.1, Kir2.2, Kir3.1-Kir3.4, Kir4.1, Kir6.1, Kir6. 2) were found. In addition, all tested auxiliary subunits (Kvbeta1-Kvbeta3, minK, SUR1, SUR2) are expressed in the pituitary. The results indicate that the macroscopic K currents in GH3/B6 and pituitary cells are presumably mediated by K channels constructed by a larger number of K channel alpha-subunits and auxiliary beta-subunits than previously distinguished electrophysiologically and pharmacologically.

The erg-like potassium current in rat lactotrophs

1. The ether-à-go-go-related gene (erg)-like K+ current in rat lactotrophs from primary culture was characterized and compared with that in clonal rat pituitary cells (GH3/B6). The class III antiarrhythmic E-4031 known to block specifically erg K+ channels was used to isolate the erg-like current as the E-4031-sensitive current. The experiments were performed in 150 mM K+ external solution using the patch-clamp technique. 2. The erg-like K+ current elicited with hyperpolarizing pulses negative to -100 mV consisted of a fast and a pronounced slowly deactivating current component. The contribution of the slow component to the total current amplitude was potential dependent and varied from cell to cell. At -100 mV it ranged from 50 to 85% and at -140 mV from 21 to 45%. 3. The potential-dependent channel availability curves determined with 2 s prepulses were fitted with the sum of two Boltzmann functions. The function related to the slowly deactivating component of the erg-like current was shifted by more than 40 mV to more negative membrane potentials compared with that of the fast component. 4. In contrast to that of native lactotrophs studied under identical conditions, the erg-like K+ current of GH3/B6 cells was characterized by a predominant fast deactivating current component, with similar kinetic and steady-state properties to the fast deactivating current component of native lactotrophs. 5. Thyrotrophin-releasing hormone reduced the erg-like current in native lactotrophs via an intracellular signal cascade which seemed to involve a pathway independent from protein kinase A and protein kinase C. 6. RT-PCR studies on cytoplasm from single lactotrophs revealed the presence of mRNA of the rat homologue of the human ether-à-go-go-related gene HERG (r-erg1) as well as mRNA of the two other cloned r-erg cDNAs (r-erg2 and r-erg3) in different combinations. In GH3/B6 cells, only the transcripts of r-erg1 and r-erg2 were found.

Separation of M-like current and ERG current in NG108-15 cells

Differentiated NG108-15 neuroblastoma x glioma hybrid cells were whole-cell voltage-clamped. Hyperpolarizing pulses, superimposed on a depolarized holding potential (-30 or -20 mV), elicited deactivation currents which consisted of two components, distinguishable by fitting with two exponential functions. Linopirdine [DuP 996, 3,3-bis(4-pyridinylmethyl)-1-phenylindolin-2-one), a neurotransmitter-release enhancer known as potent and selective blocker of the M-current of rat sympathetic neurons, in concentrations of 5 or 10 microM selectively inhibited the fast component (IC50 = 14.7 microM). The slow component was less sensitive to linopirdine (IC50>20 microM). The class III antiarrhythmics [(4-methylsulphonyl)amido]benzenesulphonamide (WAY-123.398) and 1-[2-(6-methyl-2-pyrydinil)ethyl]-4-(4-methylsulphonylaminobenz oyl) piperidine (E-4031), selective inhibitors of the inwardly rectifying ERG (ether-à-go-go-related gene) potassium channel, inhibited predominantly the slow component (IC50 = 38 nM for E-4031). The time constant of the WAY-123.398-sensitive current resembled the time constant of the slow component in size and voltage dependence. Inwardly rectifying ERG currents, recorded in K+ -rich bath at strongly negative pulse potentials, resembled the slow component of the deactivation current in their low sensitivity to linopirdine (28% inhibition at 50 microM). The size of the slow component varied greatly between cells. Accordingly, varied the effect of WAY-123.398 on deactivation current and holding current. RNA transcripts for the following members of the ether-à-go-go gene (EAG) K+ channel family were found in differentiated NG108-15 cells: ERG1, ERG2, EAGI, EAG-like (ELK)1, ELK2; ERG3 was only present in non-differentiated cells. In addition, RNA transcripts for KCNQ2 and KCNQ3 were found in differentiated and non-differentiated cells. We conclude that the fast component of the deactivation current is M-like current and the slow component is deactivating ERG current. The molecular correlates are probably KCNQ2/KCNQ3 and ERG1/ERG2, respectively.

A functional role of the erg-like inward-rectifying K+ current in prolactin secretion from rat lactotrophs

The functional role of the inward-rectifying erg-like K+ current in rat lactotrophs was studied by the use of a selective blocker, the class III antiarrhythmic agent E-4031. The erg-like current was measured as drug-sensitive current in physiological K+ gradient. In the range of the normal resting membrane potential of rat lactotrophs (around -45 mV) the erg-like current constituted a steady outward current. A selective block of this current by E-4031 resulted in a moderate (5 mV) depolarization of the membrane potential in 64% of the lactotroph cells. Measurements of basal prolactin secretion with the reverse hemolytic plaque assay showed that the number of prolactin secreting cells and the amount of prolactin secreted from single lactotrophs was significantly increased in the presence of E-4031. The data show that the contribution of the erg-like K+ current to the maintenance of the resting membrane potential is functionally important for the regulation of prolactin secretion.

