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Wright AB, Sukhanova KY, Elmslie KS. K V7 channels are potential regulators of the exercise pressor reflex. J Neurophysiol 2021; 126:1-10. [PMID: 34038189 DOI: 10.1152/jn.00700.2020] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The exercise pressor reflex (EPR) originates in skeletal muscle and is activated by exercise-induced signals to increase arterial blood pressure and cardiac output. Muscle ischemia can elicit the EPR, which can be inappropriately activated in patients with peripheral vascular disease or heart failure to increase the incidence of myocardial infarction. We seek to better understand the receptor/channels that control excitability of group III and group IV muscle afferent fibers that give rise to the EPR. Bradykinin (BK) is released within contracting muscle and can evoke the EPR. However, the mechanism is incompletely understood. KV7 channels strongly regulate neuronal excitability and are inhibited by BK. We have identified KV7 currents in muscle afferent neurons by their characteristic activation/deactivation kinetics, enhancement by the KV7 activator retigabine, and block by KV7 specific inhibitor XE991. The blocking of KV7 current by different XE991 concentrations suggests that the KV7 current is generated by both KV7.2/7.3 (high affinity) and KV7.5 (low affinity) channels. The KV7 current was inhibited by 300 nM BK in neurons with diameters consistent with both group III and group IV afferents. The inhibition of KV7 by BK could be a mechanism by which this metabolic mediator generates the EPR. Furthermore, our results suggest that KV7 channel activators such as retigabine, could be used to reduce cardiac stress resulting from the exacerbated EPR in patients with cardiovascular disease.NEW & NOTEWORTHY KV7 channels control neuronal excitability. We show that these channels are expressed in muscle afferents and generate currents that are blocked by XE991 and bradykinin (BK). The XE991 block suggests that KV7 current is generated by KV7.2/3 and KV7.5 channels. The BK inhibition of KV7 channels may explain how BK activates the exercise pressor reflex (EPR). Retigabine can enhance KV7 current, which could help control the inappropriately activated EPR in patients with cardiovascular disease.
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Affiliation(s)
- Andrew B Wright
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A.T. Still University of Health Sciences, Kirksville, Missouri
| | - Khrystyna Yu Sukhanova
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A.T. Still University of Health Sciences, Kirksville, Missouri
| | - Keith S Elmslie
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A.T. Still University of Health Sciences, Kirksville, Missouri
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Marler TL, Wright AB, Elmslie KL, Heier AK, Remily E, Kim-Han JS, Ramachandra R, Elmslie KS. Na V1.9 channels in muscle afferent neurons and axons. J Neurophysiol 2018; 120:1032-1044. [PMID: 29847236 DOI: 10.1152/jn.00573.2017] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022] Open
Abstract
The exercise pressor reflex (EPR) is activated by muscle contractions to increase heart rate and blood pressure during exercise. While this reflex is beneficial in healthy individuals, the reflex activity is exaggerated in patients with cardiovascular disease, which is associated with increased mortality. Group III and IV afferents mediate the EPR and have been shown to express both tetrodotoxin-sensitive (TTX-S, NaV1.6, and NaV1.7) and -resistant (TTX-R, NaV1.8, and NaV1.9) voltage-gated sodium (NaV) channels, but NaV1.9 current has not yet been demonstrated. Using a F--containing internal solution, we found a NaV current in muscle afferent neurons that activates at around -70 mV with slow activation and inactivation kinetics, as expected from NaV1.9 current. However, this current ran down with time, which resulted, at least in part, from increased steady-state inactivation since it was slowed by both holding potential hyperpolarization and a depolarized shift of the gating properties. We further show that, following NaV1.9 current rundown (internal F-), application of the NaV1.8 channel blocker A803467 inhibited significantly more TTX-R current than we had previously observed (internal Cl-), which suggests that NaV1.9 current did not rundown with that internal solution. Using immunohistochemistry, we found that the majority of group IV somata and axons were NaV1.9 positive. The majority of small diameter myelinated afferent somata (putative group III) were also NaV1.9 positive, but myelinated muscle afferent axons were rarely labeled. The presence of NaV1.9 channels in muscle afferents supports a role for these channels in activation and maintenance of the EPR. NEW & NOTEWORTHY Small diameter muscle afferents signal pain and muscle activity levels. The muscle activity signals drive the cardiovascular system to increase muscle blood flow, but these signals can become exaggerated in cardiovascular disease to exacerbate cardiac damage. The voltage-dependent sodium channel NaV1.9 plays a unique role in controlling afferent excitability. We show that NaV1.9 channels are expressed in muscle afferents, which supports these channels as a target for drug development to control hyperactivity of these neurons.
