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M Stocker

Publications and source records attributed to M Stocker.

At least 19 recordsLinked to original sources
PubMed2000

Molecular determinants of Ca2+-dependent K+ channel function in rat dorsal vagal neurones.

Using in situ hybridisation histochemistry in combination with patch-clamp recordings and specific pharmacological tools, the molecular nature of the channels underlying Ca2+-dependent K+ currents was determined in dorsal vagal neurones (DVNs) of rat brainstem slices. In situ hybridisation analysis at cellular resolution revealed the presence of 'big'-conductance Ca2+- and voltage-activated K+ (BK) channel alpha-subunit mRNA, and of only one 'small'-conductance Ca2+-activated K+ (SK) channel subunit transcript, SK3, at very high levels in DVNs. By contrast, SK1 and SK2 mRNAs were below the threshold limit of detection. The SK channel-mediated after-hyperpolarising current (IAHP) was blocked by apamin with a half-maximal inhibitory concentration of approximately 2.2 nM. This is consistent with homomultimeric SK3 channels mediating IAHP in DVNs. IAHP was also blocked by scyllatoxin (20-30 nM) and curare (100-200 microM). Application of apamin (100 nM) or scyllatoxin (20 nM) invariably caused a substantial increase to 146.1 +/- 10.4 and 181.8 +/- 12.9 % of control, respectively, in the spontaneous firing rate of DVNs. Action potential duration was not affected by these SK channel blockers. The selective BK channel blocker iberiotoxin (50 nM) increased action potential duration by 22.5 +/- 7.3 %, as did low concentrations of tetraethylammonium (0.5 mM; 99.3 +/- 16.4 %) and the Ca2+ channel blocker Cd2+ (100 microM; 49.5 +/- 20.9 %). BK channel blockade did not significantly affect the firing rate of DVNs. These results allow us to establish a tight correlation between the properties of cloned and native BK and SK channels, and to achieve an understanding, at the molecular level, of their role in regulating the spontaneous firing frequency and in shaping single action potentials of central neurones.

Action Potentials↗
PubMed2000

Single amino acid substitutions in kappa-conotoxin PVIIA disrupt interaction with the shaker K+ channel.

kappa-Conotoxin PVIIA (kappa-PVIIA), a 27-amino acid peptide with three disulfide cross-links, isolated from the venom of Conus purpurascens, is the first conopeptide shown to inhibit the Shaker K(+) channel (Terlau, H., Shon, K., Grilley, M., Stocker, M., Stühmer, W., and Olivera, B. M. (1996) Nature 381, 148-151). Recently, two groups independently determined the solution structure for kappa-PVIIA using NMR; although the structures reported were similar, two mutually exclusive models for the interaction of the peptide with the Shaker channel were proposed. We carried out a structure/function analysis of kappa-PVIIA, with alanine substitutions for all amino acids postulated to be key residues by both groups. Our data are consistent with the critical dyad model developed by Ménez and co-workers (Dauplais, M., Lecoq, A., Song, J. , Cotton, J., Jamin, N., Gilquin, B., Roumestand, C., Vita, C., de Medeiros, C., Rowan, E. G., Harvey, A. L., and Ménez, A. (1997) J. Biol. Chem. 272, 4802-4809) for polypeptide antagonists of K(+) channels. In the case of kappa-PVIIA, Lys(7) and Phe(9) are essential for activity as predicted by Savarin et al. (Savarin, P., Guenneugues, M., Gilquin, B., Lamthanh, H., Gasparini, S., Zinn-Justin, S., and Ménez, A. (1998) Biochemistry 37, 5407-5416); these workers also correctly predicted an important role for Lys(25). Thus, although kappa-conotoxin PVIIA has no obvious sequence homology to polypeptide toxins from other venomous animals that interact with voltage-gated K(+) channels, there may be convergent functional features in diverse K(+) channel polypeptide antagonists.

Alanine↗
PubMed2000

Differential distribution of three Ca(2+)-activated K(+) channel subunits, SK1, SK2, and SK3, in the adult rat central nervous system.