Ionic mechanisms underlying TRH-induced prolactin secretion in rat lactotrophs

Whole cell patch-clamp experiments were performed in clonal rat pituitary cells (GH3/B6 cells) to further analyze an inward-rectifying K current (IK, IR) which is suggested to be involved in the thyrotropin-releasing hormone (TRH)-induced increase in prolactin secretion from these cells. Using the class III antiarrhythmic agent E-4031 which is known as specific blocker of ether-á-go-go-related gene (ERG) K channels, the inward-rectifying K current could be isolated as the drug-sensitive current. To elucidate in molecular basis of this current, comparative experiments were performed in CHO cells which served as heterologous expression system for RERG, the rat homologue of the human ERG (HERG). It is shown that the inward-rectifying K current has properties identical to those mediated by channels encoded by RERG. In external 5 mM K+ solution, the ERG-like current IK, IR was an outward current in the physiological potential range, and this outward current could be strongly reduced by TRH. A specific block of IK, IR was able to mimick the second phase of the TRH-response which is characterized by a depolarization and/or by an increase in the frequency of Ca action potentials. These data show, that the ERG-like current in GH3/B6 cells contributes to the maintainance of the resting membrane potential and that it plays an important part in the mechanisms of the effects of TRH leading to an increase in prolactin secretion.

RERG is a molecular correlate of the inward-rectifying K current in clonal rat pituitary cells

The rat homologue of the human ether-ä-go-go-related gene (r-erg) was cloned from rat brain using homology screening. RERG has a 96% amino acid identify to HERG. Membrane currents recorded in CHO cells after previous injection of r-erg showed that the voltage- and time-dependent properties are indistinguishable from h-erg-induced currents expressed in the same system. RT-PCR revealed the presence of r-erg mRNA in clonal rat pituitary cells (GH3/B6 cells). These cells exhibit a voltage-dependent inward-rectifying K current (IK, IR) which is highly sensitive to the class III antiarrhythmic E-4031. IK, IR recorded in GH3/B6 cells and ERG currents in CHO cells were compared using similar experimental conditions (same pulse protocols and isotonic KCl as extracellular solution). The voltage- and time-dependent properties of both currents were found to be almost identical. These results strongly suggest that RERG channels mediate IK, IR in GH3/B6 cells.

The class III antiarrhythmic agent E-4031 selectively blocks the inactivating inward-rectifying potassium current in rat anterior pituitary tumor cells (GH3/B6 cells)

Hyperpolarization-elicited potassium currents in GH3/B6 cells bathed in high-potassium external solution were recorded to assess effects of the class III antiarrhythmic agent E-4031 on the inactivating inward-rectifying potassium current (IK,IR). E-4031 potently blocked IK,IR with an IC50 value of 10 nM. The complete block of IK,IR achieved with concentrations >/= 1 microM revealed the presence of a non-inactivating outward-rectifying current which contributed to the membrane currents recorded under control conditions. The time dependence of the IK,IR block depended on the concentration of E-4031. Two other methanesulfonanilides were investigated: WAY-123,398 (10 microM) also totally blocked IK,IR, while sotalol (100 microM) was almost ineffective. Also lanthanum (100 microM) had only a very small effect on IK,IR. E-4031 did not affect sodium, calcium and voltage-dependent outward-rectifying potassium currents, suggesting a selective block of IK,IR in GH3/B6 cells. In an external solution containing 16 mM potassium, the E-4031-sensitive current was present as a steady outward current within a broad potential range positive to the potassium equilibrium potential, EK. In many, but not all, cells E-4031 induced an increase in the frequency of action potentials suggesting an important role of IK,IR in controlling cell excitability. Our experiments show that E-4031 is a valuable tool in characterizing IK,IR and its physiological function.

Most lactotrophs from lactating rats are able to respond to both thyrotropin-releasing hormone and dopamine

Intracellular free calcium concentration ([Ca2+]i) was measured with video imaging in lactotrophs from lactating rats. The median resting [Ca2+]i was 24 nM (85 cells). The great majority of cells responded to thyrotropin-releasing hormone (TRH) with an increase in [Ca2+]i, (median peak [Ca2+]i after TRH = 298 nM; n = 73). In 77% of these cells this [Ca2+]i increase was biphasic, with [Ca2+]i remaining high after the initial peak (median [Ca2+]i 90 s after TRH application = 104 nM; n = 56); the second phase depended on calcium influx. Most cells also responded to dopamine (DA), after TRH had been applied. DA reduced or abolished TRH-induced calcium influx and also reduced resting [Ca2+]i if this was above its initial value. A few lactotrophs responded to TRH only after DA application and withdrawal. We conclude that the population of lactotrophs in lactating rats is heterogeneous, but is not composed of two distinct sub-groups defined by their responsiveness to TRH or DA.

An endogenous inactivating inward-rectifying potassium current in oocytes of Xenopus laevis

An endogenous inward-rectifying K+ current is described, which is present in native oocytes of some Xenopus laevis donors. Experiments were performed using defolliculated oocytes from donor frogs obtained from two different suppliers. In all oocytes from animals from one source, an inward-rectifying K+ current could be elicited with negative pulses from a holding potential of -20 mV in external solutions with a high K+ concentration. Increasing external K+ concentrations increased the amplitude of this current and shifted the reversal potential towards more positive potentials. In 118 mM KCl, the inward-rectifying K+ current partially inactivated between -20 and -80 mV and completely inactivated at more negative membrane potentials; 50% steady-state inactivation occurred near -50 mV. The time course of inactivation of the inward-rectifying current could be well fitted with two exponentials. The slow time constant had values of about 500 ms and was voltage independent. In contrast, the fast time constant and the time to reach the peak inward current decreased with more negative membrane potentials. Ba2+, Cs+, quinine (all 5 mM) and 50 mM tetraethylammonium partially blocked the inward-rectifying K+ current, whereas 10 mM 4-aminopyridine was without blocking effect. The oxidant chloramine-T blocked the inward-rectifying K+ current without slowing its inactivation.

