1
|
Menegon A, Pitassi S, Mazzocchi N, Redaelli L, Rizzetto R, Rolland JF, Poli C, Imberti M, Lanati A, Grohovaz F. A new electro-optical approach for conductance measurement: an assay for the study of drugs acting on ligand-gated ion channels. Sci Rep 2017; 7:44843. [PMID: 28322303 PMCID: PMC5359596 DOI: 10.1038/srep44843] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/21/2016] [Accepted: 02/14/2017] [Indexed: 01/02/2023] Open
Abstract
Ligand gated ion channels are involved in many pathophysiological processes and represent a relevant, although challenging, target for drug discovery. We propose an innovative electro-optical approach to their analysis able to derive membrane conductance values from the local membrane potential changes imposed by test current pulses and measured by fast voltage-sensitive fluorescent dyes. We exploited the potential of this proprietary method by developing a drug testing system called “ionChannel Optical High-content Microscope” (ionChannelΩ). This automated platform was validated by testing the responses of reference drugs on cells expressing different ligand-gated ion channels. Furthermore, a double-blind comparison with FLIPR and automated patch-clamp was performed on molecules designed to act as antagonists of the P2RX7 receptor. ionChannelΩ proved highly reliable in all tests, resulting faster and more cost-effective than electrophysiological techniques. Overall, ionChannelΩ is amenable to the study of ligand gated ion channels that are receiving less attention due to limitations in current assays.
Collapse
Affiliation(s)
- A Menegon
- San Raffaele Scientific Institute, via Olgettina 60, 20132, Milan, Italy.,San Raffaele University, via Olgettina 58, 20132, Milan, Italy
| | - S Pitassi
- Optotec, Via Zenale 44, 20024, Garbagnate Milanese, Milan, Italy
| | - N Mazzocchi
- San Raffaele Scientific Institute, via Olgettina 60, 20132, Milan, Italy
| | - L Redaelli
- Axxam SpA, via Meucci 3, 20091, Bresso, Milan, Italy
| | - R Rizzetto
- Axxam SpA, via Meucci 3, 20091, Bresso, Milan, Italy
| | - J F Rolland
- Axxam SpA, via Meucci 3, 20091, Bresso, Milan, Italy
| | - C Poli
- Valore Qualità, Via Vidari 5, 27100, Pavia, Italy.,Assing, Pavia, Viale Indipendenza 11, I-27100, Pavia, Italy
| | - M Imberti
- OPEN Sistemi, via Bonomelli 24, 26100, Cremona, Italy
| | - A Lanati
- Valore Qualità, Via Vidari 5, 27100, Pavia, Italy.,Assing, Pavia, Viale Indipendenza 11, I-27100, Pavia, Italy
| | - F Grohovaz
- San Raffaele Scientific Institute, via Olgettina 60, 20132, Milan, Italy.,San Raffaele University, via Olgettina 58, 20132, Milan, Italy
| |
Collapse
|
2
|
Three Types of Single Voltage-Dependent Potassium Channels in the Sarcolemma of Frog Skeletal Muscle. J Membr Biol 2009; 228:51-62. [DOI: 10.1007/s00232-009-9158-4] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/28/2008] [Accepted: 01/31/2009] [Indexed: 10/21/2022]
|
3
|
|
4
|
Tang XD, Hoshi T. Rundown of the hyperpolarization-activated KAT1 channel involves slowing of the opening transitions regulated by phosphorylation. Biophys J 1999; 76:3089-98. [PMID: 10354434 PMCID: PMC1300278 DOI: 10.1016/s0006-3495(99)77461-8] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022] Open
Abstract
Disappearance of the functional activity or rundown of ion channels upon patch excision in many cells involves a decrease in the number of channels available to open. A variety of cellular and biophysical mechanisms have been shown to be involved in the rundown of different ion channels. We examined the rundown process of the plant hyperpolarization-activated KAT1 K+ channel expressed in Xenopus oocytes. The decrease in the KAT1 channel activity on patch excision was accompanied by progressive slowing of the activation time course, and it was caused by a shift in the voltage dependence of the channel without any change in the single-channel amplitude. The single-channel analysis showed that patch excision alters only the transitions leading up to the burst states of the channel. Patch cramming or concurrent application of protein kinase A (PKA) and ATP restored the channel activity. In contrast, nonspecific alkaline phosphatase (ALP) accelerated the rundown time course. Low internal pH, which inhibits ALP activity, slowed the KAT1 rundown time course. The results show that the opening transitions of the KAT1 channel are enhanced not only by hyperpolarization but also by PKA-mediated phosphorylation.