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Affiliation(s)
- Tyler L Marler
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A. T. Still University of Health Sciences, Kirksville, Missouri
| | - Andrew B Wright
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A. T. Still University of Health Sciences, Kirksville, Missouri
| | - Kristina L Elmslie
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A. T. Still University of Health Sciences, Kirksville, Missouri
| | - Ankeeta K Heier
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A. T. Still University of Health Sciences, Kirksville, Missouri
| | - Ethan Remily
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A. T. Still University of Health Sciences, Kirksville, Missouri
| | - Jeong Sook Kim-Han
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A. T. Still University of Health Sciences, Kirksville, Missouri
| | - Renuka Ramachandra
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A. T. Still University of Health Sciences, Kirksville, Missouri
| | - Keith S Elmslie
- The Baker Laboratory of Pharmacology, Department of Pharmacology, Kirksville College of Osteopathic Medicine, A. T. Still University of Health Sciences, Kirksville, Missouri
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Ganapathi SB, Fox TE, Kester M, Elmslie KS. Ceramide modulates HERG potassium channel gating by translocation into lipid rafts. Am J Physiol Cell Physiol 2010; 299:C74-86. [PMID: 20375276 DOI: 10.1152/ajpcell.00462.2009] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
Human ether-à-go-go-related gene (HERG) potassium channels play an important role in cardiac action potential repolarization, and HERG dysfunction can cause cardiac arrhythmias. However, recent evidence suggests a role for HERG in the proliferation and progression of multiple types of cancers, making it an attractive target for cancer therapy. Ceramide is an important second messenger of the sphingolipid family, which due to its proapoptotic properties has shown promising results in animal models as an anticancer agent. Yet the acute effects of ceramide on HERG potassium channels are not known. In the present study we examined the effects of cell-permeable C(6)-ceramide on HERG potassium channels stably expressed in HEK-293 cells. C(6)-ceramide (10 microM) reversibly inhibited HERG channel current (I(HERG)) by 36 +/- 5%. Kinetically, ceramide induced a significant hyperpolarizing shift in the current-voltage relationship (DeltaV(1/2) = -8 +/- 0.5 mV) and increased the deactivation rate (43 +/- 3% for tau(fast) and 51 +/- 3% for tau(slow)). Mechanistically, ceramide recruited HERG channels within caveolin-enriched lipid rafts. Cholesterol depletion and repletion experiments and mathematical modeling studies confirmed that inhibition and gating effects are mediated by separate mechanisms. The ceramide-induced hyperpolarizing gating shift (raft mediated) could offset the impact of inhibition (raft independent) during cardiac action potential repolarization, so together they may nullify any negative impact on cardiac rhythm. Our results provide new insights into the effects of C(6)-ceramide on HERG channels and suggest that C(6)-ceramide can be a promising therapeutic for cancers that overexpress HERG.
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Affiliation(s)
- Sindura B Ganapathi
- Department of Pharmacology, Pennsylvania State College of Medicine, Hershey, PA 17033, USA
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Ganapathi SB, Kester M, Elmslie KS. State-dependent block of HERG potassium channels by R-roscovitine: implications for cancer therapy. Am J Physiol Cell Physiol 2009; 296:C701-10. [PMID: 19244476 DOI: 10.1152/ajpcell.00633.2008] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
Human ether-a-go-go-related gene (HERG) potassium channel acts as a delayed rectifier in cardiac myocytes and is an important target for both pro- and antiarrhythmic drugs. Many drugs have been pulled from the market for unintended HERG block causing arrhythmias. Conversely, recent evidence has shown that HERG plays a role in cell proliferation and is overexpressed both in multiple tumor cell lines and in primary tumor cells, which makes HERG an attractive target for cancer treatment. Therefore, a drug that can block HERG but that does not induce cardiac arrhythmias would have great therapeutic potential. Roscovitine is a cyclin-dependent kinase (CDK) inhibitor that is in phase II clinical trials as an anticancer agent. In the present study we show that R-roscovitine blocks HERG potassium current (human embryonic kidney-293 cells stably expressing HERG) at clinically relevant concentrations. The block (IC(50) = 27 microM) was rapid (tau = 20 ms) and reversible (tau = 25 ms) and increased with channel activation, which supports an open channel mechanism. Kinetic study of wild-type and inactivation mutant HERG channels supported block of activated channels by roscovitine with relatively little effect on either closed or inactivated channels. A HERG gating model reproduced all roscovitine effects. Our model of open channel block by roscovitine may offer an explanation of the lack of arrhythmias in clinical trials using roscovitine, which suggests the utility of a dual CDK/HERG channel block as an adjuvant cancer therapy.