An inactivating inward-rectifying K current present in prolactin cells from the pituitary of lactating rats

Primary cultures containing a high percentage of lactotrophs were obtained by dissociating the pituitary of rats following 14-18 days of lactation. Lactotrophs with a distinctive appearance were recorded after 1-35 days in vitro and identified by immunocytochemical staining for prolactin. Whole-cell voltage clamp measurements in isotonic KCl solution from a holding potential of -40 mV revealed the presence of inward-rectifying K currents with a time-dependent, Na(+)-independent inactivation at potentials negative to -60 mV. The time for complete inactivation was strikingly different between lactotrophs, varying between 1 sec and more than 5 sec at -120 mV, and was not related to time in culture. The reversal potential shifted 59 mV (25 degree C) for a tenfold change in external K+ concentration, demonstrating the selectivity of the channel for K+ over Na+. The inward-rectifying K current was blocked by 5 mM Ba2+ and partially blocked by 10 mM TEA. Chloramine-T (1 and 2 mM) produced a total block of the inward-rectifying K current in lactotrophs. Thyrotropin-releasing hormone (500 nM) significantly reduced the inward-rectifying K current in about half of the lactotrophs. This current is similar to the inward-rectifying K current previously characterized in clonal somatomammotrophic pituitary cells (GH3B6). The variability of the rate of inactivation of this current in lactotrophs and its responsiveness to TRH is discussed.

Cloning, functional expression and mRNA distribution of an inwardly rectifying potassium channel protein

In GH3/B6 cells at least two different inward K+ currents are observed that are regulated by thyrotropin-releasing hormone and somatostatin, respectively. Using a polymerase chain reaction based approach a cDNA was isolated and functionally expressed in human embryonic kidney cells that encodes an inward rectifier K+ channel, rIRK3, with a predicted molecular mass of 49.7 kDa. Corresponding transcripts of 2.6 kb have been detected in rat brain, pituitary and GH3/B6 cells. In situ hybridization revealed that rIRK3 mRNA is distributed throughout the brain and occurs predominantly in the piriform cortex, indusium griseum, supraoptic nucleus, facial nucleus and cerebellar Purkinje cells.

Action potentials and membrane currents in the human node of Ranvier

Action potentials and membrane currents were recorded in single human myelinated nerve fibres under current- and voltage-clamp conditions at room temperature. Nerve material was obtained from patients undergoing nerve graft operations. Successful recordings were made in 11 nerve fibres. In Ringer's solution, large transient Na currents were recorded, which could be blocked completely with tetrodotoxin. Partial block of these currents with 3 nM tetrodotoxin was used to reduce the voltage-clamp error due to series resistance. Outward K currents were very small in intact nerve fibres, but had a large amplitude in fibres showing signs of paranodal demyelination. In isotonic KCl, the K current could be separated into three components: two fast components (Kf1 and Kf2) and one slow component (Ks). Time constants and steady-state activation and inactivation of Na permeability and of fast and slow K conductance were measured within the potential range of -145 mV to +115 mV. From these parameters, the corresponding rate constants were calculated and a mathematical model based on the Frankenhaeuser-Huxley equations was derived. Calculated action potentials closely matched those recorded. Single calculated action potentials were little affected by removing the fast or slow K conductance, but the slow K conductance was required to limit the repetitive response of the model to prolonged stimulating currents.

Different G proteins are involved in the biphasic response of clonal rat pituitary cells to thyrotropin-releasing hormone

In rat anterior pituitary tumour cells (GH3/B6) thyrotropin-releasing hormone (TRH) elicits a biphasic response. First, a release of intracellularly stored Ca2+ induces a hyperpolarization of the cell. Second, a depolarization thought to be induced by a reduction of the inward-rectifying K+ current (KIR) causes an increase in action potential frequency and a plateau-like increase in [Ca2+]i. It has been proposed that the two phases are induced by the actions of inositol 1,4,5-trisphosphate (InsP3) and protein kinase C (PKC), respectively, but we demonstrate here that PKC is not responsible for the second phase increase in [Ca2+]i and suggest that the pathways diverge at the level of receptor and G protein coupling. Both phases of the TRH response were insensitive to pertussis toxin, but cholera toxin (CTX) selectively affected the second phase. After CTX pretreatment cells had a high spontaneous spiking frequency and smaller KIR amplitude. In these cells TRH failed to increase the action potential frequency after the first phase hyperpolarization, elicited only a transient peak increase in [Ca2+]i with no plateau phase and could only slightly reduce KIR. These effects of CTX are not mediated by its ability to increase cAMP via activation of GS, as increased cAMP levels neither inhibit KIR nor prevent its reduction by TRH. In addition, inhibition of protein kinase A activation did not block the second phase increase in [Ca2+]i induced by TRH, suggesting that the CTX-sensitive G protein mediating the second phase of the TRH response is not GS.

Effects of Buthus martensii (Karsch) scorpion venom on sodium currents in rat anterior pituitary (GH3/B6) cells

The effects of two fractions (II, containing anti-insect toxins, and III, containing eight anti-mammal toxins) isolated from the venom of the Old World scorpion Buthus martensii (Karsch) on Na+ currents of rat anterior pituitary cells (GH3/B6 cells) were investigated using the whole-cell configuration of the patch clamp technique. Fraction II induced a temporary, and fraction III a permanent increase of the Na+ current amplitude. Application of each of the venom fractions resulted in a flattening of the curve relating steady state Na+ inactivation to membrane potential. In addition, the two fractions had specific effects. Fraction II shifted the voltage dependence of Na+ current activation by -42 mV, and the voltage dependence of Na+ inactivation by -25 mV in the absence of a conditioning depolarizing pre-pulse. Slowing of Na+ inactivation was most prominent at negative membrane potentials, resulting in a steady Na+ inward current at the holding potential of -80 mV. Fraction III induced a pronounced slowing of Na+ inactivation leading to an increase of peak Na+ currents and to incomplete steady state Na+ inactivation even at positive membrane potentials.