Collapse
Affiliation(s)
- X D Tang
- Department of Physiology and Biophysics, The University of Iowa, Iowa City, Iowa 52242, USA
| | | |
Collapse
|
5
|
Hilgemann DW. Cytoplasmic ATP-dependent regulation of ion transporters and channels: mechanisms and messengers. Annu Rev Physiol 1997; 59:193-220. [PMID: 9074761 DOI: 10.1146/annurev.physiol.59.1.193] [Citation(s) in RCA: 129] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
Many ion transporters and channels appear to be regulated by ATP-dependent mechanisms when studied in planar bilayers, excised membrane patches, or with whole-cell patch clamp. Protein kinases are obvious candidates to mediate ATP effects, but other mechanisms are also implicated. They include lipid kinases with the generation of phosphatidylinositol phosphates as second messengers, allosteric effects of ATP binding, changes of actin cytoskeleton, and ATP-dependent phospholipases. Phosphatidylinositol-4,5-bisphosphate (PIP2) is a possible membrane-delimited messenger that activates cardiac sodium-calcium exchange, KATP potassium channels, and other inward rectifier potassium channels. Regulation of PIP2 by phospholipase C, lipid phosphatases, and lipid kinases would thus tie surface membrane transport to phosphatidylinositol signaling. Sodium-hydrogen exchange is activated by ATP through a phosphorylation-independent mechanism, whereas ion cotransporters are activated by several protein kinase mechanisms. Ion transport in epithelium may be particularly sensitive to changes of cytoskeleton that are regulated by ATP-dependent cell signaling mechanisms.
Collapse
Affiliation(s)
- D W Hilgemann
- University of Texas Southwestern, Medical Center at Dallas 75235-9040, USA
| |
Collapse
|
6
|
Hocherman SD, Bezanilla F. A patch-clamp study of delayed rectifier currents in skeletal muscle of control and mdx mice. J Physiol 1996; 493 ( Pt 1):113-28. [PMID: 8735698 PMCID: PMC1158954 DOI: 10.1113/jphysiol.1996.sp021368] [Citation(s) in RCA: 26] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
1. Potassium currents were measured in the extensor digitorum longus muscle of normal and mdx mice, which lack the protein dystrophin, using the cell-attached and inside-out patch clamp techniques, in the presence of asymmetrical K+ concentrations (3 mM in the pipette, 160 mM in the bath). 2. In cell-attached patches, the delayed rectifier was the most commonly found potassium channel, with a density of roughly 8 channels microns-2. Outward macroscopic currents were activated in macropatches depolarized to potentials positive to -60 mV. The probability of opening reached half-maximal values around -40 mV for control patches and -31 mV for patches from mdx mice. 3. Tail currents were linear in the range between -60 and +20 mV, reversing close to -100 mV. The single channel current at 0 mV, estimated from non-stationary analysis of variance, was used in conjunction with the slope of the linear part of the tail current to calculate the single channel conductance, yielding a value of 19 +/- 1 pS. 4. At 0 mV, the delayed rectifier inactivated with two time constants, of 70 +/- 20 ms and 600 +/- 200 ms. Prepulses of 500 ms duration to different potentials produced incomplete inactivation with inactivation reaching 50% of its maximum at -50 mV. 5. Single channel activity was recorded using small pipettes. Both single channel conductance and kinetic behaviour were in agreement with the macroscopic current data. 6. In excised patches, the delayed rectifier current ran down, unmasking other K+ channels. A Ca(2+)-dependent K+ channel of 186 pS (BK-like channel) was found frequently in patches bathed in solutions containing appropriate concentrations of calcium, especially at stronger depolarizations. A K+ channel of 63 pS was unmasked in control excised patches bathed in solutions devoid of ATP. This channel was not observed in patches excised from mdx fibers.