Ca(2+)-activated, voltage-independent K(+) channels are present in most neurons and mediate the afterhyperpolarizations (AHPs) following action potentials. They present distinct physiological and pharmacological properties and play an important role in controlling neuronal firing frequency and spike frequency adaptation. We used in situ hybridization to characterize the distribution patterns of the three cloned SK channel subunits (SK1-3), the prime candidates likely to underlie Ca(2+)-dependent AHPs in the central nervous system. We found high levels of expression in regions presenting prominent AHP currents, such as, for example, neocortex and CA1-3 layers of the hippocampus (SK1 and SK2), reticularis thalami (SK1 and SK2), supraoptic nucleus (SK3), and inferior olivary nucleus (SK2 and SK3). Our results reveal the functional role of SK channels with defined subunit compositions in some neurons and open the way to the identification of the molecular determinants of AHP currents in many brain regions.

Animals↗
PubMed2000

Successful treatment of bilateral renal fungal balls with liposomal amphotericin B and fluconazole in an extremely low birth weight infant.

UNLABELLED: At the age of 8 weeks, an extremely low birth weight infant (gestational age 26 0/7 weeks, birth weight 740 g) had non-obstructing bilateral renal fungal balls. Urine cultures had repeatedly grown Candida albicans. Combination therapy with liposomal amphotericin B intravenously and fluconazole orally was administered for 6 weeks. Monotherapy with fluconazole was then continued until complete resolution of the renal fungal balls. CONCLUSION: Combination therapy with liposomal amphotericin B and fluconazole was successful in eliminating non-obstructing bilateral renal fungal balls and obviated the need for surgical intervention.

Amphotericin B↗
PubMed2000

Educating residents about managed care: a partnership between an academic medical center and a managed care organization.

Physicians are taught to think in terms of individual patients rather than in terms of the health of a population, a view typically reinforced by residency training. Managers of managed care organizations estimate that it takes between one and two years of additional post-residency experience to prepare graduates of U.S. residency programs to practice in managed care settings. The authors describe a two-week block rotation in managed care and health system change that is required of all third-year medical residents at New York-Presbyterian Hospital/ Weill Cornell Medical Center. The program was developed through a partnership between the Joan and Sanford I. Weill Medical College of Cornell University, the New York-Presbyterian Hospital, and Empire BlueCross BlueShield. The authors discuss the rationale, curriculum content, teaching methods, and evaluation of the program, which is designed not only to educate residents about managed care but also to enable them to think critically about the changing health care system. The results of the program have exceeded expectations. Residents' knowledge of health economics, managed care concepts, and health system change, as measured by pre- and post-rotation tests, has shown a steep learning curve. Further, the residents have consistently given the rotation the highest ratings, and some residents have changed their career plans as a result of their participation. The program, which continues to expand, has demonstrated the potential of collaboration between traditional adversaries, an academic medical center and a managed care insurance company, and provides a replicable model for similar partnerships.

Academic Medical Centers↗
PubMed1999

An apamin-sensitive Ca2+-activated K+ current in hippocampal pyramidal neurons.

In hippocampal and other cortical neurons, action potentials are followed by afterhyperpolarizations (AHPs) generated by the activation of small-conductance Ca2+-activated K+ channels (SK channels). By shaping the neuronal firing pattern, these AHPs contribute to the regulation of excitability and to the encoding function of neurons. Here we report that CA1 pyramidal neurons express an AHP current that is suppressed by apamin and is involved in the control of repetitive firing. This current presents distinct kinetic and pharmacological features, and it is modulated differently than the apamin-insensitive slow AHP current. Furthermore, our in situ hybridizations show that the apamin-sensitive SK subunits are expressed in CA1 pyramidal neurons, providing a potential molecular correlate to the apamin-sensitive AHP current. Altogether, these results clarify the discrepancy between the reported high density of apamin-binding sites in the CA1 region and the apparent lack of an apamin-sensitive current in CA1 pyramidal neurons, and they may explain the effects of this toxin on hippocampal synaptic plasticity and learning.

Animals↗
PubMed1999

Heteromeric assembly of Kv2.1 with Kv9.3: effect on the state dependence of inactivation.