A Rat Pituitary Tumour K(+) Channel Expressed in Frog Oocytes Induces a Transient K(+) Current Indistinguishable from that Recorded in Native Cells

A voltage-gated K(+) channel protein has been cloned from a cDNA library derived from poly(A)(+) RNA of the rat pituitary tumour cell line GH(3) /B(6) by the polymerase chain reaction technique. The clone referred to as RGHK9 encodes a protein sequence very similar to a recently cloned K(+) channel protein from rat brain and heart, with deviations in a few amino-acid positions. In situ hybridization experiments show that RGHK9 mRNA is also present in the anterior pituitary as well as in other brain regions and that it is particularly abundant in the hippocampus. After injection of cRNA transcribed from the RGHK9 cDNA clone into Xenopus oocytes, the expressed protein induces a transient K(+) current. Except for the activation kinetics the properties of this current are indistinguishable from that of the native transient K(+) current measured in GH(3) /B(6) cells, e.g. both K(+) currents are blocked by 4-aminopyridine and show the same voltage dependence and slope of steady state activation and inactivation as well as identical time constants of, and slow recovery from, inactivation. Taken together, these data show that the outward-rectifying voltage-gated K(+) channel protein encoded by the RGHK9 cDNA correlates well in its functional properties with that of a very similar, if not identical, K(+) channel present in GH(3) /B(6) cells.

Completely functional double-barreled chloride channel expressed from a single Torpedo cDNA

We have performed an electrophysiological analysis of the recently cloned Torpedo marmorata Cl- channel. Functional expression of Cl- channels in oocytes of Xenopus laevis previously injected with cRNA yielded an outward-rectifying current activated by hyperpolarization. Replacement of Cl- with other anions significantly reduced or inhibited the current. Single-channel recordings from cell-attached patches exhibited burst-like Cl- channel activity with rapid fluctuations between three equally spaced substates (0 pS, 9 pS, and 18 pS). The properties of the cloned Cl- channel were almost identical to those of the reconstituted native T. californica Cl- channel and were in full agreement with the predictions of the double-barreled channel model [Hanke, W. & Miller, C. (1983) J. Gen. Physiol. 82, 25-45]. Our results imply that the cloned cDNA codes for the completely functional Torpedo electroplax Cl- channel.

Inositol tetrakisphosphates as second messengers induce Ca(++)-dependent chloride currents in Xenopus laevis oocytes

Microinjection of inositol 1,3,4,5-tetrakisphosphate or inositol 1,4,5-trisphosphate induced distinct chloride membrane currents in defolliculated Xenopus laevis oocytes. To decide whether these Cl(-)-currents were due to the injected compounds or their metabolic products, [3H]Ins(1,3,4,5)P4 or [3H]Ins(1,4,5)P3 were injected into oocytes and their metabolites were analyzed by HPLC. Our results indicate that Ins(1,3,4,5)P4 itself or its metabolite Ins(1,3,4,6)P4 is able to induce Cl(-)-membrane currents, most likely by increasing the cytosolic Ca(++)-concentration.

Two types of fast K+ channels in rat myelinated nerve fibres and their sensitivity to dendrotoxin

The effect of dendrotoxin (DTX), a component of the venom of the Eastern green mamba snake, Dendroaspis angusticeps, on K+ currents in rat myelinated nerve fibres was studied in voltage clamp experiments, immunocytochemistry and binding experiments. The analysis of K+ tail currents in 160 mM KCl solution revealed that K+ channels with slow gating kinetics predominate in the intact node of Ranvier. These slow K+ channels were not blocked by DTX. Intact nerve fibres additionally showed fast K+ tail currents of small amplitude which could be blocked by DTX. After enzymatic demyelination with pronase, fast K+ currents of large amplitude appeared. Analysis of the non-monotonous voltage dependence of the fast K+ conductance and the partial pharmacological block by DTX suggest the presence of two subtypes of fast K+ channels in rat nerve fibres similar to the Kf1 and Kf2 channels previously described in the frog and toad node of Ranvier. The DTX concentration required for 50% inhibition (IC50) for the Kf1 component was 8 nM. The IC50 of the blocked Kf2 component was the same as that for Kf1, but the Kf2 component was only partially blocked (about 50%). In contrast to frog nerve, these two fast K+ channel subtypes are located predominantly in the paranodal region. Immunocytochemical staining experiments with DTX using the peroxidase-antiperoxidase technique confirmed the electrophysiological data. In intact nodes, either no staining or only slight staining in some fibres was found. After demyelination, extensive staining of paranodal and internodal regions occurred.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes of ionic channel distribution in myelinated nerve fibres from rats with experimental allergic neuritis

Voltage clamp experiments were performed in single myelinated nerve fibres of rats with experimental allergic neuritis (EAN). Nerve fibres from diseased animals showed a varying degree of demyelination. In fibres with extensive demyelination large K+ currents were measured. Analysis of the contribution of slow and fast K+ channels revealed that in these fibres the relative contribution of slow K+ channels was about 50% compared to about 25% in fibres acutely demyelinated as previously reported. This indicates that chronic demyelination changes the distinct distribution of ionic channels in the rat node of Ranvier.