Collapse
Affiliation(s)
- S D Hocherman
- Department of Physiology, UCLA School of Medicine 90024, USA
| | | |
Collapse
|
7
|
Abstract
A fundamental property of ion channels is their ability to be modulated by intracellular second messenger systems acting via covalent modifications of the channel protein itself. One such important biochemical reaction is phosphorylation on serine, threonine, and tyrosine residues. Ion channels in the kidney are no exception. Moreover, many ion channels, including many amiloride-sensitive epithelial Na+ channels, are subject to modulation by a multiplicity of inputs. For example, renal Na+ channels are not gated by voltage in their unphosphorylated state. However, upon phosphorylation by PKA plus ATP, these channels become voltage-dependent as well as having their open probability increased. Phosphorylation by PKC inhibits channel activity regardless of whether the channel was previously phosphorylated by PKA. Likewise, Na+ channel ADP-ribosylation by PTX overrides the actions of cAMP-dependent phosphorylation. Consistent with this idea is the fact that the phosphorylation sites for PKA and PKC and the ADP-ribosylation sites occur on different polypeptides comprising the channel complex. Epithelial Na+ channel activity is also regulated by methylation, arachidonic acid metabolites, and by interactions with cytoskeletal components. An exciting new age in understanding renal Na+ channel function has begun. Canessa and collaborators [103, 104] and Lingueglia et al [105] have, for the first time, identified by expression cloning an amiloride-sensitive Na+ channel from rat distal colon. The messenger RNA encoding the subunits comprising this channel are expressed in the distal tubule and cortical collecting tubule of the kidney (Rossier, unpublished observations). In addition, our laboratory has successfully cloned a mammalian homologue of this same channel from bovine renal papillary collecting ducts [106].(ABSTRACT TRUNCATED AT 250 WORDS)
Collapse
Affiliation(s)
- I I Ismailov
- Department of Physiology and Biophysics, University of Alabama at Birmingham, USA
| | | |
Collapse
|
8
|
Noceti F, Ramírez AN, Possani LD, Prestipino G. Characterization of a voltage-dependent potassium channel in squid Schwann cells reconstituted in planar lipid bilayers. Glia 1995; 15:33-42. [PMID: 8847099 DOI: 10.1002/glia.440150105] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
An affinity column prepared with noxiustoxin (NTx), a K+ channel blocker from the venom of the Mexican scorpion Centruroides noxius, was used to purify a functional channel from a detergent extract of Schwann cell membrane of the giant axon of the squid Loligo vulgaris. The purified protein was reconstituted as a functional unit in a planar lipid bilayer and tested with a sequence of potentials to obtain information about single-channel amplitude and kinetics. The reconstituted channel showed delayed rectifier behavior with a slope conductance of 10 pS under 5:1 asymmetric KCl concentrations and a clear tendency to open under negative potentials. The zero-current potential was +36mV, which fitted well with the Nernst equation for the CIS/TRANS K(+)-concentration ratio of 5:1. The channel also showed a strong sensitivity to tetraethylammonium and its activity was inhibited by NTx, as expected from the purification procedure. The behavior of this protein in the presence of 0.5 mM ATP (cis side) was also tested, significantly increasing current fluctuations across the membrane. In order to compare the modulation of the Schwann cell K+ channel with that of the axonal K+ channel, a purified protein from the squid axon membrane was also tested in the presence of ATP. This 10-11 pS, delayed rectifier channel from the squid giant axon (Prestipino et al., FEBS Lett. 250:570-574, 1989) was also tested in the presence of ATP and showed a similar rise in activity.