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Affiliation(s)
- Sindura B Ganapathi
- Department of Pharmacology, Penn State College of Medicine, Milton S. Hershey Medical Center, 500 University Dr., Hershey, PA 17033, USA
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Chen Y, Penington NJ. Competition between internal AlF(4)(-) and receptor-mediated stimulation of dorsal raphe neuron G-proteins coupled to calcium current inhibition. J Neurophysiol 2000; 83:1273-82. [PMID: 10712455 DOI: 10.1152/jn.2000.83.3.1273] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Intracellular aluminum fluoride (AlF(4)(-)), placed in a patch pipette, activated a G-protein, resulting in a "tonic" inhibition of the Ca(2+) current of isolated serotonergic neurons of the rat dorsal raphe nucleus. Serotonin (5-HT) also inhibits the Ca(2+) current of these cells. After external bath application and quick removal of 5-HT to an AlF(4)(-) containing cell, there was a reversal or transient disinhibition (TD) of the inhibitory effect of AlF(4)(-) on Ca(2+) current. A short predepolarization of the membrane potential to +70 mV, a condition that is known to reverse G-protein-mediated inhibition, reversed the inhibitory effect of AlF(4)(-) on Ca(2+) current and brought the Ca(2+) current to the same level as that seen at the peak of the TD current. With AlF(4)(-) in the pipette, the TD phenomenon could be eliminated by lowering pipette MgATP, or by totally chelating pipette Al(3+). In the presence of AlF(4)(-), but with either lowered MgATP or extreme efforts to eliminate pipette Al(3+), the rate of recovery from 5-HT on wash was slowed, a condition opposite to that where a TD occurred. The putative complex of AlF(4)(-)-bound G-protein (Galpha.GDP. AlF(4)(-)) appeared to free G-betagamma-subunits, mimicking the effect on Ca(2+) channels of the G.GTP complex. The ON-rate of the inhibition of Ca(2+) current, after a depolarizing pulse, by betagamma-subunits released by AlF(4)(-) in the pipette was significantly slower than that of the agonist-activated G-protein. The OFF-rate of the AlF(4)(-)-mediated inhibition in response to a depolarizing pulse, a measure of the affinity of the free G-betagamma-subunit for the Ca(2+) channel, was slightly slower than that of the agonist stimulated G-protein. In summary, AlF(4)(-) modified the OFF-rate kinetics of G-protein activation by agonists, but had little effect on the kinetics of the interaction of the betagamma-subunit with Ca(2+) channels. Agonist application temporarily reversed the effects of AlF(4)(-), making it a complementary tool to GTP-gamma-S for the study of G-protein interactions.
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Affiliation(s)
- Y Chen
- Department of Physiology and Pharmacology, State University of New York, Health Science Center at Brooklyn, Brooklyn, New York 11203-2098, USA
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Abstract
Calcium (Ca(2+)) transport by the distal tubule (DT) luminal membrane is regulated by the parathyroid hormone (PTH) and calcitonin (CT) through the action of messengers, protein kinases, and ATP as the phosphate donor. In this study, we questioned whether ATP itself, when directly applied to the cytosolic surface of the membrane could influence the Ca(2+) channels previously detected in this membrane. We purified the luminal membranes of rabbit proximal (PT) and DT separately and measured Ca(2+) uptake by these vesicles loaded with ATP or the carrier. The presence of 100 microM ATP in the DT membrane vesicles significantly enhanced 0.5 mM Ca(2+) uptake from 0.57 +/- 0.02 to 0.71 +/- 0.02 pmol/microg per 10 sec (P < 0. 01) in the absence of Na(+) and from 0.36 +/- 0.03 to 0.59 +/- 0.01 pmol/microg per 10 sec (P < 0.01) in the presence of 100 mM Na(+). This effect was dose dependent with an EC(50) value of approximately 40 microM. ATP action involved the high-affinity component of Ca(2+) transport, decreasing the Km from 0.08 +/- 0.01 to 0.04 +/- 0.01 mM (P< 0.02). Replacement of the nucleotide by the nonhydrolyzable ATPgammas abolished this action. Because ATP has been reported to be necessary for cytoskeleton integrity, we also investigated the effect of intravesicular cytochalasin on Ca(2+) transport. Inclusion of 20 microM cytochalasin B decreased 0.5 mM Ca(2+) uptake from 0.33 +/- 0.01 to 0.15 +/- 0.01 pmol/microg per 10 sec (P< 0.01). However, when both 100 microM ATP and 20 microM cytochalasin were present in the vesicles, the uptake was not different from that observed with ATP alone. Neither ATP nor cytochalasin had any influence on Ca(2+) uptake by the PT luminal membrane. We conclude that the high-affinity Ca(2+) channel of the DT luminal membrane is regulated by ATP and that ATP plays a crucial role in the integrity of the cytoskeleton which is also involved in the control of Ca(2+) channels within this membrane.