An inward-rectifying K+ current in clonal rat pituitary cells and its modulation by thyrotrophin-releasing hormone

1. Voltage-dependent K+ currents were recorded in cultured tumour-derived anterior pituitary cells of the rat (GH3 cells) with the patch clamp technique. An inward-rectifying current is described which is found to be carried by K+. 2. In isotonic KCl, whole-cell inward K+ currents were elicited by hyperpolarizing pulses from a holding potential of -40 mV. These inward K+ currents showed time- and voltage-dependent inactivation at potentials more negative than -60 mV. Inactivation was faster and more complete at larger hyperpolarizations. Recovery from inactivation was also time- and voltage-dependent. It was faster and more complete with more positive potentials. Time course of inactivation and of recovery from inactivation could be fitted by single exponentials. 3. Two results showed that a steady inward K+ current is present at -40 mV. The holding current at -40 mV was reduced following complete inactivation of the inward K+ current during strong hyperpolarizing pulses, and the amplitude of maximum inward K+ current elicited by hyperpolarization increased after depolarizing pre-pulses of 5 s. The resting conductance was estimated to be 20-30% of the maximum inward-rectifying conductance. 4. The inward K+ current was drastically reduced by Cs+ and Ba2+, but not by Ni2+ and Co2+. Quinidine, 4-aminopyridine and tetraethylammonium chloride blocked the current. In contrast, dendrotoxin was without effect. 5. Thyrotrophin-releasing hormone (TRH) which induces biphasic secretion of prolactin in GH3 cells consistently reduced the inward K+ current in the presence of internal Ca2+. This reduction was abolished if the pipette solution contained guanosine 5'-O-(2-thiodiphosphate) (GDP beta S; 400 microM), confirming the involvement of G-proteins in the signal transduction pathway. 6. TRH shifted the voltage-dependence of inward K+ current inactivation to less negative potentials resulting in pronounced K+ current inactivation in the range of the resting potential of these cells (-40 to -60 mV). 7. In intact cells, closing of K+ channels would result in a depolarization. The existence of an inward-rectifying K+ current in GH3 cells which is able to be reduced by TRH could readily explain the TRH-induced increase in action potential firing underlying the sustained second phase of secretion.

Heterogeneous distribution of fast and slow potassium channels in myelinated rat nerve fibres

1. Potassium currents were measured in voltage-clamped single myelinated rat nerve fibres before and after paranodal demyelination with 0.2% pronase or 0.2% lysolecithin added to the external solution. Sodium currents were blocked by 300 nM-tetrodotoxin. For the purpose of comparison, intact frog nerve fibres were also investigated. 2. Our results suggest the existence of at least two distinct types of K+ channels in the intact node of Ranvier, one with slow and another with fast gating kinetics, in the ratio 4:1. 3. In the rat nodal membrane, slow K+ channels have voltage-dependent time constants of K+ deactivation with tau n = 68 ms at E = -105 mV and tau n = 26 ms at E = -150 mV at 20 degrees C. The activation curve of the slow K+ conductance is sigmoid with an inflexion point at -60 mV. This means that about 35% of the slow K+ channels are in the open state at the resting potential of -77 mV. Slow K+ channels could be blocked by 10 mM-tetraethylammonium chloride, but were insensitive to 4-aminopyridine. 4. After paranodal demyelination the ratio of fast to slow K+ channels increased from 17 to 83%. As in the frog (Dubois, 1981 alpha), the population of fast K+ channels in the rat may consist of two different subgroups, both of which can be blocked by 4-aminopyridine. 5. Demyelination was accompanied by an increase in the capacity current which was used to estimate the exposed membrane area. The density of slow and fast K+ channels was calculated from the quotient of the steady-state K+ conductance to membrane area. The density of the slow K+ channels is maximal in the nodal membrane and decreases to 1/31 in the internode. By contrast, the distribution of the fast K+ channels differs, their density being maximal in the paranode and decreasing to one-sixth in the node and internode.

Phenytoin and carbamazepine: potential- and frequency-dependent block of Na currents in mammalian myelinated nerve fibers

Voltage clamp experiments were performed in single myelinated nerve fibers of the rat and the effects of phenytoin (PHT) and carbamazepine (CBZ) on the ionic membrane currents were studied. PHT and CBZ are almost selective blockers of Na channels. The main part of this inhibition is a potential-dependent block which can be removed by hyperpolarization. The dose-response curve of PHT was described as a first-order reaction with Kd = 37 microM; 100 microM CBZ was equally as effective 100 microM PHT. Both drugs shift the steady-state Na inactivation curve (h infinity (V] to more negative membrane potentials and decrease its steepness. PHT and CBZ have equimolar effects on the shift and decrease in steepness of h infinity (V). All of the drug-induced effects depend on drug concentration. Both drugs induce a slowing of recovery from Na inactivation. PHT has a stronger slowing effect than CBZ. From this ensues a frequency-dependent block, which is more pronounced in the presence of PHT than of CBZ. The effects of both drugs can be interpreted in the framework of the modulated receptor hypothesis.

Modulation of Neuropeptide-lnduced Membrane Currents by Protein Kinase C in Xenopus Oocytes Injected with GH Pituitary Cell Poly(A) RNA

Abstract Protein kinase C was activated in Xenopus laevis oocytes by phorbol ester treatment and its effects on the inositol trisphosphate/Ca(2+) transmembrane signalling pathway analysed. Induction of the pathway was achieved by ligand stimulation of TRH receptors translated from GH(3) pituitary cell mRNA. In voltage-clamped oocytes bath application of peptide, injection of guanosine 5'-(3-O-thio) triphosphate (GTPgammaS), inositol trisphosphate or Ca(2+) all elicited inward membrane currents. Treatment of oocytes with tumour-promoting phorbol esters for 35 min almost completely abolished the ligand and GTPgammaS-induced responses. In contrast, phorbol ester treatment enhanced inositol trisphosphate-generated membrane currents. Ca(2+)-mediated responses remained unaffected by tumour promoters. The data indicate a dual role for protein kinase C in the modulation of transmembrane signalling: a feedback mechanism prevents phosphoinositide turnover whereas a feedforward reaction triggers the effect of intracellular inositol trisphosphate on the Ca(2+) release.