Collapse
Affiliation(s)
- F Noceti
- Istituto di Cibernetica e Biofisica, CNR, Genova, Italy
| | | | | | | |
Collapse
|
9
|
Ho K, Nichols CG, Lederer WJ, Lytton J, Vassilev PM, Kanazirska MV, Hebert SC. Cloning and expression of an inwardly rectifying ATP-regulated potassium channel. Nature 1993; 362:31-8. [PMID: 7680431 DOI: 10.1038/362031a0] [Citation(s) in RCA: 767] [Impact Index Per Article: 24.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
A complementary DNA encoding an ATP-regulated potassium channel has been isolated by expression cloning from rat kidney. The predicted 45K protein, which features two potential membrane-spanning helices and a proposed ATP-binding domain, represents a major departure from the basic structural design characteristic of voltage-gated and second messenger-gated ion channels. But the presence of an H5 region, which is likely to form the ion conduction pathway, indicates that the protein may share a common origin with voltage-gated potassium channel proteins.
Collapse
Affiliation(s)
- K Ho
- Harvard Center for the Study of Kidney Disease, Harvard Medical School, Boston, Massachusetts
| | | | | | | | | | | | | |
Collapse
|
10
|
Affiliation(s)
- A Stelzer
- Department of Pharmacology, State University of New York, Brooklyn 11203
| |
Collapse
|
11
|
Blatt MR. Ion channel gating in plants: physiological implications and integration for stomatal function. J Membr Biol 1991; 124:95-112. [PMID: 1662287 DOI: 10.1007/bf01870455] [Citation(s) in RCA: 64] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Affiliation(s)
- M R Blatt
- Department of Biochemistry and Biological Sciences, University of London, Wye College, England
| |
Collapse
|
12
|
Perozo E, Bezanilla F. Phosphorylation of K+ channels in the squid giant axon. A mechanistic analysis. J Bioenerg Biomembr 1991; 23:599-613. [PMID: 1917910 DOI: 10.1007/bf00785813] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
Protein phosphorylation is an important mechanism in the modulation of voltage-dependent ionic channels. In squid giant axons, the potassium delayed rectifier channel is modulated by an ATP-mediated phosphorylation mechanism, producing important changes in amplitude and kinetics of the outward current. The characteristics and biophysical basis for the phosphorylation effects have been extensively studied in this preparation using macroscopic, single-channel and gating current experiments. Phosphorylation produces a shift in the voltage dependence of all voltage-dependent parameters including open probability, slow inactivation, first latency, and gating charge transferred. The locus of the effect seems to be located in a fast 20 pS channel, with characteristics of delayed rectifier, but at least another channel is phosphorylated under our experimental conditions. These results are interpreted quantitatively with a mechanistic model that explains all the data. In this model the shift in voltage dependence is produced by electrostatic interactions between the transferred phosphate and the voltage sensor of the channel.
Collapse
Affiliation(s)
- E Perozo
- Department of Physiology, Jerry Lewis Neuromuscular Research Center, Los Angeles, California
| | | |
Collapse
|
13
|
DiPolo R, Beaugé L. Asymmetrical properties of the Na-Ca exchanger in voltage-clamped, internally dialyzed squid axons under symmetrical ionic conditions. J Gen Physiol 1990; 95:819-35. [PMID: 2362183 PMCID: PMC2216346 DOI: 10.1085/jgp.95.5.819] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
In this work we have investigated whether the asymmetrical properties of the Na/Ca exchange process found in intact preparations are intrinsic to the exchange protein(s) or the result of the asymmetric ionic environment normally prevailing in living cells. The activation of the Na/Ca exchanger by Ca2+ ions, monovalent cations, ATP gamma S and the effect of membrane potential on the different operational modes of the exchanger (Nao/Cai, Cao/Nai, Cao/Cai, and Nao/Nai) was studied in voltage-clamped squid giant axons externally perfused and internally dialyzed with symmetrical ionic solutions. Under these conditions: (a) Ca ions activate with higher affinity from the inside (K1/2 = 22 microM) than from the outside (K1/2 = 300 microM); (b) experiments measuring the Cao-dependent Ca efflux in the conditions Lio-Trisi, Lio-Lii, Triso-Trisi, and Triso-Lii, show that the activating monovalent cation site on the exchanger faces the external surface; (c) ATP gamma S activates the Cao-dependent Ca efflux (Cao/Cai exchange) only at nonsaturating [Ca2+]i. Its effect appears to be on the Ca transport site since no alteration in the apparent affinity of the activating monovalent cation site was observed. The above results show that the Na/Ca exchange process is indeed a highly asymmetric transport mechanism. Finally, the voltage dependence of the components of the different exchange modes was measured over the range of +20 to -40 mV. The voltage dependence (approximately 26% change/25 mV) was found to be similar for all modes of operation of the exchanger except Nao/Nai exchange, which was found to be voltage insensitive. The sensitivity of the Cao/Cai exchange to voltage was found to be the same in the presence and in the complete absence of monovalent cations. This finding does not support the proposition that the voltage sensitivity of the Cao/Cao exchange is induced by the binding and transport of an external monovalent cation.