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Affiliation(s)
- M G Brunette
- Maisonneuve-Rosemont Hospital, Research Centre, University of Montreal, Montreal, Quebec, Canada
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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.
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Affiliation(s)
- D W Hilgemann
- University of Texas Southwestern, Medical Center at Dallas 75235-9040, USA
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Diverse-Pierluissi M, Inglese J, Stoffel RH, Lefkowitz RJ, Dunlap K. G protein-coupled receptor kinase mediates desensitization of norepinephrine-induced Ca2+ channel inhibition. Neuron 1996; 16:579-85. [PMID: 8785055 DOI: 10.1016/s0896-6273(00)80077-x] [Citation(s) in RCA: 49] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
G protein-coupled receptors are essential signaling molecules at sites of synaptic transmission. Here, we explore the mechanisms responsible for the use-dependent termination of metabotropic receptor signaling in embryonic sensory neurons. We report that the inhibition of voltage-dependent Ca2+ channels mediated by alpha2-adrenergic receptors desensitizes slowly with prolonged exposure to the transmitter and that the desensitization is mediated by a G protein-coupled receptor kinase (GRK). Intracellular introduction of recombinant, purified kinases or synthetic blocking peptides into individual neurons demonstrates the specific involvement of a GRK3-like protein. These results suggest that GRK-mediated termination of receptor-G protein coupling is likely to regulate synaptic strength and, as such, may provide one effective mechanism for depression of synaptic transmission.
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Affiliation(s)
- M Diverse-Pierluissi
- Department of Physiology, Tufts University School of Medicine, Boston, Massachusetts 02111, USA
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Elhamdani A, Bossu JL, Feltz A. ADP exerts a protective effect against rundown of the Ca2+ current in bovine chromaffin cells. Pflugers Arch 1995; 430:401-9. [PMID: 7491265 DOI: 10.1007/bf00373916] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
In isolated chromaffin cells, the high-voltage-activated Ca2+ current, recorded using 5 mM Ca2+ as the divalent charge carrier, exhibits rundown within 10 min, which is delayed for 1 h at least by the addition of 1 mM adenosine 5'-triphosphate (ATP) to the pipette medium. The mechanism of this stabilizing action of ATP has been examined. ATP action is dose dependent; the rundown process, which was delayed at concentrations below 0.4 mM, was totally abolished at higher concentrations. The requirement for ATP was shown to be quite strict: 2 mM inosine 5'-triphosphate (ITP) could not replace ATP, whereas guanosine 5'-triphosphate (GTP) could, but at higher concentrations. This effect of ATP was shown to require the presence of MgCl2 and the liberation of a phosphate group since the ATP analogue 5'-adenylyl-imidodiphosphate (AMP-PNP) could not act as a substitute for ATP, suggesting an action through either adenosine 5'-diphosphate (ADP) or a phosphorylation step. ADP, in the presence of Mg2+ only, could replace ATP in the same concentration range. This effect was shown to be specific to ADP; it was maintained after blocking the pathways which convert ADP into ATP, and could not be mimicked by guanosine 5'-diphosphate (GDP). Similarly, ATP and ADP effects were abolished at an increased internal Ca2+ concentration (pCa 6 instead of pCa 7.7, where pCa = -log10[Ca2+]). Nevertheless, the presence of 1 mM Mg-ADP in the bathing solution did not prevent the rundown of the Ca2+ channels when going to the inside-out patch recording configuration.(ABSTRACT TRUNCATED AT 250 WORDS)
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Affiliation(s)
- A Elhamdani
- Laboratorie de Neurobiologie Cellulaire, UPR 9009, CNRS associé à l'Université Louis Pasteur, Strasbourg, France
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2‘,3‘-Dialdehyde GTP as an irreversible G protein antagonist. Disruption and reconstitution of G protein-mediated signal transduction in cells and cell membranes. J Biol Chem 1994. [DOI: 10.1016/s0021-9258(18)31794-0] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