Tocainide blocks Na currents by accumulation of inactivated Na channels

The effect of tocainide on the voltage-dependent parameters of Na permeability was studied in voltage clamp experiments performed in single myelinated rat and frog nerve fibres. The main effect of tocainide was a potential- and frequency-dependent block of Na channels in both species. There was no shift to the voltage-dependent parameters of steady state Na activation (m infinity) or of the time constants of Na activation (tau m) and inactivation (tau h) along the potential axis.

[Membrane currents and action potentials of myelinated nerve fibers and their modification by diphenylhydantoin and tocainide]

A comparison of the ionic currents in single myelinated nerve fibres isolated from the sciatic nerve of the rat and the frog was performed. The nodal membrane of rats lacked almost totally potential dependent potassium channels, whereas in the frog large potassium currents were recorded. Therefore, it was concluded that in mammalian nerve fibres repolarization of the action potential is brought about by the unspecific leakage current and the inactivation of the sodium channels. Phenytoin and tocainide reduced the Na current in nerve fibres of both species. This reduction was due to a potential and frequency dependent block of sodium channels. 50 microM phenytoin and 200 microM tocainide shifted the inactivation curve of the sodium system by 25 mV to more hyperpolarizing membrane potentials. In addition, a decrease in the time interval between repetitive depolarizations increased the amount of Na current reduction. This suggested that in the presence of the drugs the equilibrium between resting and inactive sodium channel states was shifted to more inactivation. The results were successfully interpreted in terms of the "modulated receptor hypothesis".

Na currents and action potentials in rat myelinated nerve fibres at 20 and 37 degrees C

(1) Action potentials and membrane currents were recorded in single myelinated rat nerve fibres at 20 and 37 degrees C. Three experiments were also performed in single cat nerve fibres. (2) K currents were blocked by internal CsCl and external TEA. The steady state and kinetic parameters of Na activation and inactivation were determined at both temperatures. (3) When the temperature was raised from 20 to 37 degrees C, steady state Na activation, m infinity (V), and inactivation, h infinity (V), did not change significantly. (4) The time constant of Na activation, tau m, was determined within the potential range of -40 to 125 mV at 20 degrees C and V = 40-60 mV at 37 degrees C. The temperature coefficient, Q10, of tau m was 2.2. (5) The decay in the Na current was described by two exponentials at both temperatures. The amplitude of the slow phase was 1-10%. The time constant of the fast phase of Na inactivation, tau h1, was determined at both temperatures within the potential range of -50 mV to 125 mV. The Q10 of tau h1 was 2.9 and did not depend on potential. (6) The Na equilibrium potential was 152 mV at 20 degrees C and 144 mV at 37 degrees C. The leakage conductance was 24 nS at 20 degrees C and 43 nS at 37 degrees C. These differences were interpreted as signs of fibre deterioration at higher temperature. (7) The results from the current and voltage clamp experiments performed in the cat nerve were essentially the same as those in the rat nerve fibres.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological comparison of insecticide and alkaloid agonists of Na channels

Macroscopic currents in Na channels were recorded from adult frog skeletal muscle under voltage clamp as various toxins were added to the bathing medium. Veratridine, cevadine, and 3-(4-ethoxybenzoyl)-veracevine modified the Na channels in a use-dependent manner during depolarizations and held them open for 3, 2.4, and 1.2 s, respectively, at -90 mV. The three alkaloids modified channels in the same way. Activation gating was shifted about -100 mV by the modification, and reversible closing of the channels by strong hyperpolarizations slowed reversal of the modification. The synthetic insecticides deltamethrin, EDO, GH739, and GH414 also modified channels during depolarizations that opened channels. The modification lasted 3 s with deltamethrin, but only 3-5 ms with the others. Hyperpolarization speeded the shutting off of current in insecticide-modified channels, but no reversible activation gating could be demonstrated. The ionic selectivity, PNa/PNH4, of channels was decreased by all of the toxins. This ratio was 0.11 in normal channels, 0.26 in insecticide-modified channels, and 0.7-1.6 in veratrum-alkaloid-modified channels. During use-dependent modification, the veratrum alkaloids reduced the total Na current markedly, while deltamethrin did not. Thus, alkaloid and insecticide modifications share many features but differ in how much the conducting properties of the pore are changed and whether the channel can close reversibly while the toxin remains bound.

A comparison of sodium currents in rat and frog myelinated nerve: normal and modified sodium inactivation

1. Sodium currents were measured under voltage-clamp conditions in Ranvier nodes of rat and frog nerve fibres at 20 degrees C. Voltage errors due to the resistance in series with the nodal membrane were minimized by reducing sodium currents with tetrodotoxin in the extracellular solutions. 2. The stationary and kinetic properties of sodium activation and inactivation were determined for a wide range of potentials (V) from -40 to 160 mV with respect to the initial holding level (V = 0 mV). 3. The curves m infinity(V) and h infinity(V) of stationary sodium activation and inactivation were not different in rat and frog fibres. 4. The time constants tau m, tau h of sodium activation and inactivation were normally larger in the rat than in the frog. At moderate depolarizations (0 less than or equal to V less than or equal to 80 mV) tau m in the rat was 15-50% larger; the ratio of the rat to the frog tau h values was usually smaller. Thus tau m/tau h = 0.116 for the rat and 0.0965 for the frog at V = 60 mV (potential with maximum peak sodium inward current). 5. Sodium inactivation in rat nerve was slowed and became incomplete by application of intra-axonal iodate or by treatment with external Anemonia toxin II (ATX II), chloramine-T or Ruthenium Red. Peak sodium currents were not increased by these substances. 6. Wash-out of ATX II from frog nerve was rapid and complete but partly irreversible in rat nerve. This suggests different properties or accessibilities of sodium channels in frog and rat nodes.