Modulatory alpha-subunits of Kv channels remain electrically silent after homomeric expression. Their interactions with Kv2 alpha-subunits via the amino-terminal domain promote the assembly of heteromeric functional channels. The kinetic features of these heteromers differ from those of Kv2 homomers, suggesting a distinct role in electrical signaling. This study investigates biophysical properties of channels emerging from the coexpression of Kv2.1 with the modulatory alpha-subunit Kv9.3. Changes relative to homomeric Kv2.1 concern activation, deactivation, inactivation, and recovery from inactivation. A detailed description of Kv2.1/Kv9.3 inactivation is presented. Kv2.1/Kv9.3 heteromers inactivate in a fast and complete fashion from intermediate closed states, but in a slow and incomplete manner from open states. Intermediate closed states of channel gating can be approached through partial activation or deactivation, according to a proposed qualitative model. These transitions are rate-limiting for Kv2.1/Kv9.3 inactivation. Finally, based on the kinetic description, we propose a putative function for Kv2.1/Kv9.3 heteromers in rat heart.

Animals↗
PubMed1999

Subunit assembly and domain analysis of electrically silent K+ channel alpha-subunits of the rat Kv9 subfamily.

Alpha-subunits of the voltage-gated potassium channel (Kv) subfamily Kv9 show no channel activity after homomultimeric expression in heterologous expression systems. This report shows that heteromultimeric expression of rKv9.1 and rKv9.3 specifically suppresses the currents mediated by alpha-subunits of the Kv2 and Kv3 subfamilies but does not affect currents mediated by alpha-subunits of the Kv1 and Kv4 subfamilies. To understand the molecular basis of the electrical silence of Kv9 homomultimeric channels, crucial functional domains (amino and carboxy terminus, S4 segment, and pore region) were exchanged between Kv9 alpha-subunits and rKv1.3. Electrophysiological studies of these chimeras revealed that the pore region is involved in determining the nonconductive behavior of homomultimeric Kv9 channels. This analysis was extended by protein interaction assays, aiming to identify the region of Kv9 subunits responsible for the specific suppression of rKv2.1- and rKv3.4-mediated currents. We could show that the amino-terminal domain of Kv9 alpha-subunits does not support homomultimeric assembly but interacts specifically with the rKv2.1 amino-terminal region. Conversely, the specific intersubfamily assembly of rKv3.4 with rKv9.1 or rKv9.3 is governed by the hydrophobic core and not the amino-terminal domain.

Amino Acid Sequence↗
PubMed1998

Cloning and tissue distribution of two new potassium channel alpha-subunits from rat brain.

The expressed sequence tag (EST) database is a valuable tool to identify functionally related clones, when sequence similarity is so low that standard library screening methods cannot be successfully applied. Comparing conserved protein sequences of cloned voltage-gated potassium channels led to the identification and cloning of a new putative potassium channel alpha-subunit from rat brain, Kv9.1. A homologue of this alpha-subunit was also found in human brain tissue. A second alpha-subunit, Kv9.3, most probably belonging to the same subfamily, was also isolated and sequenced. Tissue distribution and analysis of genomic DNA were performed for both channels. rKv9.1 is expressed exclusively in the central nervous system, whereas rKv9.3 shows a widespread tissue distribution. No currents could be measured in X. oocytes upon injection of rKv9.1 or rKv9.3 cRNA.

Amino Acid Sequence↗
PubMed1998

kappa-Conotoxin PVIIA is a peptide inhibiting the shaker K+ channel.