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Elmslie KS, Jones SW. Concentration dependence of neurotransmitter effects on calcium current kinetics in frog sympathetic neurones. J Physiol 1994; 481 ( Pt 1):35-46. [PMID: 7853249 PMCID: PMC1155864 DOI: 10.1113/jphysiol.1994.sp020417] [Citation(s) in RCA: 48] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
1. Noradrenaline (NA) slows the activation kinetics of N-type calcium channels, via G proteins. It has been suggested that the G proteins act by binding directly to the calcium channels. If the slow kinetics reflect binding and unbinding of G proteins, the rates should depend on the concentration of activated G protein. 2. We used different concentrations of NA, and increasing durations of intracellular dialysis with GTP-gamma-S, to vary the concentration of activated G protein. 3. At depolarized potentials (-20 or -10 mV), the slow activation kinetics showed no detectable concentration dependence. This analysis required correction for effects of inactivation on the measured time constants. 4. At -80 mV, reinhibition of calcium channel current was more rapid for larger responses. Thus, the effect appears to be concentration dependent at -80 mV, but not at more depolarized voltages. 5. This voltage dependence is actually expected from kinetic principles: the binding step is rate limiting when the position of equilibrium is toward the bound state (at -80 mV), but not when equilibrium favours unbinding (when the channel is open). 6. During inhibition, the channel appears to 'sense' directly the concentration of the modulator, possibly active G proteins.
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Affiliation(s)
- K S Elmslie
- Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, OH 44106
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Affiliation(s)
- P A Smith
- Department of Pharmacology, University of Alberta, Edmonton, Canada
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O'Rourke B. Ion channels as sensors of cellular energy. Mechanisms for modulation by magnesium and nucleotides. Biochem Pharmacol 1993; 46:1103-12. [PMID: 7692854 DOI: 10.1016/0006-2952(93)90456-7] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Affiliation(s)
- B O'Rourke
- Department of Medicine, Johns Hopkins University School of Medicine, Baltimore, MD 21205
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Werz MA, Elmslie KS, Jones SW. Phosphorylation enhances inactivation of N-type calcium channel current in bullfrog sympathetic neurons. Pflugers Arch 1993; 424:538-45. [PMID: 8255738 DOI: 10.1007/bf00374919] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
We have investigated the effects of phosphatase and protein kinase inhibitors on calcium channel currents of bullfrog sympathetic neurons using the whole cell configuration of the patch clamp technique. Intracellular dialysis with the phosphatase inhibitors okadaic acid and calyculin A markedly enhanced the decline of inward current during a depolarizing voltage step. Tail current analysis demonstrated that this was genuine inactivation of calcium channel current, not activation of an outward current. The rapidly inactivating current is N-type calcium current (blocked by omega-conotoxin and resistant to nifedipine). Staurosporine, a nonselective protein kinase inhibitor, prevented the action of okadaic acid, suggesting that protein phosphorylation is involved. Under control conditions, the time course of inactivation could be described by the sum of two exponentials (tau = 150 ms and 1200 ms), plus a constant (apparently noninactivating) component, during depolarizations lasting 2 s. Okadaic acid induced a rapid inactivation process (tau = 15 ms) that was absent or negligible under control conditions, without obvious effect on the two slower time constants. As in control cells, inactivation in okadaic-acid-treated cells was strongest near -20 mV, with less inactivation at more positive voltages. However, inactivation did not depend on calcium influx. Modulation of calcium channel activity by phosphorylation may underly the spontaneous shift between inactivating and noninactivating modes recently observed for N-type calcium channels. Differences in basal phosphorylation levels could also explain why N-type calcium channels, originally described as rapidly and completely inactivating, inactivate slowly and incompletely in many neurons.
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Affiliation(s)
- M A Werz
- Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, OH 44106
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