The effect of temperature on Na currents in rat myelinated nerve fibres

Voltage clamp experiments were performed in single myelinated nerve fibres of the rat and the effect of temperature on Na currents was investigated between 0 degrees C and 40 degrees C. The amplitude of the peak Na current changed with a Q10 = 1.1 between 40 degrees and 20 degrees C and with a Q10 = 1.3 between 20 degrees and 10 degrees C. Below 10 degrees C the peak Na current changed with a Q10 = 1.9. The temperature coefficient for time-to-peak (tp), the measure for Na activation, and tau h1 and tau h2, the time constants for Na inactivation changed throughout the temperature range. Q10 for all of these kinetic parameters increased from 1.8-2.1 between 40 degrees and 20 degrees C to 2.6-2.7 between 20 degrees and 10 degrees C. Below 10 degrees C Q10 increased to 3.7 for tau h1 and tp, and to 2.9 for tau h2. When the series resistance artifacts were minimized by addition of 6 nM TTX, the Q10's at T less than 10 degrees C were 2.9-3.0. When the temperature was decreased from 20 degrees to 0 degrees C, both the curve relating Na permeability to potential, PNa(V), and the steady state Na inactivation curve, h infinity (V), were reversibly shifted towards more negative potentials by 6 mV and 11 mV, respectively. When the temperature was increased from 20 degrees to 37 degrees C no shifts occurred. The Hodgkin-Huxley rate constants alpha h(V) and beta h(V) were calculated from h infinity (V) and tau h (or tau h1) at 20 degrees and 4 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Potassium permeability in rat myelinated nerve fibres

Potential clamp experiments were performed at 21-23 degrees C on single myelinated rat nerve fibres isolated from the sciatic nerve. A small delayed K-permeability (PK) was regularly identified in the normal rat node amounting to 0.067 cm3 s-1 X 10(-9) at large positive potentials. Measurements in isotonic KCl showed that about two-thirds of PK was already activated at resting potential. Substitution of Cs for axoplasmic K demonstrated that the rat node had large leak currents (33 nS). The leak current provided the main part of the repolarizing outward current. 10 mM tetraethylammonium chloride (TEA) slowly (tau approximately equal to 2 min) and incompletely (50%) blocked both PK activated at resting potential and the delayed increase in PK. I mM 4-aminopyridine (4-AP) had less effect on PK.

Dantrolene: evidence for effects on Na permeability properties of the nodal membrane

The effects of dantrolene on myelinated frog nerve fibers were studied in voltage clamp experiments. Dantrolene shifted the potential-dependent parameters describing Na+ permeability towards more negative membrane potentials. The findings are interpreted as a change in the negative surface charge of the membrane.

Development of Na inactivation in motor and sensory myelinated nerve fibres of Rana esculenta

Development of Na permeability inactivation was investigated in myelinated motor (N = 12) and sensory (N = 12) nerve fibres of Rana esculenta at 20 degrees C. The K currents were blocked by 10 mM tetraethylammonium chloride, added to the superfusing solution. Additionally, in 4 fibres of each group internal CsCl was applied by diffusion from the cut internodes. Development of Na inactivation was approximated by the sum of two exponentials. The time constants of the fast and slow inactivation phase (tau h1 and tau h2) were dependent on membrane potential (E) with similar values in both fibre types for a given E. In contrast, significant differences were found in the contribution of both phases. In motor nerve fibres the amplitude of the fast phase was g = 0.70 (mean value of 12 fibres) throughout the potential range investigated (-30 mV less than or equal to E less than or equal to 40 mV). In sensory fibres g was potential dependent, increasing from 0.76 (mean value of 12 fibres) at E = -30 mV to unity at E greater than 30 mV. This difference in Na permeability inactivation is a further distinguishing property between motor and sensory nerve fibres.

Flurazepam: effects on sodium and potassium currents in myelinated nerve fibres

The effect of flurazepam-HCl on single myelinated nerve fibres of the frog Rana esculenta was investigated. Flurazepam affected both Na and K currents: 0.25 mM of the drug decreased the peak Na inward current to about 50%. The initial increase and subsequent decay of the Na current was slowed down by a factor of 1.5 independent of membrane potential. The drug induced a slow phase in the recovery from Na inactivation and frequency dependence of the Na current block. The K current rose at a normal rate and was then inactivated to a sustained outward current. The time constant of block development (tau k) and the steady state block were potential-dependent. With 1 mM flurazepam, tau k decreased from 2.9 ms at E = 10 mV to 1.5 ms at E = 90 mV, and the steady state block increased from 65% at E = -20 mV to 81% at E = 90 mV. Recovery from the block proceeded faster at E = -70 mV (tau = 27 ms) than at E = -120 mV (tau = 89 ms). The effects of the drug on the K current were interpreted in terms of the reaction scheme proposed by Armstrong (J. Gen. Physiol. 54, 553; 1969).