kappa-Conotoxin PVIIA (kappa-PVIIA), a 27-amino acid toxin from Conus purpurascens venom that inhibits the Shaker potassium channel, was chemically synthesized in a biologically active form. The disulfide connectivity of the peptide was determined. kappa-Conotoxin PVIIA has the following structure. This is the first Conus peptide known to target K+ channels. [structure: see text] Although the Shaker K+ channel is sensitive to kappa-PVIIA, the rat brain Kv1.1 subtype is resistant. Chimeras between Shaker and the Kv1.1 K+ channels were constructed and expressed in Xenopus oocytes. Only channels containing the putative pore-forming region between the fifth and sixth transmembrane domains of Shaker retained toxin sensitivity, indicating that the toxin target site is in this region of the channel. Evidence is presented that kappa-PVIIA interacts with the external tetraethyl-ammonium binding site on the Shaker channel. Although both kappa-PVIIA and charybdotoxin inhibit the Shaker channel, they must interact differently. The F425G Shaker mutation increases charybdotoxin affinity by 3 orders of magnitude but abolishes kappa-PVIIA sensitivity. The precursor sequence of kappa-PVIIA was deduced from a cDNA clone, revealing a prepropeptide comprising 72 amino acids. The N-terminal region of the kappa-PVIIA prepropeptide exhibits striking homology to the omega-, muO-, and delta-conotoxins. Thus, at least four pharmacologically distinct superfamilies of Conus peptides belong to the same "O" superfamily, with the omega- and kappa-conotoxins forming one branch, and the delta- and muO-conotoxins forming a second major branch.

Amino Acid Sequence↗
PubMed1997

Interaction of midazolam with the nicotinic acetylcholine receptor of mouse myotubes.

The effects of midazolam on the peripheral embryonic nicotinergic acetylcholine receptor (nAChR) of mouse myotubes were studied to elucidate the mechanism of its effect on neuromuscular transmission. Standard patch clamp techniques on outside-out patches were used. Pulses of 10(-4) M acetylcholine (ACh) applied by a liquid filament switch technique elicited macroscopic channel currents with a peak current amplitude of approximately 40 pA within <1 ms. The current decayed with a time constant of 30-100 ms due to desensitization. When midazolam was added in stepwise increased concentrations (10(-7) M to 7 x 10(-4) M) to the pulses, the current decay became bi-exponential, and a concentration-dependent decrease of the fast component of decay was observed. The current amplitude, however, was reduced slightly, and only at high concentrations of midazolam. This may indicate that midazolam binds to the open channel to cause the block. The rate constant of block (b(+1)) was found to be 1.8 x 10(6) M/s. Recovery experiments revealed a rate of unblocking (b(-1)) of approximately 2 x 10(-1) s(-1). After preincubation of the patches with midazolam, a substantial reduction of the current amplitude was seen at very low midazolam concentrations (<10(-7) M), which suggests an additional closed channel block with a Kd of approximately 10(-6) M. This closed channel block may be responsible for the muscle-relaxing effects of midazolam.

Acetylcholine↗
PubMed1996

A1 adenosine receptors and muscarinic cholinoceptors in myocardial ischemia.

UNLABELLED: The regulation of cardiac A1 adenosine receptors and M2 muscarinic cholinoceptors was investigated in ischemic rat hearts. Ischemia was induced in isolated, perfused hearts either by stop (stop-flow) or by reduction (low-flow) of perfusion flow. Receptor densities and affinities were determined by radioligand binding. The mRNA concentrations of the receptors and of control messages were measured by quantitative polymerase chain reactions (PCR). Second messenger coupling of the receptors was evaluated by measuring their inhibition of adenylate cyclase activity. Up to 60 min of stop-flow ischemia and 6 h of low-flow ischemia, cardiac A1 adenosine receptor density and affinity, and adenosine receptor-mediated inhibition of adenylate cyclase, did not change significantly, compared to non-ischemic hearts. Receptor down-regulation, however, could be induced by perfusion with the A1 receptor agonist R-phenyl-isopropyl-adenosine (R-PIA) during normal flow. After 6 h of perfusion with R-PIA (0.1 mumol/l), A1 adenosine receptor density was reduced. Agonist-induced receptor down-regulation was not found after perfusion with R-PIA in low-flow ischemia. The density and the affinity of muscarinic cholinoceptors were not affected during stop-flow ischemia up to 1 h either, whereas the density was down-regulated to 75% of controls (P < 0.05) after 6 h of low-flow ischemia. This intervention also reduced inhibition of adenylate cyclase via muscarinic cholinoceptors. In non-ischemic hearts, perfusion with carbachol (10 mumol/l) suppressed receptor densities to 72% of control values. No significant changes in the concentration of A1 adenosine receptor or M2 cholinoceptor mRNAs occurred during normal flow, stop-flow and low-flow ischemia. Likewise, agonist stimulation with R-PIA or carbachol during normal flow did not change the respective receptor mRNA concentrations significantly. CONCLUSION: Although a down-regulation of A1 adenosine receptor density was demonstrated after receptor agonist perfusion with normal flow, adenosine did not affect the density or functional activity of cardiac A1 adenosine receptors in the ischemic myocardium. In contrast, muscarinic cholinoceptor density and function was down-regulated after prolonged ischemia. The lack of an agonist-induced down-regulation of A1 adenosine receptors in the presence of decreasing activity of m-cholinoceptors suggests a growing importance of the adenosine system in myocardial ischemia.