Collapse
Affiliation(s)
- R DiPolo
- Instituto Venezolano de Investigaciones Científicas, Centro de Biofísica y Bioquímica, Caracas, Venezuela
| | | |
Collapse
|
14
|
Augustine CK, Bezanilla F. Phosphorylation modulates potassium conductance and gating current of perfused giant axons of squid. J Gen Physiol 1990; 95:245-71. [PMID: 2307959 PMCID: PMC2216314 DOI: 10.1085/jgp.95.2.245] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
The presence of internal Mg-ATP produced a number of changes in the K conductance of perfused giant axons of squid. For holding potentials between -40 and -50 mV, steady-state K conductance increased for depolarizations to potentials more positive than approximately -15 mV and decreased for smaller depolarizations. The voltage dependencies of both steady-state activation and inactivation also appears shifted toward more positive potentials. Gating kinetics were affected by internal ATP, with the activation time constant slowed and the characteristic delay in K conductance markedly enhanced. The rate of deactivation also was hastened during perfusion with ATP. Internal ATP affected potassium channel gating currents in similar ways. The voltage dependence of gating charge movement was shifted toward more positive potentials and the time constants of ON and OFF gating current also were slowed and hastened, respectively, in the presence of ATP. These effects of ATP on the K conductance occurred when no exogenous protein kinases were added to the internal solution and persisted even after removing ATP from the internal perfusate. Perfusion with a solution containing exogenous alkaline phosphatase reversed the effects of ATP. These results provide further evidence that the effects of ATP on the K conductance are a consequence of a phosphorylation reaction mediated by a kinase present and active in perfused axons. Phosphorylation appears to alter the K conductance of squid giant axons via a minimum of two mechanisms. First, the voltage dependence of gating parameters are shifted toward positive potentials. Second, there is an increase in the number of functional closed states and/or a decrease in the rates of transition between these states of the K channels.
Collapse
Affiliation(s)
- C K Augustine
- Department of Physiology, Jerry Lewis Neuromuscular Research Center, Los Angeles, California 90024
| | | |
Collapse
|
15
|
Perozo E, Bezanilla F, Dipolo R. Modulation of K channels in dialyzed squid axons. ATP-mediated phosphorylation. J Gen Physiol 1989; 93:1195-218. [PMID: 2769224 PMCID: PMC2216247 DOI: 10.1085/jgp.93.6.1195] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023] Open
Abstract
In squid axons, internally applied ATP potentiates the magnitude of the potassium conductance and slows down its activation kinetics. This effect was characterized using internally dialyzed axons under voltage-clamp conditions. Both amplitude potentiation and kinetic slow-down effects are very selective towards ATP, other nucleotides like GTP and ITP are ineffective in millimolar concentrations. The current potentiation Km for ATP is near 10 microM with no further effects for concentrations greater than 100 microM. ATP effect is most likely produced via a phosphorylative reaction because Mg ion is an obligatory requirement and nonhydrolyzable ATP analogues are without effect. In the presence of ATP, the K current presents more delay, resembling a Cole-Moore effect due to local hyperpolarization of the channel. ATP effect induces a 10-20 mV shift in both activation and inactivation parameters towards more depolarized potentials. As a consequence of this shift, conductance-voltage curves with and without ATP cross at approximately -40 mV. This result is consistent with the hyperpolarization observed with ATP depletion, which is reversed by ATP addition. At potentials around the resting value, addition of ATP removes almost completely K current slow inactivation. It is suggested that a change in the amount of the slow inactivation is responsible for the differences in current amplitude with and without ATP, possibly as a consequence of the additional negative charge carried by the phosphate group. However, a modification of the local potential is not enough to explain completely the differences under the two conditions.