Oscillating repolarization in action potentials of frog sensory myelinated nerve fibres

Current and voltage clamp experiments were performed in single myelinated sensory nerve fibres of Rana esculenta. The K current was blocked by external tetraethylammonium-chloride and internal CsCl. Negative prepotentials led to the formation of a plateau in the repolarization phase of the action potential, and further regenerative depolarizations emerged from this plateau. A three-state model for Na inactivation based on voltage clamp data was sufficient to simulate these observations.

A three-state model for inactivation of sodium permeability

The inactivation of Na+ permeability in single myelinated motor nerve fibres of Rana esculenta was investigated under voltage and current clamp conditions at 20 degrees C in Ringer's solution and under blocked K+ currents. Development of inactivation and its recovery was described by two potential-dependent time constants: The smaller time constant followed the usual bell-shaped function of membrane potential, whereas the larger one was monotone-increasing with more negative potentials. Several three-state models for inactivation were investigated. The experiments could best be approximated by a model with two open and one closed state for inactivation following: open in equilibrium closed in equilibrium open. Rate constants were determined for all transitions shown from the voltage clamp experiments. The action potentials computed by means of the proposed model were in good agreement with those measured, both in Ringer's solution and under blocked K+ current conditions.

Short-Term Effects of South Louisiana and Kuwait Crude Oils on Glucose Utilization by Marine Bacterial Populations

Two crude oils, South Louisiana and Kuwait, were examined for their impact on glucose utilization by bacterial populations from the Gulf of Mexico. The uptake and mineralization of [ U - 14 C]glucose was assayed after a 4- to 23-h exposure to various concentrations of added crude oil (0, 0.001, 0.01, and 0.1% [vol/vol]). The effects of oil were determined in a total of 15 sediment and 13 water samples collected from offshore, open-bay, and salt marsh environments. The utilization of glucose by bacterial populations usually was not affected by added oil; in 10 sediment and 11 water samples, oil had no significant effect on either glucose uptake or mineralization. Stimulation by oil was recorded in four sediment samples. Oil inhibition occurred in one sediment and two water samples, but only in the presence of the highest concentration of added oil, i.e., 0.1%. Our data suggest that short-term exposure to either South Louisiana or Kuwait crude oil, even at 0.1%, usually has no toxic effect on glucose utilization by marine bacterial populations.

Phenobarbital induces slow recovery from sodium inactivation at the nodal membrane

Voltage-clamp experiments were performed on single myelinated nerve fibres of Rana esculenta at 20 degrees C in Ringer's solution and in solutions containing phenobarbital-sodium ([PB] less than or equal to 5 mM). The reduction of the sodium current under phenobarbital could be explained by an increase in the resting sodium inactivation; h infinity (E) was shifted towards more negative membrane potentials. The recovery from sodium inactivation proceeded with two time constants. The fast process could be described with the same time constant as in Ringer's solution, whereas the slow process had a time constant approx. 40 times larger. The slow process was also potential-dependent and could be described by 1/(0.025 alpha h + beta h), where alpha h and beta h denoted the rate constants in Ringer's solution. With the measured blockage of sodium channels by phenobarbital, both the shift of h infinity (E) and the slow recovery from sodium inactivation could be explained.

The mode of action of phenobarbital on the excitable membrane of the node of Ranvier

Single myelinated nerve fibres of Rana esculenta were investigated under current and potential clamp conditions at 20 degrees C. Under 2.5 mM phenobarbital, the amplitude of the action potential was reversibly reduced to 85.5 +/- 5% (n = 6), and the threshold potential was raised by 32% of the control in Ringer solution. The resting potential remained constant. Solutions with 2.5 mM phenobarbital caused a decrease of the Na current to 38.3 +/- 5.6% (n = 10), when the fibre had its holding potential before the test step. The effect was reversible at wash out of the drug. The Na currents were only negligibly decreased (7 +/- 4.5%; n = 10) when the test pulse was preceded by a long lasting negative polarization to Em = -140 mV. The effect of the conditioning polarization could be described by two time constants, tau 1 = 7.1 +/- 2.0 msec and tau 2 = 44.5 +/- 9.5 msec (n = 5). Experiments with 500 msec conditioning pulses showed that the Na inactivation curve, h infinity(Em), was shifted in a negative direction along the potential axis. In the range between 0.5 and 5.0 mM phenobarbital there was a shift of 8 mV for an e-fold change in drug concentration. 15 mM Ca2+ caused a shift to the h infinity(Em) curve in a positive direction along the potential axis, while simultaneous application of 2.5 mM phenobarbital and 15 mM Ca2+ caused no shift of the h afinity(Em) curve. The undissociated drug seemed to be responsible for the effects (pK = 7.3).

The diagnostic value of the short sleep EEG and other provocative methods following sleep deprivation

One hundred and eighty-five EEGs recorded after deprivation of sleep for 24 h were evaluated. Valuable diagnostic information was found in 59% of the EEG recordings; 24% of the EEGs contained seizure activity. The duration of the stages of sleep and the frequency of seizure activity, paroxysmal sharp wave groups and localizing findings were analyzed. The sleep stages A to C (based on the Loomis scale) were reached for about equal duration by an EEG recording of 30--40 min; sleep stage D was reached only shortly and stage E was not observed. Pathological EEG findings appeared for the most part in the sleep stages A and B. Localized findings were pronounced in stage C. No significant differences pertaining to the occurrence and form of EEG patterns were found between patient groups with primary generalized seizures, psychomotor seizures or those with unclarified disturbances of consciousness. The combination of the short sleep EEG following 24 h of sleep deprivation with subsequent use of the additional provocative methods of hyperventilation, photostimulation and hydration, yielded, in all, new information in 50% of the patients. Each of these additional methods contributed nearly equally to this information.