Adenylyl Cyclase Inhibitors↗
PubMed1996

MicroO-conotoxin MrVIA inhibits mammalian sodium channels, but not through site I.

1. A 31-amino-acid peptide from the venom of the snail-hunting species Conus marmoreus, microO-conotoxin MrVIA, inhibits mammalian voltage-gated sodium channels through a novel mechanism distinct from saxitoxin, tetrodotoxin, or mu-conotoxin. 2. MicroO-Conotoxin MrVIA blocks rat brain type II sodium channels expressed in Xenopus oocytes (IC50 approximately 200 nM, Hill coefficient approximately 1.6 +/- 0.2, mean +/- SE). Channel activation/inactivation kinetics and current-voltage relationships were unperturbed. 3. MicroO-Conotoxin MrVIA does not cause phasic or use-dependent inhibition of sodium currents measured in Xenopus oocytes expressing rat brain type II sodium channels, but shifts the steady-state availability of these sodium channels to more hyperpolarized potentials. 4. MicroO-Conotoxin MrVIA inhibited rapidly inactivating sodium channel conductance in rat hippocampal cells in culture. The inhibition was rapidly reversible. 5. MicroO-Conotoxin MrVIA does not displace specific [3H]saxitoxin binding to either rat brain or Electrophorus electric organ sites, indicating inhibitory effects mediated through a binding site distinct from site I.

Amino Acid Sequence↗
PubMed1994

Electrostatic distance geometry in a K+ channel vestibule.

Many voltage-gated K+ channels carry in the external vestibule a receptor for charybdotoxin, a peptide channel blocker. We use point mutagenesis of both charybdotoxin and a Shaker K+ channel to isolate the electrostatic interaction energy between chosen pairs of residues, one on the channel and one on bound toxin. The results allow estimates of physical distances between such residue pairs and, in combination with the known structure of charybdotoxin, localize specific channel residues in three-dimensional space.

Amino Acid Sequence↗
PubMed1994

Potassium currents in epilepsy: effects of the epileptogenic agent pentylenetetrazol on a cloned potassium channel.

The effect of the epileptogenic agent pentylenetetrazol (PTZ) on the cloned rat brain potassium channel Kv1.1 (labelled also RCK1) was investigated in the Xenopus laevis oocyte expression system. The Kv1.1 channel was affected by PTZ in a voltage-dependent manner. PTZ increased the potassium currents at more negative potentials and decreased them at more positive potentials. At a potential of -50 mV the potassium currents were increased by 0.97 and at -20 mV decreased by 0.21 of control value with 100 mmol/l PTZ. The potential at which the inversion from increase to decrease occurred was -33 mV. The inactivation characteristic of the current was shifted to more negative potentials by PTZ. The PTZ effect was obtained at a threshold concentration of 1 mmol/l and increased with rising PTZ concentrations. After removal of the tissues covering the oocyte membrane, the PTZ effect was augmented; with a concentration of 10 mmol/l PTZ the potassium currents at 0 mV were decreased by 0.04 in oocytes with covering tissues and by 0.27 of control value in oocytes without covering tissues. Under current-clamp conditions, PTZ decreased small depolarizations and increased larger depolarizations. This effect of PTZ represents a 'discriminatory function' that may contribute to epileptogenesis in nervous tissues.

Animals↗