Collapse
Affiliation(s)
- E Perozo
- Centro de Biofisica y Bioquimica, Instituto Venezolano de Investigaciones Cientificas, Caracas, Venezuela
| | | | | |
Collapse
|
16
|
Rakowski RF, Gadsby DC, De Weer P. Stoichiometry and voltage dependence of the sodium pump in voltage-clamped, internally dialyzed squid giant axon. J Gen Physiol 1989; 93:903-41. [PMID: 2544655 PMCID: PMC2216238 DOI: 10.1085/jgp.93.5.903] [Citation(s) in RCA: 141] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023] Open
Abstract
The stoichiometry and voltage dependence of the Na/K pump were studied in internally dialyzed, voltage-clamped squid giant axons by simultaneously measuring, at various membrane potentials, the changes in Na efflux (delta phi Na) and holding current (delta I) induced by dihydrodigitoxigenin (H2DTG). H2DTG stops the Na/K pump without directly affecting other current pathways: (a) it causes no delta I when the pump lacks Na, K, Mg, or ATP, and (b) ouabain causes no delta I or delta phi Na in the presence of saturating H2DTG. External K (Ko) activates Na efflux with Michaelis-Menten kinetics (Km = 0.45 +/- 0.06 mM [SEM]) in Na-free seawater (SW), but with sigmoid kinetics in approximately 400 mM Na SW (Hill coefficient = 1.53 +/- 0.08, K1/2 = 3.92 +/- 0.29 mM). H2DTG inhibits less strongly (Ki = 6.1 +/- 0.3 microM) in 1 or 10 mM K Na-free SW than in 10 mM K, 390 mM Na SW (1.8 +/- 0.2 microM). Dialysis with 5 mM each ATP, phosphoenolpyruvate, and phosphoarginine reduced Na/Na exchange to at most 2% of the H2DTG-sensitive Na efflux. H2DTG sensitive but nonpump current caused by periaxonal K accumulation upon stopping the pump, was minimized by the K channel blockers 3,4-diaminopyridine (1 mM), tetraethylammonium (approximately 200 mM), and phenylpropyltriethylammonium (20-25 mM) whose adequacy was tested by varying [K]o (0-10 mM) with H2DTG present. Two ancillary clamp circuits suppressed stray current from the axon ends. Current and flux measured from the center pool derive from the same membrane area since, over the voltage range -60 to +20 mV, tetrodotoxin-sensitive current and Na efflux into Na-free SW, under K-free conditions, were equal. The stoichiometry and voltage dependence of pump Na/K exchange were examined at near-saturating [ATP], [K]o and [Na]i in both Na-free and 390 mM Na SW. The H2DTG-sensitive F delta phi Na/delta I ratio (F is Faraday's constant) of paired measurements corrected for membrane area match, was 2.86 +/- 0.09 (n = 8) at 0 mV and 3.05 +/- 0.13 (n = 6) at -60 to -90 mV in Na-free SW, and 2.72 +/- 0.09 (n = 7) at 0 mV and 2.91 +/- 0.21 (n = 4) at -60 mV in 390 mM Na SW. Its overall mean value was 2.87 +/- 0.07 (n = 25), which was not significantly different from the 3.0 expected of a 3 Na/2 K pump.(ABSTRACT TRUNCATED AT 400 WORDS)
Collapse
Affiliation(s)
- R F Rakowski
- Marine Biological Laboratory, Woods Hole, Massachusetts 02543
| | | | | |
Collapse
|
17
|
|
18
|
Ashford ML, Sturgess NC, Trout NJ, Gardner NJ, Hales CN. Adenosine-5'-triphosphate-sensitive ion channels in neonatal rat cultured central neurones. Pflugers Arch 1988; 412:297-304. [PMID: 2460821 DOI: 10.1007/bf00582512] [Citation(s) in RCA: 219] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
ATP-sensitive channels were observed in isolated inside-out membrane patches from rat cultured central neurones. Two types of ATP-sensitive K+ channels were present in cortical neurones, one which had its open-state probability increased, the other its open-state probability decreased by application of ATP to the cytoplasmic membrane surface. Another, ATP-sensitive channel differing in ion conductance from all previously reported ATP-sensitive channels was also seen in patches from cortical neurones. This channel was nonselective with respect to Na+, K+ and Cl- ions and ATP produced a "flickery" type of block. The non-hydrolysable analogue, AMPPNP, did not mimic ATP and prevented ATP action. Preliminary experiments indicate that similar, but not, identical ATP-sensitive channels exist in cerebellar neurones.
Collapse
Affiliation(s)
- M L Ashford
- Department of Pharmacology, University of Cambridge, Great Britain
| | | | | | | | | |
Collapse
|
19
|
Stelzer A, Kay AR, Wong RK. GABAA-receptor function in hippocampal cells is maintained by phosphorylation factors. Science 1988; 241:339-41. [PMID: 2455347 DOI: 10.1126/science.2455347] [Citation(s) in RCA: 236] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
Gamma aminobutyric acid (GABA) mediates fast synaptic inhibition in the central nervous system by activating the chloride-permeable GABAA channel. The GABAA conductance progressively diminishes with time when the intracellular contents of hippocampal neurons are perfused with a minimal intracellular medium. This "run down" of the GABA-activated conductance can be prevented by the inclusion of magnesium adenosine triphosphate and calcium buffer in the intracellular medium. The amount of chloride conductance that can be activated by GABA is determined by competition between a calcium-dependent process that reduces the conductance and a phosphorylation process that maintains the conductance.
Collapse
Affiliation(s)
- A Stelzer
- Department of Neurology, College of Physicians and Surgeons, Columbia University, New York, NY 10032
| | | | | |
Collapse
|
20
|
|
21
|
Bkaily G, Peyrow M, Yamamoto T, Sculptoreanu A, Jacques D, Sperelakis N. Macroscopic Ca2+ -Na+ and K+ currents in single heart and aortic cells. Mol Cell Biochem 1988; 80:59-72. [PMID: 3173339 DOI: 10.1007/bf00231004] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023]
Abstract
The whole-cell voltage clamp technique was used to study the slow inward currents and K+ outward currents in single heart cells of embryonic chick and in rabbit aortic cells. In single heart cells of 3-day-old chick embryo three types of slow inward Na+ currents were found. The kinetics and the pharmacology of the slow INa were different from those of the slow ICa in older embryos. Two types of slow inward currents were found in aortic single cells of rabbit; angiotensin II increased the sustained type and d-cAMP and d-cGMP decreased the slow transient component. Two types of outward K+ currents were found in both aortic and heart cells. Single channel analysis demonstrated the presence of a high single K+ channel conductance in aortic cells. In cardiac and vascular smooth muscles, slow inward currents do share some pharmacological properties, although the regulation of these channels by cyclic nucleotides and several drugs seems to be different.
Collapse
Affiliation(s)
- G Bkaily
- Department of Physiology and Biophysics, Faculty of Medicine, University of Sherbrooke (CHUS), Québec, Canada
| | | | | | | | | | | |
Collapse
|
22
|
Arreola J, Calvo J, García MC, Sánchez JA. Modulation of calcium channels of twitch skeletal muscle fibres of the frog by adrenaline and cyclic adenosine monophosphate. J Physiol 1987; 393:307-30. [PMID: 2451739 PMCID: PMC1192395 DOI: 10.1113/jphysiol.1987.sp016825] [Citation(s) in RCA: 105] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023] Open
Abstract
1. Modulation of fast and slow Ca2+ channels of frog skeletal muscle by adrenaline (10(-6) to 10(-5) M) and cyclic AMP was investigated using intracellular voltage recordings in intact fibres and a voltage-clamp technique in cut fibres. 2. In tetraethylammonium (TEA), Cl(-)-free Ringer solution, adrenaline increased the maximum rate of rise of Ca2+ spikes by 85% and in a similar solution, peak slow Ca2+ current (ICa,s) by 51%. 3. Application of cyclic AMP to the cut ends of fibres, produced a relative increase of ICa,s of ca. 24%. The effect was maintained for ca. 2 h. 4. Changes in the time course of ICa,s were produced by adrenaline and cyclic AMP: the limiting values of time-to-peak current measured as a function of membrane potential were lower (ca. 41% in adrenaline and ca. 34% in cyclic AMP) than those found in control experiments. Also, ICa,s decayed faster in the presence of adrenaline or cyclic AMP. These changes can be explained by exhaustion of Ca2+ in the lumen of transverse tubular system and do not require the assumption of kinetic variations. 5. Fast Ca2+ currents (ICa,f) which could not be blocked by nifedipine were also recorded. Cyclic AMP greatly increased the amplitude of ICa,f but had no obvious effects on ICa,f kinetics. 6. Application of catalytic subunit of cyclic AMP-dependent protein kinase by diffusion or by pressure injection also increased the amplitude of ICa,s and ICa,f. Pressure injection brought about modifications in the time course of ICa,s that cannot be explained by depletion of Ca2+. 7. Mechanical experiments were performed on single fibres. Nominally Ca2+-free solutions prevented the development and the maintenance of positive inotropic effect of adrenaline on twitch tension. Development of twitch potentiation was dependent upon the frequency of stimulation. Adrenaline was practically ineffective if no stimulation was applied. 8. It is concluded that both populations of Ca2+ channels are modulated by adrenergic stimulation probably via cyclic AMP, and that twitch potentiation may be mediated by a Ca2+ entry through Ca2+ channels.
Collapse
Affiliation(s)
- J Arreola
- Department of Pharmacology, Centro de Investigación y de Estudios Avanzados del I.P.N., México, D.F
| | | | | | | |
Collapse
|
23
|
Dudai Y. The cAMP cascade in the nervous system: molecular sites of action and possible relevance to neuronal plasticity. CRC CRITICAL REVIEWS IN BIOCHEMISTRY 1987; 22:221-81. [PMID: 2445527 DOI: 10.3109/10409238709101484] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
Many intercellular messages regulate the activity of their target cells by altering the intracellular level of cAMP and, as a consequence, the phosphorylation state of proteins which serve as substrates for cAMP-dependent protein kinase. Such regulation plays a crucial role in neuronal development, neuronal function, and neuronal plasticity (e.g., elementary learning mechanisms). Ample information has been accumulated in recent years on the enzymes that regulate the level of cAMP or respond to it, on the regulation of cAMP synthesis by neurohormones, neurotransmitters, ions, and toxins, on neuronal-specific substrate proteins that are phosphorylated by the cAMP-dependent kinase, and on the interaction of the cAMP-cascade with other second-messenger systems within neurons. Such data, obtained by a combination of molecular-biological, biochemical, and cellular approaches, shed light on the detailed mechanisms by which modulation of a ubiquitous molecular cascade leads to a great variety of short-term as well as long-term specific neuronal responses and alterations.
Collapse
Affiliation(s)
- Y Dudai
- Department of Neurobiology, Weizmann Institute of Science, Rehovot, Israel
| |
Collapse
|